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RBSE Praveshika Science study materials
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RBSE Praveshika Science practice questions

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Question 1Matter and Its Nature

A sample of matter has a fixed volume but no fixed shape, and it takes the shape of the vessel into which it is poured. In which state is the sample?

  1. Solid, since its particles vibrate about fixed mean positions.
  2. Liquid, since its particles can move about while staying in contact. Correct answer.
  3. Gas, since its particles are very far apart and move at high speed.
  4. Plasma, since its particles are ionised and conduct electricity.

Explanation

In a liquid the particles are still close together, so the volume is fixed, but the forces between them are not strong enough to hold a fixed shape. The particles slide past one another, so the liquid flows and takes the shape of its container.

Question 2Matter and Its Nature

Which of the following samples is a pure substance in the chemical sense?

  1. Nitrogen gas collected in a cylinder Correct answer.
  2. Brass, an alloy of copper and zinc metals
  3. Sea water taken directly from an open sea beach
  4. Air that has been filtered to remove all dust particles

Explanation

A pure substance is either an element or a compound with a fixed composition. Nitrogen contains only nitrogen molecules, so it is an element and therefore pure, while air, sea water and brass each contain two or more substances mixed together.

Question 3Matter and Its Nature

In which of the following does a new substance with different properties form?

  1. Ice melting into liquid water at room temperature
  2. Petrol evaporating slowly from an open dish left in the sun
  3. A magnesium ribbon burning in air Correct answer.
  4. Sugar dissolving completely in a glass of water

Explanation

A chemical change produces one or more new substances whose properties differ from those of the starting substance. Magnesium burns in air to give white magnesium oxide, a new substance, whereas melting, dissolving and evaporation only change the state or the distribution of the same substance.

Question 4Matter and Its Nature

A jar of bromine vapour is placed mouth to mouth with a jar of air. After some time the brown colour of bromine is seen evenly in both jars. Which explanation fits this observation best?

  1. Air particles are pushed out because bromine particles expand on their own.
  2. Bromine reacts chemically with the gases of air to give a brown product.
  3. Bromine particles move randomly and diffuse among the air particles. Correct answer.
  4. Bromine is heavier than air, so it spreads upward into the upper jar.

Explanation

Diffusion is the spreading of the particles of one substance through another because of their random motion. Bromine particles move randomly and mix with air particles until they are evenly distributed through both jars, and no reaction or expansion of the particles themselves is involved.

Question 5Matter and Its Nature

Water kept in an unglazed earthen pot (matka) stays cooler than water kept in a metal vessel on the same day. Why?

  1. Water seeping through the pores evaporates and takes latent heat from the water inside. Correct answer.
  2. Clay chemically reacts with the water and absorbs heat from the water during the reaction.
  3. Clay is a poor conductor of heat, so the pot stops the outside air from warming the water inside it.
  4. The tiny pores of the pot let cool air enter and push the warm air out of the pot.

Explanation

Water that seeps through the porous clay evaporates from the outer surface, and evaporation requires latent heat, which it takes from the water remaining inside the pot. This loss of heat lowers the temperature of the water, so it stays cooler than water in a metal vessel.

Question 6Matter and Its Nature

Which statement about the particles of a gas kept in a closed container is correct?

  1. They are packed closely together but can slide past one another.
  2. They are held together by strong forces and form a rigid lattice.
  3. They are very far apart and move rapidly in random directions. Correct answer.
  4. They are held in fixed positions and only vibrate about those positions.

Explanation

Gas particles are separated by distances much larger than the particles themselves and move rapidly and randomly, colliding with the walls and with one another. The forces of attraction between them are weak, so a gas fills any container and has neither a fixed shape nor a fixed volume.

Question 7Matter and Its Nature

A mixture of sand and water is cleared by filtration, but salt water cannot be cleared of salt in the same way. What is the reason?

  1. Salt particles are dissolved and pass through the filter paper. Correct answer.
  2. Salt is lighter than water, so it floats on the surface and is not stopped by the paper.
  3. Salt is heavier than sand, so it settles down at the bottom of the funnel.
  4. Salt water is a compound, so its components cannot be separated by any physical method.

Explanation

Dissolved salt is present as individual ions that are far smaller than the pores of the filter paper, so they pass through with the water. Filter paper can hold back only undissolved particles, such as grains of sand, that are large enough to be trapped in its pores.

Question 8Matter and Its Nature

Which feature belongs to a compound and not to a mixture?

  1. The composition can be varied within a wide range.
  2. The constituents are combined in a fixed ratio by mass. Correct answer.
  3. The constituents can be separated by filtration or evaporation.
  4. The sample shows the properties of all its constituents.

Explanation

In a compound the elements combine chemically in a definite ratio by mass, and the compound has properties different from those of its constituents. A mixture can be made in any proportion, keeps the properties of its constituents and can be separated by physical methods.

Question 9Matter and Its Nature

Air is trapped in a sealed syringe. The piston is pushed in until the volume of the trapped air is halved, while the temperature is kept constant. Which description of what happens inside the syringe is correct?

  1. The particles join together into a liquid, which occupies the smaller volume inside.
  2. The air particles themselves shrink to half their size, so the air fits into the smaller volume.
  3. Half of the air particles leave through the walls of the syringe, so the air takes less space.
  4. The same particles are crowded into a smaller space and strike the walls more often. Correct answer.

Explanation

The particles of a gas are separated by large empty spaces, so when the piston is pushed in, the same particles fit into a smaller volume without changing their size or number. They strike the walls of the syringe more frequently, which is why the pressure of the trapped air rises.

Question 10Matter and Its Nature

A piece of ice floats in water, with most of it lying below the surface. Which statement explains why ice floats on water?

  1. Upthrust acts only on solid bodies, and it lifts the ice upwards through the water.
  2. Water molecules split into gases when the liquid freezes at zero degrees Celsius.
  3. Ice particles are arranged in an open cage, so ice is less dense than water. Correct answer.
  4. Ice particles are lighter than water particles because cooling removes some of their mass.

Explanation

In ice, hydrogen bonding holds the water molecules in an open, cage-like framework containing many empty spaces, so a given volume of ice contains fewer molecules than the same volume of liquid water. This open arrangement gives ice a lower density than water, so ice floats with most of its volume below the surface.

Question 11Matter and Its Nature

Which state of matter has particles that are closely packed in a regular pattern and can only vibrate about fixed positions?

  1. Gas
  2. Plasma
  3. Solid Correct answer.
  4. Liquid

Explanation

In a solid, particles are held close together by strong forces and occupy fixed mean positions; they vibrate but do not move past one another. This regular, closely packed arrangement gives a solid a definite shape and a definite volume.

Question 12Matter and Its Nature

Which of the following changes is a physical change?

  1. Burning of a piece of paper
  2. Melting of wax Correct answer.
  3. Rusting of an iron nail
  4. Digestion of food

Explanation

Melting wax is a physical change because only the state of the wax changes from solid to liquid; no new substance is formed and the wax can be solidified again. Burning paper, rusting iron and digesting food all produce new substances with different properties.

Question 13Matter and Its Nature

Which of the following can be compressed easily by pressing a piston?

  1. Honey kept in a jar
  2. Water filled in a syringe
  3. A block of iron
  4. Air trapped in a syringe Correct answer.

Explanation

A gas has large empty spaces between its particles, so pressing the piston brings the particles closer together and reduces the volume. Solids and liquids are already closely packed, so they are almost incompressible.

Question 14Matter and Its Nature

Which statement correctly distinguishes an element from a compound?

  1. An element can be broken down by chemical methods, while a compound cannot be broken down.
  2. An element has one kind of atom, while a compound has two or more kinds combined. Correct answer.
  3. An element is always a gas at room temperature, while a compound is always a solid.
  4. An element has a composition that varies, while a compound has a fixed composition by mass.

Explanation

An element is a pure substance made of only one kind of atom and cannot be broken down into simpler substances by chemical means. A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio by mass.

Question 15Matter and Its Nature

A mixture of oil and water is shaken well and then left to stand. Which method is most suitable for separating the two liquids?

  1. Evaporation
  2. Using a separating funnel Correct answer.
  3. Filtration through filter paper
  4. Sublimation

Explanation

Oil and water are immiscible liquids; on standing they form two separate layers. A separating funnel lets the lower layer run out first, after which the upper layer is collected separately. Filtration, evaporation and sublimation are not designed to separate two immiscible liquids.

Question 16Matter and Its Nature

Which change will increase the rate of evaporation of water kept in an open dish?

  1. Covering the dish with a lid
  2. Decreasing the surface area of the water
  3. Increasing the humidity of the surrounding air
  4. Increasing the temperature of the water Correct answer.

Explanation

Evaporation is faster at higher temperature because more molecules at the liquid surface gain enough energy to escape into the vapour phase. A larger surface area, dry air and wind also increase evaporation, whereas a smaller surface area, high humidity and a lid reduce it.

Question 17Matter and Its Nature

A drop of ink spreads faster through hot water than through cold water. What is the best explanation for this observation?

  1. At a higher temperature, particles move faster, so diffusion occurs more quickly. Correct answer.
  2. Ink dissolves only in hot water and not in cold water.
  3. Cold water particles attract ink particles strongly and stop them from moving.
  4. Hot water has larger spaces between its particles than cold water.

Explanation

Diffusion is the spontaneous spreading of particles due to their random motion. At a higher temperature the particles have greater average kinetic energy and move faster, so ink particles mix with hot water more quickly than with cold water.

Question 18Matter and Its Nature

The random zig-zag movement of tiny pollen grains suspended in water is known as Brownian motion. What causes this motion?

  1. Unequal collisions of fast-moving water molecules with the pollen grains Correct answer.
  2. The pollen grains swimming actively in the water
  3. A chemical reaction between pollen and water
  4. Gravity pulling the pollen grains downwards

Explanation

Water molecules are in constant random motion and collide with the suspended pollen grains from all sides. Because the collisions are unequal at any instant, the grains move in an irregular zig-zag path. This is evidence that matter is made of moving particles.

Question 19Matter and Its Nature

Ice at 0°C is heated continuously, but its temperature remains at 0°C until all the ice melts. Why does the temperature not rise during melting?

  1. Ice is a poor conductor, so its temperature cannot rise.
  2. The heat changes the state by breaking forces between particles. Correct answer.
  3. The heat is completely lost to the surroundings.
  4. The thermometer cannot record temperatures above 0°C.

Explanation

The heat supplied during melting is latent heat; it is used to overcome the forces holding water molecules in the ice lattice. This energy changes the state from solid to liquid without increasing the average kinetic energy of the molecules, so the temperature remains at 0°C until all the ice has melted.

Question 20Matter and Its Nature

Two immiscible liquids, P and Q, are placed in a container. The density of P is 1.2 g/cm³ and the density of Q is 0.8 g/cm³. Which statement describes the arrangement after they settle?

  1. Q floats on P because it has a lower density. Correct answer.
  2. Q sinks below P because its density is lower.
  3. P floats on Q because it has a higher density.
  4. P and Q mix completely because both are liquids.

Explanation

For immiscible liquids, the less dense liquid forms the upper layer and the denser liquid forms the lower layer. Q has a lower density (0.8 g/cm³) than P (1.2 g/cm³), so Q floats on P. Volume alone does not decide the order of layers.

Question 21Matter and Its Nature

A solid air freshener made of naphthalene is kept in a closed cupboard. After some weeks the solid has become much smaller and its smell has spread through the cupboard, but no liquid is ever seen on the shelf. The change of state taking place here is called

  1. Condensation
  2. Melting
  3. Evaporation
  4. Sublimation Correct answer.

Explanation

Sublimation is the direct change of a solid into vapour on warming, with no liquid stage in between. Naphthalene, camphor and ammonium chloride behave in this way, which is why the solid slowly shrinks while its vapour spreads and gives the smell. Since no liquid is formed at any point, the change cannot be called evaporation or melting.

Question 22Matter and Its Nature

A person suffers more serious burns from steam at 100 °C than from boiling water at 100 °C. Which statement explains this?

  1. It is at a temperature higher than that of boiling water
  2. It contains more air dissolved in it than boiling water
  3. Its specific heat capacity is greater than that of water
  4. It releases latent heat when it condenses Correct answer.

Explanation

Steam at 100 °C first condenses to water at 100 °C on the cooler skin, and in doing so it gives out its latent heat of vaporisation, which is very large. The skin therefore receives this extra heat in addition to the heat that boiling water alone would give up while cooling. Both steam and boiling water are at the same temperature, so a temperature difference cannot be the reason.

Question 23Matter and Its Nature

A drop of green ink is placed near the lower end of a strip of filter paper whose lower tip dips into water. As the water rises up the paper, the green spot separates into a blue band and a yellow band at different heights. The method used here is

  1. Filtration through filter paper
  2. Separating funnel
  3. Simple distillation
  4. Paper chromatography Correct answer.

Explanation

In chromatography the solvent creeps up the paper and carries the components of the mixture with it. Each coloured component travels at a different speed depending on how strongly it is held by the paper and how well it dissolves in the solvent, so the dyes separate into bands at different heights. The same principle is used to test the dyes in inks and food colours.

Question 24Matter and Its Nature

50 mL of water is mixed with 50 mL of ethanol in a measuring cylinder. The total volume of the mixture is found to be slightly less than 100 mL, although no liquid is spilt or lost. Which explanation is correct?

  1. The molecules of both liquids shrink in size when they are mixed
  2. Part of the ethanol evaporates as soon as the two liquids are mixed
  3. The smaller molecules fit into the spaces between the larger molecules Correct answer.
  4. The two liquids react chemically and form a new compound of smaller volume

Explanation

Particles of matter are not packed tightly together; there are empty spaces between them. When water and ethanol are mixed, the smaller molecules slip into these gaps, so the particles occupy less total space and the volume of the mixture falls slightly short of the sum of the two volumes. The particles themselves keep their size and no new substance is formed, so the change is physical.

Question 25Motion, Force and Gravitation

A boy walks 4 m due east and then 3 m due north. What are the total distance travelled and the magnitude of the displacement?

  1. distance = 5 m, displacement = 7 m
  2. distance = 7 m, displacement = 7 m
  3. distance = 7 m, displacement = 5 m Correct answer.
  4. distance = 12 m, displacement = 5 m

Explanation

Distance is a scalar, so the two legs simply add: 4 m + 3 m = 7 m. Displacement is the straight-line vector from the starting point to the finishing point, and the two legs are at right angles, so its magnitude is the square root of (4 squared + 3 squared), that is 5 m, directed towards the north-east.

Question 26Motion, Force and Gravitation

A passenger standing in a stationary bus falls backward when the bus starts suddenly. This is best explained by

  1. the backward reaction force exerted by the road on the bus
  2. the gravitational pull of the Earth on the passenger
  3. the inertia of rest of the passenger's upper body Correct answer.
  4. the inertia of motion of the bus

Explanation

When the bus starts, the lower part of the passenger's body moves forward with the floor while the upper body tends to stay at rest because of its inertia of rest. The upper body therefore lags behind relative to the bus, and the passenger appears to fall backward.

Question 27Motion, Force and Gravitation

According to Newton's law of universal gravitation, the gravitational force between two point masses is proportional to the

  1. difference of the two masses and inversely proportional to the square of the distance between them
  2. product of the two masses and directly proportional to the distance between them
  3. product of the two masses and inversely proportional to the square of the distance between them Correct answer.
  4. sum of the two masses and inversely proportional to the distance between them

Explanation

The law is written as F = G m1 m2 / r squared, where r is the separation between the centres of the two masses. Hence the force grows with the product of the masses and falls off as the inverse square of the distance between them.

Question 28Motion, Force and Gravitation

The velocity of a scooter increases uniformly from 10 m/s to 30 m/s in 5 s. What is its acceleration?

  1. 3 m/s²
  2. 2 m/s²
  3. 4 m/s² Correct answer.
  4. 5 m/s²

Explanation

Acceleration is the rate of change of velocity, a = (v - u) / t. Substituting u = 10 m/s, v = 30 m/s and t = 5 s gives a = 20 / 5 = 4 m/s², and the change is uniform so this is the constant acceleration.

Question 29Motion, Force and Gravitation

A bullet of mass 0.02 kg leaves a gun of mass 4 kg with a velocity of 400 m/s. What is the speed of recoil of the gun?

  1. 2 m/s Correct answer.
  2. 4 m/s
  3. 0.5 m/s
  4. 8 m/s

Explanation

Before firing, the total momentum of the bullet-gun system is zero, and no external horizontal force acts, so momentum is conserved. The bullet's momentum is 0.02 × 400 = 8 kg m/s, so the gun must carry 8 kg m/s in the opposite direction, giving 8 / 4 = 2 m/s.

Question 30Motion, Force and Gravitation

A stone is dropped from rest from a tower and falls freely. Taking g = 10 m/s², how far does it fall in the first 3 s?

  1. 30 m
  2. 90 m
  3. 15 m
  4. 45 m Correct answer.

Explanation

For free fall from rest, the distance is given by s = ½ g t² because the initial velocity is zero. Substituting g = 10 m/s² and t = 3 s gives s = ½ × 10 × 9 = 45 m.

Question 31Motion, Force and Gravitation

An astronaut travels from the Earth to the Moon. Compared with the Earth, on the Moon's surface

  1. the mass remains the same and the weight becomes about six times
  2. the mass remains the same and the weight becomes about one-sixth Correct answer.
  3. both the mass and the weight remain unchanged
  4. the astronaut's mass becomes one-sixth of its Earth value and the weight stays the same

Explanation

Mass measures the quantity of matter in a body and does not change with location. Weight is the gravitational force W = mg, and since g on the Moon is about one-sixth of g on the Earth, the astronaut's weight there is about one-sixth of the Earth weight.

Question 32Motion, Force and Gravitation

A footballer kicks a stationary ball. The ball pushes back on the footballer's foot with a force equal in magnitude to the kick, yet the ball accelerates far more than the foot. Why?

  1. The ball has far less mass, so the same force produces a much greater acceleration in it. Correct answer.
  2. The reaction force from the ball acts on the foot only after the ball has started moving.
  3. The foot exerts a larger force on the ball than the ball exerts on the foot.
  4. The ball is lighter, so gravity pulls it towards the ground with a greater force.

Explanation

Newton's third law says the two forces are equal in magnitude and opposite in direction, and they act simultaneously on the two different bodies. Newton's second law then gives a = F/m, so for the same force the body with much smaller mass, the ball, gets the much larger acceleration.

Question 33Motion, Force and Gravitation

A person of mass 50 kg stands on a spring scale inside a lift that is accelerating upward at 2 m/s². Taking g = 10 m/s², what reading does the scale show?

  1. 500 N
  2. 100 N
  3. 600 N Correct answer.
  4. 400 N

Explanation

The scale reads the normal force N exerted by the floor. For an upward acceleration, Newton's second law gives N - mg = ma, so N = m(g + a) = 50 × (10 + 2) = 600 N, which is greater than the true weight of 500 N.

Question 34Motion, Force and Gravitation

Two spheres of masses m and 4m have their centres separated by a distance r. At what point on the line joining them is the net gravitational field zero?

  1. At a distance r/3 from the sphere of mass m Correct answer.
  2. At a distance 2r/3 from the sphere of mass m
  3. At a distance r/5 from the sphere of mass m
  4. At a distance r/2 from the sphere of mass m

Explanation

At the neutral point the fields of the two spheres must be equal in magnitude and opposite in direction, so Gm/x² = G(4m)/(r - x)². Taking the square root gives (r - x) = 2x, hence x = r/3, measured from the smaller mass and inside the region between the spheres.

Question 35Motion, Force and Gravitation

Which of the following quantities is a vector quantity?

  1. Speed
  2. Displacement Correct answer.
  3. Distance
  4. Mass

Explanation

Displacement has both magnitude and direction, so it is a vector. Speed, distance and mass have magnitude only and are scalars.

Question 36Motion, Force and Gravitation

One newton is the force that produces an acceleration of 1 m/s² in a mass of

  1. 1 g
  2. 100 g
  3. 1 kg Correct answer.
  4. 10 kg

Explanation

From Newton's second law, F = ma. Putting F = 1 N and a = 1 m/s² gives m = F/a = 1 kg.

Question 37Motion, Force and Gravitation

The acceleration of a body is defined as the rate of change of its

  1. velocity Correct answer.
  2. momentum
  3. displacement
  4. force

Explanation

Acceleration measures how quickly velocity changes with time, so a = (v − u)/t. Change of displacement with time is velocity, not acceleration.

Question 38Motion, Force and Gravitation

A car covers the first half of a straight road at 20 m/s and the second half at 30 m/s. Its average speed for the whole journey is

  1. 30 m/s
  2. 24 m/s Correct answer.
  3. 25 m/s
  4. 26 m/s

Explanation

For equal distances, average speed is 2v1v2/(v1+v2) = 2×20×30/(20+30) = 24 m/s, not the arithmetic mean. The slower half takes more time, so the average is below 25 m/s.

Question 39Motion, Force and Gravitation

A body starts from rest and moves with a uniform acceleration of 2 m/s². The distance covered in 5 s is

  1. 50 m
  2. 25 m Correct answer.
  3. 10 m
  4. 20 m

Explanation

Using s = ut + ½at² with u = 0, a = 2 m/s² and t = 5 s gives s = ½ × 2 × 25 = 25 m.

Question 40Motion, Force and Gravitation

A constant force of 10 N acts for 4 s on a 2 kg body initially at rest. The final speed of the body is

  1. 5 m/s
  2. 20 m/s Correct answer.
  3. 10 m/s
  4. 40 m/s

Explanation

Impulse = F × t = 10 × 4 = 40 N s, which equals the change in momentum. Since the body starts from rest, 2 × v = 40, so v = 20 m/s.

Question 41Motion, Force and Gravitation

An object of mass 10 kg is taken to a planet where the acceleration due to gravity is 4 m/s². Its weight on that planet is

  1. 40 N Correct answer.
  2. 10 N
  3. 100 N
  4. 4 N

Explanation

Weight is W = mg, where g is the local gravitational acceleration. Here W = 10 kg × 4 m/s² = 40 N, whereas on Earth it would be about 100 N.

Question 42Motion, Force and Gravitation

The gravitational force between two bodies is F. If the mass of one body is doubled and the distance between their centres is halved, the new force is

  1. 8F Correct answer.
  2. 4F
  3. 2F
  4. F/2

Explanation

By Newton's law of gravitation F ∝ m1m2/r². Doubling one mass multiplies the force by 2, and halving r multiplies it by 4, giving 2 × 4 = 8F.

Question 43Motion, Force and Gravitation

A body falls freely from rest. The ratio of the distances covered by it in the first, second and third seconds of its fall is

  1. 1 : 3 : 5 Correct answer.
  2. 1 : 1 : 1
  3. 1 : 4 : 9
  4. 1 : 2 : 3

Explanation

The distance in the nth second from rest is proportional to (2n − 1), giving 1, 3 and 5 for n = 1, 2, 3. The ratio 1 : 4 : 9 is the ratio of total distances after 1, 2 and 3 seconds, not the distances in each second.

Question 44Motion, Force and Gravitation

A bus moving at 15 m/s is brought to rest in 5 s. The retardation of the bus is

  1. 0.3 m/s²
  2. 3 m/s² Correct answer.
  3. 5 m/s²
  4. 15 m/s²

Explanation

Retardation is the magnitude of the negative acceleration: a = (v − u)/t = (0 − 15)/5 = −3 m/s². Its magnitude is 3 m/s².

Question 45Motion, Force and Gravitation

The SI unit of the universal gravitational constant G is

  1. N m kg⁻²
  2. N m² kg²
  3. N m² kg⁻² Correct answer.
  4. N m² kg⁻¹

Explanation

Newton's law of gravitation gives F = G m₁m₂/r², so G = Fr²/(m₁m₂). Substituting the SI units of force, distance and mass gives N m² kg⁻². The squared distance contributes m², and both masses contribute kg² in the denominator.

Question 46Motion, Force and Gravitation

A body moves along a circular path of radius 5 m with a constant speed of 10 m/s. What is the magnitude of its centripetal acceleration?

  1. 2 m/s²
  2. 50 m/s²
  3. 100 m/s²
  4. 20 m/s² Correct answer.

Explanation

For uniform circular motion the centripetal acceleration has magnitude a = v²/r. Here v² = 100 m²/s² and r = 5 m, so a = 100/5 = 20 m/s². This acceleration is directed towards the centre of the circle.

Question 47Motion, Force and Gravitation

A stone is thrown vertically upwards with a speed of 20 m/s. Taking g = 10 m/s² and ignoring air resistance, what maximum height does the stone reach?

  1. 20 m Correct answer.
  2. 40 m
  3. 10 m
  4. 15 m

Explanation

At the highest point the stone's velocity is zero. Using v² = u² − 2gh with v = 0 gives h = u²/(2g) = (20)²/(2 × 10) = 400/20 = 20 m. The upward speed decreases uniformly because gravity acts downwards throughout the rise.

Question 48Motion, Force and Gravitation

A body weighs 50 N on the Earth's surface. At a height above the surface equal to the Earth's radius, its weight will be

  1. 50.0 N
  2. 25.0 N
  3. 12.5 N Correct answer.
  4. 100.0 N

Explanation

At height h above the surface, g′ = g(R/(R + h))². With h = R, the distance from the Earth's centre is 2R, so g′ = g/4 and the weight becomes 50/4 = 12.5 N. Weight decreases with altitude because the gravitational field weakens.

Question 49Work, Energy and Sound

The SI unit of work and energy is the:

  1. joule Correct answer.
  2. watt
  3. newton
  4. pascal

Explanation

Work is force multiplied by displacement along the force, so its unit is the newton-metre, called the joule. Energy is the capacity to do work and is measured in the same unit. Hence the SI unit of both work and energy is the joule.

Question 50Work, Energy and Sound

A ringing electric bell is placed inside a jar from which air is gradually removed. The sound becomes fainter and finally cannot be heard because:

  1. the bell stops vibrating in vacuum
  2. the frequency of the bell falls to zero
  3. sound needs a material medium to travel Correct answer.
  4. sound waves are reflected by the jar

Explanation

Sound is a mechanical wave, so it requires a material medium such as air, water, or steel to carry its vibrations. When the air is removed, there are too few particles to transmit the disturbance to the listener. The bell itself continues to vibrate, but no sound reaches the ear through vacuum.