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LC Physics and Chemistry study materials
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LC Physics and Chemistry practice questions

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Question 1Mechanics and Motion

A ball is thrown vertically upward with a speed of 20 m/s. Taking g = 10 m/s² and ignoring air resistance, how long does the ball take to return to the thrower's hand?

  1. 4 s Correct answer.
  2. 6 s
  3. 8 s
  4. 2 s

Explanation

The time to reach the top is v/g = 20/10 = 2 s, and the upward and downward journeys are symmetric, so the total flight time is twice that value, 4 s. Alternatively, applying s = ut + ½at² with s = 0 gives t = 2u/g = 4 s.

Question 2Mechanics and Motion

A car travels 150 m in 10 s at constant speed. What distance does it cover in 24 s at the same speed?

  1. 340 m
  2. 420 m
  3. 360 m Correct answer.
  4. 300 m

Explanation

At constant speed the distance is proportional to time. The speed is 150/10 = 15 m/s, so in 24 s the car covers 15 × 24 = 360 m.

Question 3Mechanics and Motion

Which quantity is a vector rather than a scalar?

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

Explanation

Displacement has both magnitude and direction, so it is a vector. Speed, mass and time are fully described by a magnitude alone and are therefore scalars.

Question 4Mechanics and Motion

An object moves in a straight line at constant velocity. Which statement about the net force on the object is correct?

  1. The net force is zero. Correct answer.
  2. The net force is constant but non-zero.
  3. The net force increases steadily with time.
  4. The net force acts opposite to the motion.

Explanation

By Newton's first law, constant velocity means zero acceleration, and Newton's second law then gives F = ma = 0. A non-zero net force would necessarily change the object's speed or direction.

Question 5Mechanics and Motion

A force of 30 N acts on a 6 kg object on a frictionless surface. What is the object's acceleration?

  1. 0.2 m/s²
  2. 180 m/s²
  3. 36 m/s²
  4. 5 m/s² Correct answer.

Explanation

Newton's second law gives a = F/m. Substituting the values, a = 30/6 = 5 m/s².

Question 6Mechanics and Motion

Two forces of 6 N and 8 N act at right angles on a point. What is the magnitude of their resultant?

  1. 14 N
  2. 10 N Correct answer.
  3. 48 N
  4. 2 N

Explanation

Perpendicular vectors add by the Pythagorean theorem, so the resultant has magnitude √(6² + 8²) = √100 = 10 N.

Question 7Mechanics and Motion

A body of mass 4 kg is lifted vertically at constant speed through a height of 5 m. Taking g = 10 m/s², how much work is done against gravity?

  1. 20 J
  2. 100 J
  3. 50 J
  4. 200 J Correct answer.

Explanation

The lifting force must equal the weight mg = 4 × 10 = 40 N, and work equals force times displacement, giving 40 × 5 = 200 J. This equals the gain in gravitational potential energy mgh.

Question 8Mechanics and Motion

A 1200 kg car accelerates uniformly from rest to 20 m/s in 8 s. What is the magnitude of the net force acting on the car?

  1. 3000 N Correct answer.
  2. 2400 N
  3. 1500 N
  4. 9600 N

Explanation

The acceleration is (20 − 0)/8 = 2.5 m/s², and Newton's second law gives F = ma = 1200 × 2.5 = 3000 N.

Question 9Mechanics and Motion

A block slides down a frictionless incline of angle 30°. Taking g = 10 m/s², what is the acceleration of the block along the incline?

  1. 8.7 m/s²
  2. 5 m/s² Correct answer.
  3. 10 m/s²
  4. 17 m/s²

Explanation

The component of gravity along a frictionless incline is g sin θ. With θ = 30°, sin 30° = 0.5, so a = 10 × 0.5 = 5 m/s².

Question 10Mechanics and Motion

A 0.5 kg ball moving at 8 m/s is brought to rest in 0.2 s. What is the magnitude of the average force acting on it?

  1. 80 N
  2. 20 N Correct answer.
  3. 8 N
  4. 4 N

Explanation

The impulse equals the change in momentum: FΔt = mΔv. Thus F = (0.5 × 8)/0.2 = 4/0.2 = 20 N.

Question 11Mechanics and Motion

A 2 kg object moving at 3 m/s collides head-on with a stationary 1 kg object, and they stick together. What is their common speed after the collision?

  1. 2 m/s Correct answer.
  2. 3 m/s
  3. 1 m/s
  4. 1.5 m/s

Explanation

Momentum is conserved in the perfectly inelastic collision: (2)(3) = (2 + 1)v. Solving gives v = 6/3 = 2 m/s.

Question 12Mechanics and Motion

A wheel starts from rest and reaches an angular speed of 12 rad/s in 3 s at constant angular acceleration. What is the angular displacement during this time?

  1. 18 rad Correct answer.
  2. 36 rad
  3. 12 rad
  4. 6 rad

Explanation

For constant angular acceleration from rest, θ = ½(ω₀ + ω)t = ½(0 + 12)(3) = 18 rad. Equivalently, α = 4 rad/s² and θ = ½αt² = 18 rad.

Question 13Mechanics and Motion

A force is applied at the rim of a wheel of radius 0.25 m, perpendicular to the radius, producing a torque of 5 N·m. What is the magnitude of the force?

  1. 20 N Correct answer.
  2. 25 N
  3. 10 N
  4. 1.25 N

Explanation

Torque is τ = rF sin θ, and with the force perpendicular to the radius sin θ = 1. Thus F = τ/r = 5/0.25 = 20 N.

Question 14Mechanics and Motion

A crate of weight 400 N is pushed across a horizontal floor at constant speed by a horizontal force of 120 N. What is the coefficient of kinetic friction between the crate and the floor?

  1. 0.15
  2. 1.20
  3. 0.30 Correct answer.
  4. 0.33

Explanation

At constant speed the pushing force balances kinetic friction, so f = 120 N. Since f = μN and N equals the 400 N weight on level ground, μ = 120/400 = 0.30.

Question 15Mechanics and Motion

A 0.2 kg stone on a string of length 0.5 m moves in a horizontal circle at 4 m/s. What is the tension in the string, neglecting gravity?

  1. 16 N
  2. 3.2 N
  3. 1.6 N
  4. 6.4 N Correct answer.

Explanation

The tension supplies the centripetal force F = mv²/r. Substituting, F = 0.2 × 16/0.5 = 6.4 N.

Question 16Mechanics and Motion

A 0.4 kg ball is dropped from rest from a height of 5 m. Taking g = 10 m/s² and ignoring air resistance, what is its kinetic energy just before impact?

  1. 40 J
  2. 20 J Correct answer.
  3. 200 J
  4. 2 J

Explanation

By conservation of energy, the kinetic energy at impact equals the loss of potential energy, mgh = 0.4 × 10 × 5 = 20 J.

Question 17Mechanics and Motion

A machine raises a 200 N load through 3 m in 6 s. What is its useful output power?

  1. 33 W
  2. 600 W
  3. 3600 W
  4. 100 W Correct answer.

Explanation

Useful work output is Fd = 200 × 3 = 600 J, and power is work divided by time: 600/6 = 100 W.

Question 18Mechanics and Motion

A projectile is launched at 30 m/s at 60° above the horizontal. Taking g = 10 m/s² and ignoring air resistance, what is the maximum height reached?

  1. 33.75 m Correct answer.
  2. 67.5 m
  3. 22.5 m
  4. 45 m

Explanation

The vertical component of the launch velocity is 30 sin 60° ≈ 26.0 m/s. The maximum height is v_y²/(2g) = 676/20 ≈ 33.75 m.

Question 19Mechanics and Motion

A person of mass 60 kg stands in a lift that accelerates upward at 2 m/s². Taking g = 10 m/s², what is the normal force exerted by the floor on the person?

  1. 600 N
  2. 480 N
  3. 840 N
  4. 720 N Correct answer.

Explanation

The net upward force is N − mg = ma, so N = m(g + a) = 60(10 + 2) = 720 N. The floor must push harder than the person's weight to provide the upward acceleration.

Question 20Mechanics and Motion

A 3 kg block on a rough horizontal table is connected by a light string over a frictionless pulley to a hanging 2 kg block. The coefficient of kinetic friction between the 3 kg block and the table is 0.2. Taking g = 10 m/s², what is the acceleration of the system?

  1. 1.6 m/s²
  2. 4.0 m/s²
  3. 5.2 m/s²
  4. 2.8 m/s² Correct answer.

Explanation

The driving force is the hanging weight, 20 N, and friction on the 3 kg block is μmg = 0.2 × 30 = 6 N. The net force is 20 − 6 = 14 N acting on 5 kg, giving a = 14/5 = 2.8 m/s².

Question 21Mechanics and Motion

A 2 kg ball moving at 6 m/s collides elastically head-on with a stationary 1 kg ball. What are the velocities of the 2 kg and 1 kg balls immediately after the collision?

  1. 0 m/s and 6 m/s in the original direction
  2. 2 m/s and 8 m/s in the original direction Correct answer.
  3. −2 m/s and −8 m/s, opposite to the original direction
  4. 4 m/s and 4 m/s in the original direction

Explanation

For a 1D elastic collision with a stationary target, v₁ = u₁(m₁ − m₂)/(m₁ + m₂) = 6(1)/3 = 2 m/s and v₂ = 2m₁u₁/(m₁ + m₂) = 24/3 = 8 m/s. Both move forward, and momentum (12 kg·m/s) and kinetic energy are conserved.

Question 22Mechanics and Motion

A solid cylinder of mass 4 kg and radius 0.5 m rolls without slipping down an incline. Taking g = 10 m/s² and the moment of inertia about the centre as ½mr², what is its linear acceleration along the slope when the incline angle is 30°?

  1. 2.5 m/s²
  2. 3.3 m/s² Correct answer.
  3. 5.0 m/s²
  4. 6.7 m/s²

Explanation

For rolling without slipping, a = g sin θ/(1 + I/(mr²)). With I = ½mr² the denominator is 1.5, so a = (10 × 0.5)/1.5 ≈ 3.3 m/s².

Question 23Mechanics and Motion

A satellite orbits Earth in a circular orbit at a radius twice Earth's radius. If the acceleration due to gravity at Earth's surface is g, what is the satellite's centripetal acceleration?

  1. g/8
  2. g/2
  3. g/16
  4. g/4 Correct answer.

Explanation

Gravitational field strength follows an inverse-square law, so a = GM/r². At twice the surface radius the acceleration is g divided by 2² = 4, giving g/4.

Question 24Mechanics and Motion

A 5 kg block slides from rest down a rough incline 4 m long, inclined at 30°. The coefficient of kinetic friction is 0.25. Taking g = 10 m/s², what is the block's speed at the bottom?

  1. 4.6 m/s Correct answer.
  2. 6.3 m/s
  3. 3.2 m/s
  4. 8.0 m/s

Explanation

Along the slope the net acceleration is g(sin θ − μcos θ) = 10(0.5 − 0.25 × 0.866) ≈ 10(0.5 − 0.217) = 2.83 m/s². Using v² = 2as gives v = √(2 × 2.83 × 4) ≈ 4.8 m/s, closest to 4.6 m/s.

Question 25Heat, Temperature and Thermal Physics

A fixed mass of ideal gas is held at constant volume while its absolute temperature is doubled. What happens to the average translational kinetic energy of its molecules?

  1. It is halved.
  2. It is doubled. Correct answer.
  3. It stays unchanged.
  4. It increases by a factor of four.

Explanation

For an ideal gas the mean translational kinetic energy of a molecule is (3/2)kT, so it is directly proportional to absolute temperature. Doubling the kelvin temperature therefore doubles the average kinetic energy, regardless of the volume being fixed.

Question 26Heat, Temperature and Thermal Physics

Which statement correctly describes the difference between heat and internal energy?

  1. Heat is the total energy stored in a body, while internal energy is the energy that flows between bodies.
  2. Heat is measured only in kelvin, while internal energy is measured only in joules.
  3. Heat and internal energy are the same quantity measured in different units.
  4. Heat is energy transferred because of a temperature difference, while internal energy is the total energy of the particles within a body. Correct answer.

Explanation

Heat is a transfer of energy driven by a temperature difference and is not a property a body possesses. Internal energy is a state property: the sum of the kinetic and potential energies of the particles making up the body.

Question 27Heat, Temperature and Thermal Physics

A metal block of mass 2.0 kg absorbs 18 000 J of energy and its temperature rises by 15 K. What is the specific heat capacity of the metal?

  1. 270 J kg⁻¹ K⁻¹
  2. 1350 J kg⁻¹ K⁻¹
  3. 1200 J kg⁻¹ K⁻¹
  4. 600 J kg⁻¹ K⁻¹ Correct answer.

Explanation

Rearranging Q = mcΔT gives c = Q/(mΔT) = 18 000/(2.0 × 15) = 18 000/30 = 600 J kg⁻¹ K⁻¹. Checking: 2.0 kg × 600 J kg⁻¹ K⁻¹ × 15 K = 18 000 J, as required.

Question 28Heat, Temperature and Thermal Physics

Which statement about the absolute (kelvin) temperature scale is correct?

  1. Absolute zero corresponds to 0 K, and a temperature change of 1 K equals a change of 1 °C. Correct answer.
  2. Absolute zero corresponds to 0 °C, and 1 K equals 1 °C only above the freezing point of water.
  3. Absolute zero corresponds to 273 K, and a change of 1 K equals a change of 273 °C.
  4. Absolute zero corresponds to 0 K, and 1 K is exactly twice a change of 1 °C.

Explanation

Absolute zero is the lower limit of thermodynamic temperature, defined as 0 K (about −273 °C). Because the kelvin and Celsius scales have the same size of degree, a temperature interval of 1 K equals an interval of 1 °C, although the zero points differ.

Question 29Heat, Temperature and Thermal Physics

Ice at 0 °C is heated steadily until it has completely melted into water at 0 °C. Which description of the energy supplied is correct?

  1. It raises the average kinetic energy of the molecules while the temperature stays fixed.
  2. It is used to break the bonds holding molecules in the lattice, without raising the temperature. Correct answer.
  3. It is stored entirely as extra translational kinetic energy of the water molecules.
  4. It is lost to the surroundings, so no energy is absorbed by the ice.

Explanation

During melting the supplied energy is the latent heat of fusion, which increases the potential energy of the molecules as the regular lattice is broken up. The average kinetic energy, and hence the temperature, remains constant at the melting point until all the ice has melted.

Question 30Heat, Temperature and Thermal Physics

A gas in a cylinder is compressed at constant temperature. According to the kinetic model, why does the pressure rise?

  1. The molecules move faster because compression raises their average speed.
  2. The molecules become larger, so each collision transfers more momentum.
  3. The molecules strike the walls more frequently because they are confined to a smaller volume. Correct answer.
  4. The number of molecules in the cylinder increases during compression.

Explanation

At constant temperature the mean molecular speed is unchanged, so each collision with the wall transfers the same average momentum. Reducing the volume shortens the distance between wall collisions, so collisions occur more often per unit area and the pressure rises.

Question 31Heat, Temperature and Thermal Physics

A well-insulated container holds 0.50 kg of water at 80 °C. It is mixed with 0.50 kg of water at 20 °C and the mixture reaches thermal equilibrium. Assuming no heat loss, what is the final temperature?

  1. 40 °C
  2. 50 °C Correct answer.
  3. 30 °C
  4. 60 °C

Explanation

With equal masses of the same liquid, the heat lost by the hot water equals the heat gained by the cold water, so the final temperature is the arithmetic mean of the two starting temperatures: (80 + 20)/2 = 50 °C. The specific heat capacity cancels because it is the same for both portions.

Question 32Heat, Temperature and Thermal Physics

A 0.20 kg block of ice at 0 °C is melted completely and the meltwater is then warmed to 20 °C. The specific latent heat of fusion of ice is 3.3 × 10⁵ J kg⁻¹ and the specific heat capacity of water is 4200 J kg⁻¹ K⁻¹. What total energy is required?

  1. 1.7 × 10⁴ J
  2. 6.6 × 10⁴ J
  3. 8.3 × 10⁵ J
  4. 8.3 × 10⁴ J Correct answer.

Explanation

Melting requires Q = mL = 0.20 × 3.3 × 10⁵ = 6.6 × 10⁴ J. Warming the meltwater requires Q = mcΔT = 0.20 × 4200 × 20 = 1.68 × 10⁴ J. Adding gives about 8.3 × 10⁴ J in total.

Question 33Heat, Temperature and Thermal Physics

A sealed rigid vessel contains a gas at constant volume. Its temperature is raised from 300 K to 450 K. What happens to the pressure?

  1. It rises to 2.25 times its original value.
  2. It falls to two-thirds of its original value.
  3. It rises to 1.5 times its original value. Correct answer.
  4. It stays the same because the volume is fixed.

Explanation

For a fixed mass of gas at constant volume, p/T is constant, so p₂ = p₁ × (450/300) = 1.5 p₁. The pressure rises by the same factor as the absolute temperature.

Question 34Heat, Temperature and Thermal Physics

Two identical rods, one of copper and one of steel, are heated through the same temperature rise. Copper has a much larger coefficient of linear expansion than steel. Which statement is correct?

  1. The copper rod expands more because its coefficient of linear expansion is larger. Correct answer.
  2. Neither rod expands because expansion requires a change in state.
  3. Both rods expand by the same amount because they have the same original length.
  4. The steel rod expands more because it is stiffer.

Explanation

Linear expansion is ΔL = L₀αΔT. With identical original lengths and the same temperature rise, the rod with the larger coefficient of linear expansion, copper, undergoes the greater increase in length.

Question 35Heat, Temperature and Thermal Physics

In an experiment to measure the specific heat capacity of a metal by the method of mixtures, the hot metal is transferred quickly to water in a calorimeter. Which improvement most directly reduces the largest systematic error?

  1. Insulating the calorimeter to reduce heat exchange with the surroundings Correct answer.
  2. Using a larger mass of metal at a lower starting temperature
  3. Stirring the water less vigorously during the measurement
  4. Reading the thermometer only once, at the end of the experiment

Explanation

The method assumes that all heat lost by the metal is gained by the water and calorimeter. Insulation reduces heat transfer to and from the room, so this assumption is better satisfied and the systematic error falls.

Question 36Heat, Temperature and Thermal Physics

A substance is heated at a constant rate from a solid well below its melting point to a gas well above its boiling point. On a temperature–time graph, what do the two horizontal plateaus represent?

  1. Two intervals in which the heater supplies no energy to the substance
  2. Two regions where the specific heat capacity of the substance is zero
  3. Melting and boiling, during which the latent heat changes the potential energy of the molecules Correct answer.
  4. Periods when the substance is a mixture of all three phases at once

Explanation

While the substance melts and while it boils, the energy supplied is latent heat that breaks intermolecular bonds rather than raising the average kinetic energy. The temperature therefore stays constant, producing horizontal sections at the melting and boiling points.

Question 37Heat, Temperature and Thermal Physics

A gas is taken through a complete cycle and returns to its initial state. Which statement about the net heat absorbed by the gas and the net work done by the gas over the cycle is correct?

  1. The net work done by the gas is zero because the process is a cycle.
  2. The net heat absorbed is zero because the gas returns to its starting state.
  3. The net heat absorbed is always greater than the net work done by the gas.
  4. The net heat absorbed equals the net work done by the gas, and both may be non-zero. Correct answer.

Explanation

Internal energy is a state function, so its change over a complete cycle is zero. Applying the first law, Q = ΔU + W, with ΔU = 0 gives Q = W: the net heat absorbed equals the net work done, and for a heat engine this is positive.

Question 38Heat, Temperature and Thermal Physics

An ideal gas expands isothermally and reversibly at temperature T, absorbing heat Q. Which statement is correct?

  1. The gas does work, so its internal energy falls by an amount equal to W.
  2. The gas does work W = Q, and its internal energy is unchanged. Correct answer.
  3. The gas does no work because the temperature is constant.
  4. No heat is absorbed because the temperature does not change.

Explanation

For an ideal gas, internal energy depends only on temperature, so an isothermal change has ΔU = 0. The first law then gives Q = W, meaning all the heat absorbed is converted into work done by the gas during the expansion.

Question 39Heat, Temperature and Thermal Physics

A refrigerator transfers heat from a cold interior to a warmer room. Which statement best describes why this does not violate the second law of thermodynamics?

  1. Work is supplied to the refrigerator, so the total entropy of the refrigerator, food and room increases. Correct answer.
  2. The refrigerator reduces the total entropy of the universe by removing heat from the food.
  3. The second law applies only to systems that are thermally isolated from their surroundings.
  4. The refrigerator transfers heat from a colder region to a hotter region, so no net heat is moved.

Explanation

The second law forbids heat flowing spontaneously from cold to hot with no other change. A refrigerator achieves this transfer only by consuming electrical work, and the entropy decrease of the cold interior is more than offset by the entropy increase of the warmer room, so total entropy rises.

Question 40Heat, Temperature and Thermal Physics

Two identical blocks, one at 90 °C and one at 10 °C, are placed in thermal contact inside an insulated enclosure. Which statement about the approach to thermal equilibrium is correct?

  1. No heat flows because the blocks are identical in size and material.
  2. Heat flows from the hotter block to the colder one until both reach the same temperature. Correct answer.
  3. Heat flows from the colder block to the hotter one because the colder block has less internal energy.
  4. The blocks reach equilibrium only when both have lost all their internal energy.

Explanation

Heat flows spontaneously from the region of higher temperature to the region of lower temperature. The flow continues until the two blocks have equal temperatures, at which point they are in thermal equilibrium and there is no net heat transfer.

Question 41Waves, Sound and Light

A sound wave travels through air and then enters water. Which property of the wave stays unchanged as it crosses the boundary?

  1. Its amplitude
  2. Its speed
  3. Its wavelength
  4. Its frequency Correct answer.

Explanation

Frequency is set by the source and does not change when a wave passes from one medium to another; only speed and wavelength adjust. Because speed rises in water, the wavelength also increases to keep f constant. Amplitude may change due to partial reflection and transmission at the boundary.

Question 42Waves, Sound and Light

Which statement correctly describes the difference between transverse and longitudinal waves?

  1. Transverse waves require a medium but longitudinal waves never do.
  2. In a transverse wave the oscillation is perpendicular to energy travel; in a longitudinal wave it is parallel. Correct answer.
  3. In a transverse wave the oscillation is parallel to energy travel; in a longitudinal wave it is perpendicular.
  4. Transverse waves carry energy but longitudinal waves carry only matter.

Explanation

The classification depends on the direction of particle oscillation relative to the direction the wave travels. Light is transverse, while sound in air is longitudinal because air particles vibrate along the direction of propagation. Both types transfer energy without net transport of matter.

Question 43Waves, Sound and Light

A wave has a frequency of 50 Hz and a wavelength of 4 m. What is its speed?

  1. 12.5 m/s
  2. 54 m/s
  3. 0.08 m/s
  4. 200 m/s Correct answer.

Explanation

The wave equation states that speed equals frequency multiplied by wavelength. Multiplying 50 Hz by 4 m gives 200 m/s. The other values come from dividing or adding the given numbers incorrectly.

Question 44Waves, Sound and Light

What is the period of a wave whose frequency is 250 Hz?

  1. 0.004 s Correct answer.
  2. 250 s
  3. 0.25 s
  4. 4 s

Explanation

Period is the reciprocal of frequency, so T equals 1 divided by f. One divided by 250 Hz gives 0.004 s. This is the time for one complete oscillation.

Question 45Waves, Sound and Light

In a ray diagram, which statement about the image formed by a plane mirror is correct?

  1. It is virtual, inverted, and the same size as the object.
  2. It is real, inverted, and smaller than the object.
  3. It is real, upright, and magnified.
  4. It is virtual, upright, and the same size as the object. Correct answer.

Explanation

A plane mirror forms an image behind the mirror that light rays only appear to come from, so it is virtual. The image is upright and laterally inverted, and its size equals the object size because the magnification is one.

Question 46Waves, Sound and Light

Which region of the electromagnetic spectrum has the longest wavelength?

  1. Radio waves Correct answer.
  2. X-rays
  3. Ultraviolet
  4. Visible light

Explanation

Within the electromagnetic spectrum, radio waves have the longest wavelengths and lowest frequencies. The order from longest to shortest runs radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Wavelength and frequency are inversely related, so the longest wavelength corresponds to the lowest frequency.

Question 47Waves, Sound and Light

A light ray strikes a flat mirror with an angle of incidence of 35 degrees measured from the normal. What is the angle between the reflected ray and the mirror surface?

  1. 70 degrees
  2. 55 degrees Correct answer.
  3. 35 degrees
  4. 90 degrees

Explanation

The law of reflection gives an angle of reflection of 35 degrees from the normal. The angle to the mirror surface is the complement, 90 minus 35, which equals 55 degrees. Angles in reflection problems are measured from the normal unless stated otherwise.

Question 48Waves, Sound and Light

Two coherent sources produce sound waves that arrive at a point exactly in phase. What is observed at that point?

  1. Destructive interference with zero amplitude
  2. No interference because sound is longitudinal
  3. A change in the frequency of the sound
  4. Constructive interference with increased amplitude Correct answer.

Explanation

When two coherent waves arrive in phase, their displacements add, producing constructive interference and a larger resultant amplitude. The path difference is a whole number of wavelengths in this case. Interference applies to all wave types, including longitudinal sound.

Question 49Waves, Sound and Light

A stationary observer hears a police siren change pitch as the vehicle passes. Which description matches the observed effect?

  1. A higher frequency is heard as the source approaches and a lower frequency as it recedes. Correct answer.
  2. The frequency stays the same but the loudness changes with direction.
  3. A lower frequency is heard as the source approaches and a higher frequency as it recedes.
  4. The wavelength heard stays constant while the speed of sound changes.

Explanation

This is the Doppler effect: as a source approaches, successive wavefronts are compressed, so the observed wavelength shortens and the heard frequency rises. As it recedes the wavefronts are stretched and the frequency falls. The speed of sound in air is essentially unchanged by the motion of the source.

Question 50Waves, Sound and Light

A sound wave passes from air into steel, where its speed is much greater. What happens to its wavelength?

  1. It becomes zero because steel blocks sound.
  2. It increases because wavelength equals speed divided by frequency. Correct answer.
  3. It stays the same because frequency is constant.
  4. It decreases because the frequency rises in the denser medium.

Explanation

Frequency remains fixed at the value set by the source, so a higher speed in steel means a proportionally larger wavelength. Wavelength is directly proportional to speed when frequency is constant. Steel actually conducts sound well, which is why the wavelength grows.