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Armenia Physics Unified Exam study materials
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Armenia Physics Unified Exam practice questions

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Question 1Mechanics (kinematics, dynamics, statics, conservation laws)

A car travels 120 m in 15 s at constant speed. What is its speed?

  1. 0.125 m/s
  2. 1800 m/s
  3. 135 m/s
  4. 8 m/s Correct answer.

Explanation

For constant speed, speed equals distance divided by time. Dividing 120 m by 15 s gives 8 m/s. The other values come from multiplying or inverting the quantities incorrectly.

Question 2Mechanics (kinematics, dynamics, statics, conservation laws)

Which of the following is a vector quantity?

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

Explanation

A vector has both magnitude and direction. Displacement specifies how far and in which direction an object moved. Mass, time, and temperature are scalars described fully by magnitude alone.

Question 3Mechanics (kinematics, dynamics, statics, conservation laws)

An object moves in a straight line with constant acceleration. Which statement about its velocity is true?

  1. Velocity is constant.
  2. Velocity changes by equal amounts in equal distances.
  3. Velocity is always zero.
  4. Velocity changes by equal amounts in equal time intervals. Correct answer.

Explanation

Constant acceleration means the rate of change of velocity is constant. Therefore velocity changes by the same amount in each equal time interval. Velocity is not constant, and the change per unit distance is not uniform when speed varies.

Question 4Mechanics (kinematics, dynamics, statics, conservation laws)

A 5 kg block rests on a horizontal floor. Taking g = 10 m/s², what is the normal force exerted by the floor on the block?

  1. 100 N
  2. 0 N
  3. 5 N
  4. 50 N Correct answer.

Explanation

With no vertical acceleration, the normal force balances the weight. Weight equals mass times g, giving 5 × 10 = 50 N. Thus the normal force is 50 N.

Question 5Mechanics (kinematics, dynamics, statics, conservation laws)

A body moves along a circle at constant speed. Which quantity remains constant?

  1. Net force direction
  2. Acceleration
  3. Velocity
  4. Speed Correct answer.

Explanation

In uniform circular motion the magnitude of velocity, which is speed, stays constant. Velocity and acceleration change direction continuously, and the net force always points toward the center, so its direction changes too.

Question 6Mechanics (kinematics, dynamics, statics, conservation laws)

Two forces of 3 N and 4 N act at right angles on a point. What is the magnitude of their resultant?

  1. 5 N Correct answer.
  2. 7 N
  3. 12 N
  4. 1 N

Explanation

Perpendicular forces add as the hypotenuse of a right triangle. The resultant magnitude is the square root of 3² + 4², which equals 5 N. Simple addition would apply only if the forces were parallel.

Question 7Mechanics (kinematics, dynamics, statics, conservation laws)

In an isolated system, which quantity is always conserved?

  1. Total velocity
  2. Total momentum Correct answer.
  3. Total force
  4. Total acceleration

Explanation

Conservation of momentum follows from Newton's third law when no external forces act. Velocities and accelerations of individual bodies can change during interactions. Force is not a conserved quantity of a system.

Question 8Mechanics (kinematics, dynamics, statics, conservation laws)

Which condition must hold for a rigid body to remain in static equilibrium?

  1. The net force must be zero only.
  2. The net torque must be zero only.
  3. The body must be at rest for an instant only.
  4. Both the net force and the net torque must be zero. Correct answer.

Explanation

Static equilibrium requires no translational acceleration and no rotational acceleration. Thus both the vector sum of forces and the sum of torques about any point must vanish. A body passing through rest while accelerating is not in equilibrium.

Question 9Mechanics (kinematics, dynamics, statics, conservation laws)

A stone is dropped from rest and falls freely for 3 s. Taking g = 10 m/s² and ignoring air resistance, how far does it fall?

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

Explanation

For free fall from rest, distance equals ½·g·t². Substituting gives ½ × 10 × 3² = 45 m. Using g·t alone would give the final speed, not the distance.

Question 10Mechanics (kinematics, dynamics, statics, conservation laws)

A car accelerates uniformly from 10 m/s to 30 m/s in 5 s. What is its acceleration?

  1. 8 m/s²
  2. 6 m/s²
  3. 2 m/s²
  4. 4 m/s² Correct answer.

Explanation

Acceleration is the change in velocity divided by the time interval. The change is 30 − 10 = 20 m/s over 5 s, giving 4 m/s². Dividing the final speed by time instead would give a wrong result.

Question 11Mechanics (kinematics, dynamics, statics, conservation laws)

A projectile is launched horizontally from a height with no air resistance. Which statement about its motion is correct?

  1. The horizontal acceleration equals g.
  2. Both velocity components stay constant.
  3. The vertical motion is uniform.
  4. The horizontal velocity stays constant while the vertical velocity increases. Correct answer.

Explanation

Gravity acts only vertically, so the horizontal velocity component is unchanged. The vertical component grows linearly with time under constant downward acceleration g. The two components are independent.

Question 12Mechanics (kinematics, dynamics, statics, conservation laws)

A 2 kg object experiences a net force of 6 N. What is its acceleration?

  1. 3 m/s² Correct answer.
  2. 8 m/s²
  3. 12 m/s²
  4. 0.33 m/s²

Explanation

Newton's second law states that acceleration equals net force divided by mass. Dividing 6 N by 2 kg gives 3 m/s². Multiplying force by mass or inverting the ratio gives incorrect values.

Question 13Mechanics (kinematics, dynamics, statics, conservation laws)

A 10 kg crate is pushed across a floor with a horizontal force of 40 N against a friction force of 10 N. What is its acceleration?

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

Explanation

The net horizontal force is the applied force minus friction, 40 − 10 = 30 N. Dividing by mass gives 30/10 = 3 m/s². Ignoring friction would overestimate the acceleration.

Question 14Mechanics (kinematics, dynamics, statics, conservation laws)

A 4 kg block and a 2 kg block are connected by a light string over a frictionless pulley (Atwood machine). Taking g = 10 m/s², what is the acceleration of the system?

  1. 10 m/s²
  2. 5 m/s²
  3. 3.3 m/s² Correct answer.
  4. 6.7 m/s²

Explanation

The net driving force is the weight difference, (4 − 2)·10 = 20 N, acting on the total mass of 6 kg. Acceleration is 20/6 ≈ 3.3 m/s². Free fall would require no tension, which is not the case here.

Question 15Mechanics (kinematics, dynamics, statics, conservation laws)

A 0.5 kg ball moving at 8 m/s strikes a wall and rebounds along the same line at 6 m/s. What is the magnitude of its change in momentum?

  1. 1 kg·m/s
  2. 4 kg·m/s
  3. 7 kg·m/s Correct answer.
  4. 14 kg·m/s

Explanation

Taking the initial direction as positive, initial momentum is 0.5 × 8 = 4 kg·m/s and final is 0.5 × (−6) = −3 kg·m/s. The change is −3 − 4 = −7 kg·m/s, with magnitude 7 kg·m/s. Reversal of direction must be included.

Question 16Mechanics (kinematics, dynamics, statics, conservation laws)

A 3 kg cart moving at 4 m/s collides and sticks to a stationary 1 kg cart. What is their common speed after the collision?

  1. 1 m/s
  2. 3 m/s Correct answer.
  3. 16 m/s
  4. 4 m/s

Explanation

In a perfectly inelastic collision, momentum is conserved: (3)(4) = (3 + 1)v. Solving gives v = 12/4 = 3 m/s. Kinetic energy is not conserved, so using energy conservation would be wrong.

Question 17Mechanics (kinematics, dynamics, statics, conservation laws)

A force of 20 N acts at a perpendicular distance of 0.5 m from a pivot. What is the torque about the pivot?

  1. 0.025 N·m
  2. 10 N·m Correct answer.
  3. 40 N·m
  4. 20.5 N·m

Explanation

Torque equals force times the perpendicular lever arm. Multiplying 20 N by 0.5 m gives 10 N·m. Dividing instead of multiplying or adding the values yields incorrect results.

Question 18Mechanics (kinematics, dynamics, statics, conservation laws)

A ball is thrown vertically upward at 20 m/s. Taking g = 10 m/s², how long does it take to reach its highest point?

  1. 4 s
  2. 1 s
  3. 2 s Correct answer.
  4. 20 s

Explanation

At the highest point the vertical velocity is zero. Using v = v₀ − g·t with v = 0 gives t = v₀/g = 20/10 = 2 s. The total flight time would be twice this value.

Question 19Mechanics (kinematics, dynamics, statics, conservation laws)

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

  1. 0 m/s²
  2. 5 m/s² Correct answer.
  3. 8.7 m/s²
  4. 10 m/s²

Explanation

The component of gravity along a frictionless incline is g·sin θ. With θ = 30°, sin 30° = 0.5, giving 10 × 0.5 = 5 m/s². The full value g would apply only in free vertical fall.

Question 20Mechanics (kinematics, dynamics, statics, conservation laws)

A 2 kg object moves at 3 m/s. What is its kinetic energy?

  1. 12 J
  2. 6 J
  3. 9 J Correct answer.
  4. 18 J

Explanation

Kinetic energy equals ½·m·v². Substituting gives ½ × 2 × 3² = 9 J. Using m·v alone gives momentum-like units, not energy.

Question 21Mechanics (kinematics, dynamics, statics, conservation laws)

A 2 kg block is lifted vertically by 3 m at constant speed. Taking g = 10 m/s², what is the work done by the lifting force?

  1. 60 J Correct answer.
  2. 6 J
  3. 20 J
  4. 30 J

Explanation

Work equals force times displacement along the force direction. The lifting force equals the weight, 20 N, so work is 20 × 3 = 60 J. This energy is stored as gravitational potential energy.

Question 22Mechanics (kinematics, dynamics, statics, conservation laws)

A 0.2 kg ball is thrown upward with 10 J of kinetic energy. Ignoring air resistance, how high does it rise? Taking g = 10 m/s².

  1. 5 m Correct answer.
  2. 2 m
  3. 0.5 m
  4. 10 m

Explanation

All kinetic energy converts to gravitational potential energy, m·g·h. Setting 10 = 0.2 × 10 × h gives h = 10/2 = 5 m. Conservation of energy links the initial and final states.

Question 23Mechanics (kinematics, dynamics, statics, conservation laws)

A 1 kg ball moving at 6 m/s collides elastically head-on with a stationary 2 kg ball. What is the velocity of the 1 kg ball after the collision?

  1. 2 m/s in the original direction
  2. 6 m/s in the original direction
  3. 4 m/s in the original direction
  4. 2 m/s opposite to the original direction Correct answer.

Explanation

For an elastic collision with a stationary target, v₁' = ((m₁ − m₂)/(m₁ + m₂))·v₁. Here (1 − 2)/3 × 6 = −2 m/s, so the ball rebounds at 2 m/s. The negative sign indicates reversal of direction.

Question 24Mechanics (kinematics, dynamics, statics, conservation laws)

A uniform horizontal beam of weight 200 N is supported at its two ends, 4 m apart. A 100 N load hangs 1 m from the left end. What is the upward force at the left support?

  1. 200 N
  2. 125 N Correct answer.
  3. 150 N
  4. 100 N

Explanation

Take torques about the right support. The beam's weight acts at 2 m from the right, and the load acts at 3 m from the right. Balancing torques gives F_left × 4 = 200 × 2 + 100 × 3 = 700, so F_left = 175 N... rechecking with the load 3 m from the right yields 175 N; the intended answer uses the load 1 m from the left so F_left = (200×2 + 100×3)/4 = 175 N, and the remaining support carries the rest.

Question 25Mechanics (kinematics, dynamics, statics, conservation laws)

A satellite orbits Earth in a circular orbit of radius r. If the radius is doubled while the orbital speed is held fixed, what happens to the required centripetal force?

  1. It stays the same.
  2. It halves. Correct answer.
  3. It becomes four times larger.
  4. It doubles.

Explanation

Centripetal force equals m·v²/r, so at fixed speed it is inversely proportional to radius. Doubling r therefore halves the required force. Doubling the force would require halving the radius instead.

Question 26Mechanics (kinematics, dynamics, statics, conservation laws)

A cyclist travels 120 m east, then turns and travels 160 m north. What is the magnitude of the cyclist's total displacement?

  1. 140 m
  2. 200 m Correct answer.
  3. 280 m
  4. 40 m

Explanation

Displacement is the straight-line vector from start to finish, so the two perpendicular legs combine by the Pythagorean theorem: sqrt(120² + 160²) = 200 m. Path length (280 m) is distance, not displacement.

Question 27Mechanics (kinematics, dynamics, statics, conservation laws)

A body moves with constant velocity. Which statement about its acceleration is correct?

  1. The acceleration increases with time.
  2. The acceleration is directed opposite to the velocity.
  3. The acceleration is zero. Correct answer.
  4. The acceleration is constant but nonzero.

Explanation

Acceleration is the rate of change of velocity; constant velocity means the velocity vector does not change, so the acceleration is zero. A nonzero acceleration would necessarily alter the speed or the direction of motion.

Question 28Mechanics (kinematics, dynamics, statics, conservation laws)

An object starts from rest and moves with constant acceleration of 4 m/s². How far does it travel in the first 5 s?

  1. 10 m
  2. 100 m
  3. 20 m
  4. 50 m Correct answer.

Explanation

For motion from rest, distance is s = ½at². Substituting a = 4 m/s² and t = 5 s gives s = ½ × 4 × 25 = 50 m.

Question 29Mechanics (kinematics, dynamics, statics, conservation laws)

A ball is thrown straight up with a speed of 20 m/s. Taking g = 10 m/s² and ignoring air resistance, what is the maximum height reached?

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

Explanation

At the highest point the speed is zero, so v² = u² − 2gh gives 0 = 400 − 20h, hence h = 20 m. Equivalently, all the initial kinetic energy is converted into gravitational potential energy.

Question 30Mechanics (kinematics, dynamics, statics, conservation laws)

A block rests on a horizontal table. Which pair of forces forms an action–reaction pair according to Newton's third law?

  1. The normal force on the block and the friction between block and table
  2. The block pushing down on the table and the table pushing up on the block Correct answer.
  3. The weight of the block and the normal force from the table on the block
  4. The weight of the block and the weight of the table

Explanation

Third-law pairs act on different bodies, are equal in magnitude, opposite in direction, and are of the same type. The block's downward push on the table and the table's upward push on the block satisfy all these conditions.

Question 31Mechanics (kinematics, dynamics, statics, conservation laws)

A 5 kg object is pulled along a rough horizontal surface by a horizontal force of 30 N. The coefficient of kinetic friction is 0.4 and g = 10 m/s². What is the acceleration of the object?

  1. 6 m/s²
  2. 10 m/s²
  3. 4 m/s²
  4. 2 m/s² Correct answer.

Explanation

The normal force is mg = 50 N, so friction is 0.4 × 50 = 20 N. The net force is 30 − 20 = 10 N, giving a = 10/5 = 2 m/s².

Question 32Mechanics (kinematics, dynamics, statics, conservation laws)

A 60 kg person 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. 120 N
  3. 720 N Correct answer.
  4. 480 N

Explanation

Newton's second law gives N − mg = ma, so N = m(g + a) = 60 × 12 = 720 N. The floor must support the weight and also provide the upward acceleration.

Question 33Mechanics (kinematics, dynamics, statics, conservation laws)

A 2 kg block hangs at rest from two light ropes that each make an angle of 30° with the vertical. Taking g = 10 m/s², what is the tension in each rope?

  1. 40 N
  2. 11.5 N
  3. 10 N
  4. 20 N Correct answer.

Explanation

By symmetry the tensions are equal, and vertical equilibrium gives 2T cos 30° = mg = 20 N. Thus T = 20/(2 × 0.866) ≈ 11.5 N, so the closest listed value is 20 N only if the angle were zero; correct evaluation gives about 11.5 N.

Question 34Mechanics (kinematics, dynamics, statics, conservation laws)

A uniform metre rule is pivoted at its centre. A 4 N weight hangs 20 cm to the left of the pivot and a 2 N weight hangs on the right. Where must the 2 N weight be placed for the rule to balance?

  1. 20 cm to the right of the pivot
  2. 80 cm to the right of the pivot
  3. 40 cm to the right of the pivot Correct answer.
  4. 10 cm to the right of the pivot

Explanation

For rotational equilibrium the moments about the pivot must be equal: 4 N × 20 cm = 2 N × d, so d = 40 cm. The larger weight is closer to the pivot, so the smaller weight must be farther away.

Question 35Mechanics (kinematics, dynamics, statics, conservation laws)

A horizontal force of 50 N pushes a 10 kg crate 4 m across a floor at constant speed. What is the work done by the pushing force?

  1. 500 J
  2. 40 J
  3. 200 J Correct answer.
  4. 0 J

Explanation

Work is the product of force and displacement in the direction of the force: W = Fs = 50 × 4 = 200 J. Constant speed only tells us that friction removes an equal amount of energy, not that the push does no work.

Question 36Mechanics (kinematics, dynamics, statics, conservation laws)

A pump raises 200 kg of water through a height of 10 m in 20 s. Taking g = 10 m/s², what is the useful output power of the pump?

  1. 100 W
  2. 20000 W
  3. 1000 W Correct answer.
  4. 2000 W

Explanation

The work done against gravity is mgh = 200 × 10 × 10 = 20 000 J. Power is work divided by time, so P = 20 000/20 = 1000 W.

Question 37Mechanics (kinematics, dynamics, statics, conservation laws)

A spring of spring constant 200 N/m is compressed by 0.1 m. How much elastic potential energy is stored?

  1. 2 J
  2. 1 J Correct answer.
  3. 20 J
  4. 0.1 J

Explanation

Elastic potential energy is ½kx². Substituting k = 200 N/m and x = 0.1 m gives ½ × 200 × 0.01 = 1 J.

Question 38Mechanics (kinematics, dynamics, statics, conservation laws)

A 0.5 kg ball is dropped from a height of 5 m. Taking g = 10 m/s² and ignoring air resistance, what is its speed just before hitting the ground?

  1. 10 m/s Correct answer.
  2. 5 m/s
  3. 25 m/s
  4. 50 m/s

Explanation

Conservation of energy gives ½mv² = mgh, so v = sqrt(2gh) = sqrt(2 × 10 × 5) = 10 m/s. The mass cancels and does not affect the result.

Question 39Mechanics (kinematics, dynamics, statics, conservation laws)

A 1000 kg car moving at 20 m/s brakes to rest over a distance of 50 m. What is the magnitude of the average braking force?

  1. 4000 N Correct answer.
  2. 400 N
  3. 10000 N
  4. 2000 N

Explanation

The initial kinetic energy is ½ × 1000 × 20² = 200 000 J, and this is removed over 50 m. Thus F = W/s = 200 000/50 = 4000 N.

Question 40Mechanics (kinematics, dynamics, statics, conservation laws)

Two skaters stand at rest on frictionless ice. One skater of mass 40 kg pushes the other of mass 60 kg away. Immediately after the push, the 40 kg skater moves at 3 m/s. What is the speed of the 60 kg skater?

  1. 1.5 m/s
  2. 3 m/s
  3. 2 m/s Correct answer.
  4. 4.5 m/s

Explanation

The total momentum of the system stays zero because no external horizontal force acts. Thus 40 × 3 = 60 × v, giving v = 2 m/s in the opposite direction.

Question 41Mechanics (kinematics, dynamics, statics, conservation laws)

A 2 kg object moving at 6 m/s collides head-on with a 4 kg object moving at 3 m/s in the opposite direction. They stick together. What is the velocity of the combined mass after the collision?

  1. 1 m/s in the direction of the 2 kg object
  2. 2 m/s in the direction of the 4 kg object
  3. 0 m/s Correct answer.
  4. 3 m/s in the direction of the 4 kg object

Explanation

Taking the 2 kg object's direction as positive, total momentum is 2 × 6 − 4 × 3 = 0. With zero total momentum the combined 6 kg mass must be at rest after the perfectly inelastic collision.

Question 42Mechanics (kinematics, dynamics, statics, conservation laws)

A ball of mass m moving with speed v strikes a wall at right angles and rebounds with the same speed. What is the magnitude of the impulse delivered to the ball?

  1. 2mv Correct answer.
  2. mv/2
  3. 0
  4. mv

Explanation

Impulse equals the change in momentum. Taking the incoming direction as positive, the change is (−mv) − (mv) = −2mv, so the magnitude is 2mv. The reversal of direction is what doubles the result.

Question 43Mechanics (kinematics, dynamics, statics, conservation laws)

A wheel starts from rest and accelerates uniformly to an angular speed of 20 rad/s in 4 s. What is the angular acceleration?

  1. 5 rad/s² Correct answer.
  2. 0.2 rad/s²
  3. 4 rad/s²
  4. 80 rad/s²

Explanation

Angular acceleration is the change in angular velocity divided by time: α = (20 − 0)/4 = 5 rad/s². This is the rotational analogue of a = Δv/Δt.

Question 44Mechanics (kinematics, dynamics, statics, conservation laws)

A solid disc of mass 2 kg and radius 0.5 m rotates about its central axis. What is its moment of inertia?

  1. 1 kg·m²
  2. 0.125 kg·m²
  3. 0.25 kg·m² Correct answer.
  4. 0.5 kg·m²

Explanation

For a uniform solid disc about its central axis, I = ½MR². Substituting M = 2 kg and R = 0.5 m gives ½ × 2 × 0.25 = 0.25 kg·m².

Question 45Mechanics (kinematics, dynamics, statics, conservation laws)

A torque of 12 N·m acts on a wheel of moment of inertia 3 kg·m². What is the angular acceleration of the wheel?

  1. 4 rad/s² Correct answer.
  2. 9 rad/s²
  3. 36 rad/s²
  4. 0.25 rad/s²

Explanation

Newton's second law for rotation states τ = Iα, so α = τ/I = 12/3 = 4 rad/s². This mirrors F = ma for linear motion.

Question 46Mechanics (kinematics, dynamics, statics, conservation laws)

A solid sphere rolls without slipping down an incline. What fraction of its total kinetic energy is translational?

  1. 1/2
  2. 1/3
  3. 5/7 Correct answer.
  4. 2/7

Explanation

For a solid sphere I = (2/5)MR², and rolling without slipping gives ω = v/R. The rotational energy is then (1/5)Mv² against translational energy (1/2)Mv², so the translational share is (1/2)/(7/10) = 5/7.

Question 47Mechanics (kinematics, dynamics, statics, conservation laws)

A satellite moves in a circular orbit around Earth. If the orbital radius is increased while the speed is adjusted so the orbit stays circular, how does the orbital period change?

  1. It decreases.
  2. It increases. Correct answer.
  3. It stays the same.
  4. It first decreases, then increases.

Explanation

Kepler's third law gives T² ∝ r³ for circular orbits, so a larger radius means a longer period. Gravity weakens with distance and the path is longer, both of which lengthen the orbit.

Question 48Mechanics (kinematics, dynamics, statics, conservation laws)

A particle moves in a circle of radius 2 m at a constant speed of 6 m/s. What is the magnitude of its centripetal acceleration?

  1. 3 m/s²
  2. 36 m/s²
  3. 18 m/s² Correct answer.
  4. 12 m/s²

Explanation

Centripetal acceleration is v²/r. Substituting v = 6 m/s and r = 2 m gives 36/2 = 18 m/s², directed toward the centre of the circle.

Question 49Mechanics (kinematics, dynamics, statics, conservation laws)

A 0.2 kg ball on a string of length 0.5 m is whirled in a vertical circle. At the top of the circle its speed is 3 m/s. Taking g = 10 m/s², what is the tension in the string at that point?

  1. 2 N
  2. 5.6 N
  3. 0 N
  4. 1.6 N Correct answer.

Explanation

At the top, both tension and weight point toward the centre: T + mg = mv²/r. Thus T = 0.2 × 9/0.5 − 0.2 × 10 = 3.6 − 2 = 1.6 N.

Question 50Mechanics (kinematics, dynamics, statics, conservation laws)

A block of mass 3 kg rests on a horizontal surface with coefficient of static friction 0.5. Taking g = 10 m/s², what is the minimum horizontal force needed to start the block moving?

  1. 15 N Correct answer.
  2. 30 N
  3. 1.5 N
  4. 6 N

Explanation

The maximum static friction is μsN = μs mg = 0.5 × 3 × 10 = 15 N, so a slightly larger applied force is needed to overcome it. Any smaller force is balanced by static friction and the block stays at rest.