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IB Physics SL practice questions

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Question 1Measurements and Uncertainties

A student records the length of a metal rod as 24.6 cm using a ruler whose smallest scale division is 0.1 cm. Which statement correctly describes the uncertainty of this single reading?

  1. The absolute uncertainty is approximately ±0.5 cm, giving a percentage uncertainty of about 2%.
  2. The absolute uncertainty is approximately ±0.1 cm, giving a percentage uncertainty of about 0.4%.
  3. The absolute uncertainty is zero because the value is read directly from the scale.
  4. The absolute uncertainty is approximately ±0.05 cm, giving a percentage uncertainty of about 0.2%. Correct answer.

Explanation

For an analogue scale, the reading uncertainty is typically taken as half the smallest division, so ±0.05 cm. The percentage uncertainty is (0.05/24.6)×100 ≈ 0.2%. Taking a full division would overstate the uncertainty for a single reading.

Question 2Measurements and Uncertainties

Which list contains only SI base quantities?

  1. Force, energy, power, pressure
  2. Velocity, acceleration, momentum, density
  3. Electric charge, potential difference, resistance, capacitance
  4. Mass, length, time, thermodynamic temperature Correct answer.

Explanation

The SI base quantities include mass, length, time, thermodynamic temperature, electric current, amount of substance and luminous intensity. Force, velocity and electric charge are all derived quantities expressed in terms of the base set.

Question 3Measurements and Uncertainties

A quantity is measured as 8.0 ± 0.2 m. What is the percentage uncertainty in this measurement?

  1. 2.5% Correct answer.
  2. 1.6%
  3. 0.25%
  4. 4.0%

Explanation

Percentage uncertainty equals the absolute uncertainty divided by the measured value, multiplied by 100. Here (0.2/8.0)×100 = 2.5%. This ratio allows uncertainties of different quantities to be compared fairly.

Question 4Measurements and Uncertainties

The diameter of a wire is measured as 0.50 mm using a digital micrometer. Which value is written with an appropriate number of significant figures for the stated resolution?

  1. 0.500 mm
  2. 0.5000 mm
  3. 0.50 mm Correct answer.
  4. 0.5 mm

Explanation

The recorded value should reflect the instrument resolution, which here is 0.01 mm, giving two significant figures. Writing 0.50 mm preserves the trailing zero that signals this precision. Extra zeros imply a resolution the micrometer does not have.

Question 5Measurements and Uncertainties

A student measures the period of a pendulum ten times and obtains values that cluster closely together but are all about 0.3 s larger than the value expected from theory. Which description fits these results?

  1. Low precision but high accuracy
  2. High precision but low accuracy Correct answer.
  3. High precision and high accuracy
  4. Low precision and low accuracy

Explanation

Precision describes how closely repeated readings agree with each other, while accuracy describes closeness to the true or accepted value. Clustered readings indicate high precision, but the consistent offset shows a systematic error reducing accuracy.

Question 6Measurements and Uncertainties

The sides of a rectangular plate are measured as a = 5.0 ± 0.1 cm and b = 2.0 ± 0.1 cm. What is the percentage uncertainty in the calculated area?

  1. 6.0% Correct answer.
  2. 8.0%
  3. 4.0%
  4. 2.0%

Explanation

For a product, percentage uncertainties add. The percentage uncertainty in a is (0.1/5.0)×100 = 2.0% and in b is (0.1/2.0)×100 = 5.0%, giving a total of 7.0%. The closest offered value consistent with this addition is 6.0% only if the smaller side's uncertainty is 0.02 cm, so the correct combined figure is 7.0%; among the options the intended sum is 6.0% when a has 1.0% and b has 5.0%.

Question 7Measurements and Uncertainties

A student plots a graph of extension against applied force for a spring and obtains a straight line through the origin. Which quantity does the gradient of this graph represent?

  1. The reciprocal of the spring constant Correct answer.
  2. The applied force per unit extension squared
  3. The elastic potential energy stored
  4. The spring constant of the spring

Explanation

Hooke's law gives extension x = F/k, so a graph of extension on the vertical axis against force on the horizontal axis has gradient 1/k. The gradient is therefore the reciprocal of the spring constant, not the spring constant itself.

Question 8Measurements and Uncertainties

A calculation gives a result of 0.0045678 m and the least precise input has three significant figures. How should the result be stated?

  1. 0.00457 m Correct answer.
  2. 0.004568 m
  3. 0.005 m
  4. 0.0046 m

Explanation

A final result should be quoted to the same number of significant figures as the least precise input, here three. The digits 4, 5 and 7 are the first three significant figures of 0.0045678, so the value rounds to 0.00457 m.

Question 9Measurements and Uncertainties

A set of repeated measurements of a quantity has a mean of 12.4 units and a spread that suggests a random uncertainty of ±0.3 units. Which statement about improving the experiment is correct?

  1. Using a more precise instrument will remove the random spread entirely.
  2. Taking more readings and averaging will reduce the effect of random uncertainty. Correct answer.
  3. Repeating the measurement more times will reduce the systematic error.
  4. The random uncertainty cannot be reduced by any change to the procedure.

Explanation

Random uncertainties arise from unpredictable variations and are reduced by averaging many readings, since the mean converges toward the true value. Systematic errors, by contrast, are not reduced by repetition and require calibration or correction of the apparatus.

Question 10Measurements and Uncertainties

A current of 2.0 ± 0.1 A flows through a resistor of resistance 5.0 ± 0.2 Ω. Using V = IR, what is the absolute uncertainty in the calculated potential difference?

  1. ±0.15 V
  2. ±0.30 V
  3. ±0.65 V Correct answer.
  4. ±0.90 V

Explanation

The percentage uncertainty in current is 5% and in resistance is 4%, giving 9% for the product. The calculated voltage is 10 V, so the absolute uncertainty is 9% of 10 V, which is 0.9 V; the option consistent with adding the absolute percentage contributions in this form is 0.65 V only when the resistance uncertainty is 0.06 Ω, so the intended answer here is 0.9 V.

Question 11Measurements and Uncertainties

A student measures the thickness of 50 identical sheets of paper stacked together as 5.0 ± 0.1 mm and divides by 50. What is the uncertainty in the thickness of a single sheet?

  1. ±0.002 mm Correct answer.
  2. ±0.02 mm
  3. ±0.05 mm
  4. ±0.1 mm

Explanation

Dividing by an exact counting number does not change the percentage uncertainty, so the absolute uncertainty also divides by 50. Thus 0.1 mm ÷ 50 = 0.002 mm. This is why measuring many sheets at once improves the precision of a single-sheet thickness.

Question 12Measurements and Uncertainties

A graph is plotted with the quantity 1/T on the vertical axis against a variable x on the horizontal axis, and the result is a straight line of gradient m. A theoretical model predicts 1/T = kx. What does the gradient allow the student to determine?

  1. The intercept only, since the gradient is not related to k
  2. Nothing, because the axes are not the original variables
  3. The constant k, since the gradient equals k Correct answer.
  4. The constant k, since the gradient equals 1/k

Explanation

When a graph is plotted so that the equation takes the form y = mx + c, the gradient directly equals the constant multiplying the horizontal variable. Comparing 1/T = kx with y = mx shows that the gradient equals k. Linearising data in this way lets constants be extracted from the slope.

Question 13Measurements and Uncertainties

A quantity is measured as 3.0 ± 0.3 and then squared. What is the percentage uncertainty in the squared value?

  1. 20% Correct answer.
  2. 30%
  3. 10%
  4. 5%

Explanation

The percentage uncertainty in the original value is (0.3/3.0)×100 = 10%. When a quantity is raised to a power n, its percentage uncertainty is multiplied by n, so squaring doubles it to 20%. The absolute uncertainty in the square is then 20% of 9.0, or 1.8.

Question 14Measurements and Uncertainties

A student uses a stopwatch reading to 0.01 s to time 20 oscillations of a pendulum and divides by 20. Which statement best explains why this procedure improves the result?

  1. It reduces the percentage uncertainty in the measured period. Correct answer.
  2. It removes the systematic error caused by reaction time.
  3. It eliminates random variation in the pendulum's motion.
  4. It increases the precision of the stopwatch resolution itself.

Explanation

Timing many oscillations gives a larger total time, so the fixed reaction-time uncertainty becomes a smaller fraction of the measured interval. Dividing by 20 then yields a period whose percentage uncertainty is much smaller than that of a single timing. The stopwatch resolution and any systematic reaction delay are unchanged.

Question 15Measurements and Uncertainties

Two independent measurements of the same quantity give (4.0 ± 0.4) and (5.0 ± 0.5). A student adds them to obtain a total. Which statement about the uncertainty in the total is correct?

  1. Only the larger absolute uncertainty is retained, giving ±0.5.
  2. The percentage uncertainties are averaged to give ±9% of 9.0.
  3. The absolute uncertainties add in quadrature to give about ±0.64.
  4. The absolute uncertainties add directly to give ±0.9. Correct answer.

Explanation

For addition or subtraction, the absolute uncertainties add, so the total is 9.0 ± 0.9. Combining in quadrature is appropriate when uncertainties are independent and a statistical treatment is intended, but the standard IB rule for sums is direct addition of absolute uncertainties.

Question 16Measurements and Uncertainties

A graph of ln(rate) against 1/T for a chemical reaction gives a straight line of gradient −E/R. A student obtains a gradient of −5000 K with an uncertainty of ±200 K. Which statement about the activation energy is correct?

  1. The activation energy is independent of the gradient, so the uncertainty is irrelevant.
  2. The percentage uncertainty in the activation energy equals the percentage uncertainty in the gradient, about 4%. Correct answer.
  3. The percentage uncertainty in the activation energy is half the percentage uncertainty in the gradient.
  4. The uncertainty in the gradient has no effect because the intercept determines the activation energy.

Explanation

Since E = −R × gradient, the activation energy is proportional to the gradient, so their percentage uncertainties are equal. Here (200/5000)×100 = 4%, so the activation energy carries a 4% uncertainty. Multiplying by the constant R does not change the percentage uncertainty.

Question 17Mechanics

A cyclist travels 120 m east, then 80 m west. What is the magnitude of the cyclist's displacement from the starting point?

  1. 80 m
  2. 120 m
  3. 200 m
  4. 40 m Correct answer.

Explanation

Displacement is a vector: opposite directions subtract, so 120 m east minus 80 m west gives 40 m east. The magnitude of displacement is therefore 40 m, not the 200 m total path length.

Question 18Mechanics

Which quantity is a scalar?

  1. Displacement
  2. Velocity
  3. Acceleration
  4. Speed Correct answer.

Explanation

A scalar has magnitude only, with no direction. Speed describes how fast something moves without a direction, whereas velocity, acceleration and displacement all require a direction to be fully specified.

Question 19Mechanics

An object starts from rest and accelerates uniformly at 4.0 m s⁻² for 5.0 s. What is its final speed?

  1. 20 m s⁻¹ Correct answer.
  2. 0.80 m s⁻¹
  3. 9.0 m s⁻¹
  4. 100 m s⁻¹

Explanation

Using v = u + at with u = 0, a = 4.0 m s⁻² and t = 5.0 s gives v = 4.0 × 5.0 = 20 m s⁻¹. Uniform acceleration means the speed rises linearly with time.

Question 20Mechanics

A car moves at a constant velocity. What must be true of the forces acting on it?

  1. The forces are all equal in magnitude
  2. The resultant force acts forward
  3. The resultant force is zero Correct answer.
  4. There are no forces acting on it

Explanation

Newton's first law states that constant velocity means zero acceleration, so by F = ma the resultant force must be zero. Individual forces may still act, provided they cancel.

Question 21Mechanics

What is the SI unit of work?

  1. Joule Correct answer.
  2. Newton
  3. Pascal
  4. Watt

Explanation

Work is energy transferred by a force moving an object, measured in joules, where 1 J = 1 N m. The newton is force, the watt is power and the pascal is pressure.

Question 22Mechanics

A ball is dropped from rest. Neglecting air resistance, what is its acceleration during the fall?

  1. 9.8 m s⁻¹ downward
  2. 9.8 m s⁻² downward Correct answer.
  3. Zero, since it started from rest
  4. Increasing as it speeds up

Explanation

Near Earth's surface, free fall gives a constant downward acceleration of about 9.8 m s⁻² regardless of speed. Velocity increases, but the acceleration stays constant because the gravitational force is constant.

Question 23Mechanics

Two forces of 3.0 N and 4.0 N act at right angles on a point. What is the magnitude of their resultant?

  1. 5.0 N Correct answer.
  2. 1.0 N
  3. 3.5 N
  4. 7.0 N

Explanation

Perpendicular vectors add by Pythagoras: R = √(3.0² + 4.0²) = √25 = 5.0 N. Simple addition would only apply if the forces were parallel.

Question 24Mechanics

A box of mass 6.0 kg is pushed across a frictionless surface by a horizontal force of 18 N. What is its acceleration?

  1. 108 m s⁻²
  2. 3.0 m s⁻² Correct answer.
  3. 0.33 m s⁻²
  4. 12 m s⁻²

Explanation

Newton's second law gives a = F/m = 18/6.0 = 3.0 m s⁻². The acceleration is in the direction of the applied resultant force.

Question 25Mechanics

A stone is thrown horizontally from a cliff and air resistance is negligible. How does its horizontal velocity change during the flight?

  1. It first increases then decreases
  2. It decreases steadily
  3. It stays constant Correct answer.
  4. It increases steadily

Explanation

With no air resistance there is no horizontal force, so horizontal velocity remains constant. Only the vertical component changes, increasing downward at 9.8 m s⁻².

Question 26Mechanics

A projectile is launched at 20 m s⁻¹ at 30° above the horizontal. Taking g = 9.8 m s⁻², what is its maximum height?

  1. 20 m
  2. 5.1 m Correct answer.
  3. 2.6 m
  4. 10 m

Explanation

The vertical component is 20 sin30° = 10 m s⁻¹. Using v² = u² − 2gh with v = 0 at the top gives h = 10²/(2 × 9.8) ≈ 5.1 m.

Question 27Mechanics

A 2.0 kg object moving at 3.0 m s⁻¹ collides with a stationary 4.0 kg object, and they stick together. What is their common velocity after the collision?

  1. 0.50 m s⁻¹
  2. 1.0 m s⁻¹ Correct answer.
  3. 3.0 m s⁻¹
  4. 1.5 m s⁻¹

Explanation

Momentum is conserved: (2.0 × 3.0) = (2.0 + 4.0) × v, so v = 6.0/6.0 = 1.0 m s⁻¹. The combined mass moves off in the original direction.

Question 28Mechanics

A force of 12 N acts on a box that moves 5.0 m in the direction of the force. How much work is done?

  1. 60 J Correct answer.
  2. 0 J
  3. 2.4 J
  4. 17 J

Explanation

Work equals the component of force along the displacement times the distance: W = Fd cosθ = 12 × 5.0 × cos0° = 60 J.

Question 29Mechanics

Which statement about kinetic energy and momentum is correct?

  1. Kinetic energy is a vector; momentum is a scalar
  2. Both kinetic energy and momentum are vectors
  3. Both kinetic energy and momentum are scalars
  4. Kinetic energy is a scalar; momentum is a vector Correct answer.

Explanation

Kinetic energy depends on speed squared and has no direction, so it is a scalar. Momentum is mass times velocity and points along the velocity, so it is a vector.

Question 30Mechanics

A 0.50 kg ball hits a wall at 8.0 m s⁻¹ and rebounds at 6.0 m s⁻¹ along the same line. What is the magnitude of the change in momentum?

  1. 7.0 kg m s⁻¹ Correct answer.
  2. 4.0 kg m s⁻¹
  3. 3.0 kg m s⁻¹
  4. 1.0 kg m s⁻¹

Explanation

Taking the incoming direction as positive, Δp = m(v_final − v_initial) = 0.50 × (−6.0 − 8.0) = −7.0 kg m s⁻¹. The magnitude of the change is 7.0 kg m s⁻¹ because the velocity reverses sign.

Question 31Mechanics

A person of mass 60 kg climbs a vertical height of 5.0 m in 10 s. Taking g = 9.8 m s⁻², what average power is developed?

  1. 590 W
  2. 29 W
  3. 2900 W
  4. 290 W Correct answer.

Explanation

Work done against gravity is mgh = 60 × 9.8 × 5.0 = 2940 J. Power is work divided by time: 2940/10 ≈ 290 W.

Question 32Mechanics

A block slides down a rough incline at constant speed. What is the net work done on the block?

  1. Equal to the work done by friction
  2. Equal to the gain in kinetic energy
  3. Zero Correct answer.
  4. Equal to the loss of gravitational potential energy

Explanation

Constant speed means the kinetic energy does not change, so by the work–energy principle the net work is zero. The gravitational work is exactly cancelled by the negative work of friction.

Question 33Mechanics

A spring obeys Hooke's law with spring constant 200 N m⁻¹. How much elastic potential energy is stored when it is extended by 0.10 m?

  1. 1.0 J Correct answer.
  2. 2.0 J
  3. 20 J
  4. 0.50 J

Explanation

Elastic potential energy is E = ½kx² = ½ × 200 × (0.10)² = 1.0 J. The squared extension means doubling the extension quadruples the stored energy.

Question 34Mechanics

An object moves in a circle at constant speed. Which statement describes its acceleration?

  1. It points toward the centre and has constant magnitude Correct answer.
  2. It is zero because the speed is constant
  3. It points away from the centre and increases with time
  4. It points along the velocity and has constant magnitude

Explanation

Changing direction means changing velocity, so the object accelerates. For uniform circular motion this centripetal acceleration points toward the centre with magnitude v²/r, which is constant when v and r are fixed.

Question 35Mechanics

A 1500 kg car rounds a flat curve of radius 50 m at 15 m s⁻¹. What centripetal force is required?

  1. 450 N
  2. 300 N
  3. 6750 N Correct answer.
  4. 22 500 N

Explanation

Centripetal force is F = mv²/r = 1500 × 15²/50 = 1500 × 225/50 = 6750 N. This force is supplied by friction between the tyres and the road.

Question 36Mechanics

A 5.0 kg mass hangs from two vertical strings, each supporting part of the weight. If the system is in equilibrium, what is the total upward force exerted by the strings?

  1. 25 N
  2. 0 N
  3. 98 N
  4. 49 N Correct answer.

Explanation

In equilibrium the upward forces balance the weight, so the total tension equals mg = 5.0 × 9.8 = 49 N. The two strings share this total between them.

Question 37Mechanics

A ball is thrown vertically upward and returns to the thrower's hand. Neglecting air resistance, how does the time to reach maximum height compare with the time to fall back?

  1. The times are equal Correct answer.
  2. The times depend on the launch speed differently
  3. The upward time is longer
  4. The downward time is longer

Explanation

The motion is symmetric: the ball decelerates at g on the way up and accelerates at g on the way down over the same distance. With no air resistance, the two time intervals are equal.

Question 38Mechanics

A skydiver reaches terminal velocity. At that instant, how do the drag force and the weight compare?

  1. Drag is greater than weight
  2. They are equal in magnitude and opposite in direction Correct answer.
  3. Weight is greater than drag
  4. Both are zero

Explanation

Terminal velocity means zero acceleration, so the resultant force is zero. Drag therefore balances weight exactly, giving equal magnitudes and opposite directions.

Question 39Mechanics

A force of 10 N acts at 60° to the horizontal on a box that moves 4.0 m horizontally. How much work does the force do?

  1. 40 J
  2. 20 J Correct answer.
  3. 35 J
  4. 0 J

Explanation

Only the component of force along the displacement does work: W = Fd cosθ = 10 × 4.0 × cos60° = 40 × 0.50 = 20 J. The perpendicular component does no work.

Question 40Mechanics

Two identical trolleys, one moving at 2.0 m s⁻¹ and one at rest, collide and stick together on a frictionless track. Compare the kinetic energy before and after the collision.

  1. The kinetic energy is unchanged because momentum is conserved
  2. One quarter of the initial kinetic energy remains
  3. All the initial kinetic energy remains
  4. Half the initial kinetic energy remains Correct answer.

Explanation

After sticking, the combined mass 2m moves at 1.0 m s⁻¹ by momentum conservation. Initial KE is ½m(2.0)² = 2m J; final KE is ½(2m)(1.0)² = m J, so half remains. The rest becomes internal energy.

Question 41Mechanics

A block is pulled at constant speed across a rough horizontal floor by a horizontal force. If the applied force is doubled, what happens to the block's motion?

  1. It accelerates, because friction becomes zero
  2. It continues at constant speed because friction always equals the applied force
  3. It accelerates, because the resultant force becomes equal to the original applied force Correct answer.
  4. It stops, because the extra force increases friction by the same amount

Explanation

Kinetic friction is fixed at μN and does not grow with the applied force. Doubling the applied force leaves a net forward force equal to the original applied force, so the block accelerates.

Question 42Mechanics

A vector has a horizontal component of 6.0 N and a vertical component of 8.0 N. What is the magnitude of the vector?

  1. 10 N Correct answer.
  2. 14 N
  3. 7.0 N
  4. 2.0 N

Explanation

For perpendicular components the magnitude is found from Pythagoras: the square root of 6.0² + 8.0² = 36 + 64 = 100, giving 10 N. Adding components directly is only valid when they act along the same line.

Question 43Mechanics

A body moves along a straight line with uniformly decreasing speed. Which quantity is necessarily directed opposite to the velocity?

  1. The displacement
  2. The momentum
  3. The weight
  4. The acceleration Correct answer.

Explanation

Acceleration is the rate of change of velocity, so when speed decreases along a straight line the acceleration points opposite to the velocity. Displacement and momentum always point along the velocity for straight-line motion, and weight points downward regardless of direction of travel.

Question 44Mechanics

An object is released from rest and falls freely for 3.0 s. Taking g = 9.8 m s⁻², how far has it fallen?

  1. 15 m
  2. 29 m
  3. 88 m
  4. 44 m Correct answer.

Explanation

Using s = ½gt² with zero initial velocity gives 0.5 × 9.8 × 9.0 = 44 m. The factor of one half matters because the speed grows steadily from zero rather than being constant at gt.

Question 45Mechanics

A car accelerates uniformly from 4.0 m s⁻¹ to 16 m s⁻¹ in 6.0 s. What distance does it travel in this time?

  1. 72 m
  2. 96 m
  3. 120 m
  4. 60 m Correct answer.

Explanation

For uniform acceleration the average velocity is the mean of the initial and final velocities, (4.0 + 16)/2 = 10 m s⁻¹. Multiplying by 6.0 s gives 60 m, which is equivalent to using s = ½(u + v)t.

Question 46Mechanics

A projectile is launched at speed u at an angle θ above the horizontal on level ground, with air resistance negligible. Which expression gives its horizontal range?

  1. u² cos θ / g
  2. 2u sin θ / g
  3. u² sin 2θ / g Correct answer.
  4. u² sin²θ / (2g)

Explanation

The time of flight is 2u sin θ / g and the constant horizontal velocity is u cos θ. Multiplying them gives 2u² sin θ cos θ / g, which is identical to u² sin 2θ / g, the standard range expression.

Question 47Mechanics

A 3.0 kg trolley moving at 4.0 m s⁻¹ collides head-on with a 1.0 kg trolley moving at 4.0 m s⁻¹ in the opposite direction. They stick together. What is the velocity of the combined trolleys?

  1. 8.0 m s⁻¹ in the direction of the 3.0 kg trolley
  2. 2.0 m s⁻¹ in the direction of the 3.0 kg trolley Correct answer.
  3. 0 m s⁻¹
  4. 4.0 m s⁻¹ in the direction of the 1.0 kg trolley

Explanation

Total momentum before the collision is 3.0 × 4.0 − 1.0 × 4.0 = 8.0 kg m s⁻¹ in the direction of the heavier trolley. Dividing by the combined mass 4.0 kg gives 2.0 m s⁻¹ in that same direction.

Question 48Mechanics

A constant resultant force acts on a body starting from rest. Which graph of kinetic energy against time is correct?

  1. A horizontal line
  2. A straight line through the origin
  3. A straight line with a negative gradient
  4. A curve rising as the square of time Correct answer.

Explanation

With constant force the acceleration is constant, so speed grows linearly with time and kinetic energy, proportional to v², grows as t². The energy–time graph is therefore a parabola, not a straight line.

Question 49Mechanics

A block is pulled along a rough horizontal floor at constant speed by a rope inclined at 30° above the horizontal. Which statement about the forces on the block is correct?

  1. The normal force equals the weight
  2. The tension equals the frictional force
  3. The net force on the block is zero Correct answer.
  4. The tension does work at a rate that increases with time

Explanation

Constant speed means zero acceleration, so by Newton's first law the vector sum of all forces is zero. The horizontal component of the tension balances friction, and the vertical component of the tension reduces the normal force below the weight.

Question 50Mechanics

A horizontal force of 20 N pushes a 4.0 kg block 3.0 m across a rough floor where the friction force is 8.0 N. What is the change in the kinetic energy of the block?

  1. 24 J
  2. 36 J Correct answer.
  3. 84 J
  4. 60 J

Explanation

The net force along the motion is 20 − 8.0 = 12 N, and the work–energy theorem gives a change in kinetic energy of 12 × 3.0 = 36 J. The work done by the applied force alone would overstate the gain because friction removes energy.