Free Force and Motion MCQs with Answers

11. In an elastic collision, which quantity is conserved in addition to momentum?

• A. Kinetic energy
• B. Mass alone
• C. Velocity of each body
• D. Potential energy

Explanation: An elastic collision is defined by the conservation of kinetic energy as well as momentum, and collisions between gas molecules are treated this way. In an inelastic collision momentum is still conserved but some kinetic energy is converted into heat, sound or deformation. Individual velocities change in every collision.

12. A car accelerates uniformly from rest to 20 m s-1 in 8 s. The distance covered is

• A. 40 m
• B. 80 m
• C. 160 m
• D. 320 m

Explanation: With uniform acceleration from rest the average velocity is half the final velocity, which is 10 m s-1, and over 8 s that gives 80 m. The same answer comes from finding a as 20 divided by 8, which is 2.5 m s-2, then using S equal to half a t squared. Using the final velocity of 20 m s-1 for the whole 8 s gives the wrong answer of 160 m.

13. The area under a velocity-time graph represents

• A. acceleration
• B. displacement
• C. force
• D. momentum

Explanation: Multiplying velocity by time gives displacement, so the area enclosed between the curve and the time axis is the displacement travelled. The gradient of the same graph gives acceleration. On a displacement-time graph, by contrast, the gradient is velocity and the area has no useful physical meaning.

14. Newton's first law of motion is essentially a statement about

• A. the acceleration produced by a force
• B. inertia, the tendency of a body to resist a change in its state of motion
• C. action and reaction pairs
• D. the conservation of energy

Explanation: A body continues at rest or in uniform motion in a straight line unless a resultant force acts, so the law defines what a force actually does: it changes motion rather than maintaining it. Mass is the measure of that inertia. This is why a passenger lurches forward when a bus brakes suddenly.

15. A body of mass 5 kg is acted on by a resultant force of 20 N. Its acceleration is

• A. 4 m per second squared
• B. 100 m per second squared
• C. 0.25 m per second squared
• D. 25 m per second squared

Explanation: From F equals ma, the acceleration is 20 divided by 5, giving 4 m per second squared in the direction of the resultant force. Multiplying instead of dividing produces the 100 distractor. The force used must always be the resultant, not any single force acting on the body.

16. Newton's third law pair of forces always

• A. act on the same body and therefore cancel
• B. act on two different bodies, so they never cancel each other
• C. are unequal in magnitude
• D. act in the same direction

Explanation: Action and reaction are equal and opposite but act on different objects, which is why a book on a table is held up by the table's push rather than by any cancellation. If they acted on one body nothing could ever accelerate. The weight of the book and the table's normal force are not a third law pair, though they happen to be equal here.

17. The second law can also be written as force equals

• A. the rate of change of momentum
• B. momentum multiplied by time
• C. mass divided by acceleration
• D. work divided by distance only

Explanation: F equals ma is the special case of this for constant mass; the momentum form is more general and handles situations such as a rocket losing mass as it burns fuel. It also leads directly to the impulse momentum theorem, that force multiplied by time equals change in momentum. That is why a longer collision time means a smaller force.

18. The impulse delivered to a body equals

• A. its change in kinetic energy
• B. its change in momentum
• C. the work done on it
• D. its mass times its velocity

Explanation: Impulse is force multiplied by the time for which it acts, and it equals the change in momentum, which is why airbags and crumple zones extend the collision time and so reduce the force on the occupants. A cricketer moving the hands back while catching does the same thing. Change in kinetic energy is related to work rather than to impulse.

19. In an isolated system, the total linear momentum

• A. always increases
• B. is conserved, whatever type of collision occurs
• C. is conserved only in elastic collisions
• D. is zero

Explanation: Momentum conservation follows from Newton's third law and holds in every collision and explosion provided no external force acts, whether the collision is elastic or not. Kinetic energy, by contrast, is conserved only in a perfectly elastic collision. This distinction is what most collision questions are really testing.

20. In a perfectly inelastic collision, the two bodies

• A. bounce apart with no loss of kinetic energy
• B. stick together and move with a common velocity, losing some kinetic energy
• C. conserve kinetic energy but not momentum
• D. come to rest

Explanation: Momentum is still conserved, so the common velocity follows from the total momentum divided by the total mass, but some kinetic energy is converted to heat, sound and deformation. A bullet embedding in a block is the standard example. The bodies come to rest only in the special case where the total momentum happens to be zero.