Free Newton's Laws of Motion MCQs with Answers

1. What is the SI unit of force?

• A. Joule
• B. Watt
• C. Newton
• D. Pascal

Explanation: The SI unit of force is the newton (N), defined as the force that accelerates a one-kilogram mass at one metre per second squared. The joule is the unit of energy, the watt of power and the pascal of pressure.

2. Newton's second law of motion is most generally stated as

• A. force equals mass times velocity
• B. force equals the rate of change of momentum
• C. force equals mass times distance
• D. force equals momentum times time

Explanation: The general statement is that the net force equals the rate of change of momentum, which reduces to F equal to ma only when the mass is constant. The general form is needed for a rocket, whose mass falls as fuel burns. Mass times velocity is momentum itself, not force.

3. A body moving with uniform velocity has

• A. a net force acting on it in the direction of motion
• B. zero net force acting on it
• C. a net force acting opposite to its motion
• D. an increasing acceleration

Explanation: Uniform velocity means zero acceleration, and by Newton's second law zero acceleration requires zero net force. The individual forces need not be absent: a car at steady speed has thrust and friction that cancel exactly. A non-zero net force would necessarily change either the speed or the direction.

4. A force of 20 N acts on a mass of 4 kg. The acceleration produced is

• A. 0.2 m s-2
• B. 5 m s-2
• C. 16 m s-2
• D. 80 m s-2

Explanation: From F equal to ma, the acceleration is 20 divided by 4, which is 5 m s-2. The answer 80 comes from multiplying instead of dividing, and 0.2 from inverting the ratio. Always check that the units resolve to metres per second squared.

5. The action and reaction forces described by Newton's third law

• A. act on the same body and cancel out
• B. act on different bodies and therefore never cancel
• C. are unequal in magnitude
• D. act in the same direction

Explanation: The pair is equal in magnitude and opposite in direction but acts on two different bodies, which is why they cannot cancel each other and why motion is possible at all. When you push a wall, your force acts on the wall and the wall's force acts on you. Forces only cancel when they act on the same body.

6. 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.

7. 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.

8. 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.

9. The apparent weight of a person standing in a lift accelerating upwards is

• A. greater than their true weight
• B. less than their true weight
• C. equal to their true weight
• D. zero

Explanation: The floor must both support the weight and supply the extra upward force to accelerate the person, so the normal reaction, which is what the scales read, is m times g plus a. Accelerating downwards makes the reading smaller, and in free fall it drops to zero, which is the weightlessness astronauts experience. The true weight, mg, is unchanged throughout.

10. Friction between two surfaces in contact depends on

• A. the area of contact only
• B. the normal force pressing them together and the nature of the surfaces
• C. the speed only, in all cases
• D. the volume of the bodies

Explanation: Limiting friction is the coefficient of friction multiplied by the normal reaction, and to a good approximation it is independent of the apparent area of contact, which surprises most students. Pressing harder or roughening the surfaces increases it. Kinetic friction is usually slightly less than the maximum static friction, which is why a pushed object suddenly lurches once it starts to slide.