Newton's laws quiz: 25 questions with answers

Questions and answers

By CanQuiz

An AI wrote the questions in this quiz from its creator’s material. It may contain mistakes.

1. 1.According to Newton's first law, what happens to an object when the net force acting on it is zero?

   • It stays at rest or keeps moving at a constant velocity
   • It gradually slows down until it stops
   • It speeds up in the direction it is already moving
   • It moves in a circle at constant speed
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   Answer: It stays at rest or keeps moving at a constant velocity

   Explanation: With no net force there is no acceleration, so the velocity does not change. A resting object stays at rest and a moving one keeps the same speed and direction.

2. 2.Which property of an object measures how strongly it resists a change in its motion?

   • Weight
   • Volume
   • Mass
   • Speed
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   Answer: Mass

   Explanation: This resistance is called inertia, and mass is its measure: the greater the mass, the harder it is to speed the object up, slow it down or change its direction.

3. 3.A net force of 12 N acts on a 3 kg cart. What is the cart's acceleration?

   • 0.25 m/s²
   • 4 m/s²
   • 9 m/s²
   • 36 m/s²
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   Answer: 4 m/s²

   Explanation: From the second law, a = F / m = 12 N / 3 kg = 4 m/s².

4. 4.What net force is needed to give a 1,200 kg car an acceleration of 2 m/s²?

   • 600 N
   • 1,202 N
   • 2,400 N
   • 24,000 N
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   Answer: 2,400 N

   Explanation: F = m × a = 1,200 kg × 2 m/s² = 2,400 N.

5. 5.Which combination of SI base units is equal to one newton?

   • kg·m/s
   • kg·m²/s²
   • kg/s²
   • kg·m/s²
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   Answer: kg·m/s²

   Explanation: Since F = m × a, a force unit is a mass unit times an acceleration unit: 1 N = 1 kg × 1 m/s². The unit kg·m²/s² is the joule, a unit of energy.

6. 6.What is the weight on Earth of a student whose mass is 50 kg? Take g = 9.8 N/kg.

   • 50 N
   • 5.1 N
   • 490 N
   • 4,900 N
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   Answer: 490 N

   Explanation: Weight is the gravitational force on a mass: W = m × g = 50 kg × 9.8 N/kg = 490 N.

7. 7.An astronaut has a mass of 70 kg on Earth. What is her mass on the Moon, where the pull of gravity is about one sixth as strong?

   • 70 kg
   • About 11.7 kg
   • About 420 kg
   • 0 kg
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   Answer: 70 kg

   Explanation: Mass is the amount of matter in a body and does not depend on where it is. Her weight on the Moon is about one sixth of her weight on Earth, but her mass stays 70 kg.

8. 8.Mass and weight are two names for the same physical quantity.

   • True
   • False
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   Answer: False

   Explanation: Mass is measured in kilograms and is the same everywhere. Weight is a force, measured in newtons, and changes with the strength of gravity.

9. 9.A swimmer pushes the water backwards with her hands. According to Newton's third law, what does the water do?

   • It pushes the swimmer backwards as well
   • It pushes the swimmer forwards with a force of equal size
   • It pushes the swimmer forwards with a much smaller force
   • It exerts no force, because water is a liquid
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   Answer: It pushes the swimmer forwards with a force of equal size

   Explanation: Forces come in pairs: if the swimmer pushes the water backwards, the water pushes the swimmer forwards with a force of the same size in the opposite direction. That forward push is what moves her.

10. 10.Why do the two forces of an action-reaction pair not cancel each other out?

    • They have different sizes
    • They act at different moments in time
    • One of them is weakened by friction
    • They act on different objects
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    Answer: They act on different objects

    Explanation: Forces can only cancel when they act on the same object. The two forces of a third-law pair act on two different bodies, so each one can change the motion of the body it acts on.

11. 11.A book rests on a table, and the Earth pulls it downwards with a gravitational force. According to Newton's third law, what is the partner force of this pull?

    • The table pushing the book upwards
    • The book pressing down on the table
    • The book pulling the Earth upwards
    • The friction between the book and the table
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    Answer: The book pulling the Earth upwards

    Explanation: A third-law pair involves the same two bodies and the same type of force. The Earth pulls the book down by gravity, so the book pulls the Earth up by gravity. The table's upward push is a different force that happens to balance the weight.

12. 12.When a heavy truck collides head-on with a small car, the truck exerts a larger force on the car than the car exerts on the truck.

    • True
    • False
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    Answer: False

    Explanation: By the third law the two forces are equal in size and opposite in direction. The car suffers more because its smaller mass gives it a much larger acceleration from the same force.

13. 13.A box is pushed across a level floor at constant velocity with a horizontal force of 40 N. What is the friction force on the box?

    • 40 N, opposite to the direction of motion
    • 0 N, because the box is moving
    • Less than 40 N, otherwise the box could not move
    • More than 40 N, opposite to the push
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    Answer: 40 N, opposite to the direction of motion

    Explanation: Constant velocity means zero acceleration, so the net force is zero. The friction force must therefore balance the 40 N push exactly, acting in the opposite direction.

14. 14.When a bus brakes suddenly, the standing passengers lurch forwards. Which law best explains this?

    • Newton's first law
    • Newton's second law
    • Newton's third law
    • The law of universal gravitation
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    Answer: Newton's first law

    Explanation: The passengers' bodies tend to keep moving at the bus's original speed because of their inertia. The brakes slow the bus, not the passengers, so they keep going forwards until a force stops them.

15. 15.The net force on an object is doubled while its mass stays the same. What happens to its acceleration?

    • It halves
    • It stays the same
    • It doubles
    • It becomes four times as large
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    Answer: It doubles

    Explanation: From a = F / m, acceleration is directly proportional to the net force when the mass is constant, so twice the force gives twice the acceleration.

16. 16.On a block sliding across a table, friction acts in the same direction as the block's motion.

    • True
    • False
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    Answer: False

    Explanation: Friction between two surfaces opposes their sliding past each other. On a block sliding across a table it points opposite to the motion, which is why the block slows down if nothing else pushes it.

17. 17.Two horizontal forces act on a sledge: 30 N to the right and 18 N to the left. What is the net force on the sledge?

    • 48 N to the right
    • 12 N to the left
    • 18 N to the left
    • 12 N to the right
    Show answer

    Answer: 12 N to the right

    Explanation: Forces in opposite directions are subtracted: 30 N − 18 N = 12 N, in the direction of the larger force, which is to the right.

18. 18.A space probe far from any star or planet switches off its engines while it is moving. What does the probe do next?

    • It slows down and eventually stops
    • It keeps moving in a straight line at the same speed
    • It starts to move in a circle
    • It immediately stops moving
    Show answer

    Answer: It keeps moving in a straight line at the same speed

    Explanation: Far from any planet or star there is practically no gravity or air resistance, so the net force is zero. By the first law the probe keeps its velocity without needing an engine.

19. 19.An object moving at a constant velocity can have several forces acting on it.

    • True
    • False
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    Answer: True

    Explanation: Constant velocity only requires the net force to be zero. Several forces can act at once as long as they balance, like the engine's push and air resistance on a car cruising at a steady speed.

20. 20.How can a rocket accelerate in space, where there is no air to push against?

    • It pushes exhaust gases backwards, and the gases push the rocket forwards
    • Its exhaust gases push against the empty space behind it
    • Its engines cancel out the rocket's inertia
    • The Sun's gravity pulls it in the direction it is pointing
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    Answer: It pushes exhaust gases backwards, and the gases push the rocket forwards

    Explanation: The engine forces the hot gases out of the back, and by the third law the gases push on the rocket with an equal and opposite force. No air is needed for this action-reaction pair.

21. 21.In which direction does an object accelerate?

    • In the direction of its velocity
    • Opposite to the net force
    • At right angles to the net force
    • In the direction of the net force
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    Answer: In the direction of the net force

    Explanation: The second law, F = m × a, links two vectors: the acceleration points the same way as the net force. It need not point along the velocity, as a car braking shows.

22. 22.A 2 kg ball is dropped near the Earth's surface. Ignoring air resistance, what is its acceleration? Take g = 9.8 m/s².

    • 4.9 m/s²
    • 9.8 m/s²
    • 19.6 m/s²
    • 2 m/s²
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    Answer: 9.8 m/s²

    Explanation: The only force is its weight, m × g. Dividing by its mass gives a = m × g / m = g = 9.8 m/s², whatever the mass of the ball.

23. 23.In a vacuum, a hammer and a feather dropped together from the same height reach the ground at the same time.

    • True
    • False
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    Answer: True

    Explanation: Without air resistance, gravity gives every object the same acceleration, because a larger weight is matched by a larger mass. An Apollo 15 astronaut showed this on the Moon in 1971.

24. 24.Which change would increase the friction force on a crate sliding across a floor?

    • Polishing the floor
    • Placing a heavy load on top of the crate
    • Putting the crate on wheels
    • Turning the crate onto a smaller face made of the same material
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    Answer: Placing a heavy load on top of the crate

    Explanation: Sliding friction grows with the force pressing the surfaces together (the normal force), so a heavier load means more friction. Polishing and wheels reduce friction, and in the simple model the contact area does not change it.

25. 25.A skydiver falls at her terminal velocity. Which statement about the forces on her is correct?

    • Her weight is greater than the air resistance
    • The air resistance is greater than her weight
    • No forces act on her any more
    • Her weight and the air resistance are equal in size, so the net force is zero
    Show answer

    Answer: Her weight and the air resistance are equal in size, so the net force is zero

    Explanation: At terminal velocity her speed no longer changes, so her acceleration is zero. By the second law the net force is zero: the upward air resistance balances her weight.