5.18 Newton's third law
9 exam questions worth 22 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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State Newton's third law of motion.
Mark scheme
when two objects interact, the forces they exert on each other (1); are equal in size and opposite in direction (1)
Model answer
When two objects interact, the forces they exert on each other are equal in size and opposite in direction.
A student pushes a wall with a force of 30 N. State the size and direction of the force that the wall exerts on the student.
Mark scheme
30 N (1); in the opposite direction / back towards the student (1)
Model answer
30 N, in the opposite direction to the student's push.
A book rests on a table. The weight of the book and the normal contact force on the book are equal and opposite. Explain why they are not a Newton's third law pair.
Mark scheme
they both act on the same object (the book) (1); they are different types of force (gravity and contact) (1)
Model answer
Both forces act on the book, not on different objects, and they are different types of force.
Name the force that is the pair of the Earth pulling on a person (the person's weight).
Mark scheme
the person pulls on the Earth (1); with an equal and opposite gravitational force (1)
Model answer
The person pulls on the Earth with a gravitational force of the same size, in the opposite direction.
Use Newton's third law to explain how a rocket accelerates upwards.
Mark scheme
the engine pushes hot gas downwards (1); the gas pushes the rocket upwards with an equal and opposite force (1); the resultant force is upwards so the rocket accelerates upwards (1)
Model answer
The rocket's engine pushes hot gases downwards. By Newton's third law, the gases push the rocket upwards with an equal force, and this gives a resultant upward force, so the rocket accelerates.
A swimmer pushes backwards on the water. Explain how this makes the swimmer move forwards.
Mark scheme
the water pushes forwards on the swimmer (1); with an equal and opposite force (1)
Model answer
The water exerts an equal force forwards on the swimmer, in the opposite direction.
A gun fires a bullet. The force on the bullet and the force on the gun are equal and opposite. Explain why the bullet's acceleration is much bigger than the gun's.
Mark scheme
the forces are equal (1); the bullet has a much smaller mass (1); a = F รท m, so a smaller mass has a larger acceleration (1)
Model answer
The forces are the same size, but the bullet has a much smaller mass, so it has a much larger acceleration (a = F / m).
Two ice skaters stand facing each other, and one pushes the other. Both move backwards. Explain why.
Mark scheme
the first skater pushes the second with a force (1); the second pushes back with an equal and opposite force, so both move apart (1)
Model answer
The first skater exerts a force on the second, and by Newton's third law the second exerts an equal and opposite force on the first, so both move backwards.
A horse pulls a cart. A student says that, by Newton's third law, the cart pulls back on the horse with an equal force, so the horse and cart can never accelerate. Explain why the student is wrong.
Mark scheme
the pair of forces act on different objects (1); resultant force on the cart is the pull of the horse minus friction (1); the horse also has a forward force from the ground (friction) which is larger than the backward pull of the cart (1); so there is a resultant forward force on the horse-cart system and it accelerates (1)
Model answer
The two forces in the pair act on different objects, so they do not cancel. For the cart, the pull from the horse is larger than the friction on it, so it accelerates. The horse accelerates because the ground's forward push on its hooves is larger than the backward pull of the cart.
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