Newton's Third Law | Exit Quiz | Oak National Academy

Match the following key terms to their definition.

• contact force

• one force representing the overall effect of all forces on an object

• resultant force

• a force that exists between objects even if they are not touching

• non–contact force

• a force that only exists between objects when they are touching

Answer explanation

Contact forces only act between objects when they are touching. Non–contact forces can act whether the objects are touching or not. The resultant force is the single force that has the same effect as all of the forces acting on an object.

A block rests on a table. Which of the following diagrams correctly shows the weight of the block?

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Answer explanation

A non–contact force arrow starts at the centre of the object.

A footballer kicks a football. Which of the following diagrams correctly shows the force of the boot on the football?

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Answer explanation

This is a contact force, so its arrow should start where the boot touches the ball.

Newton’s Third Law explains that interactions can be described by a pair of forces, sometimes called a ‘third law pair’. Which of the following are characteristics of the forces in a third law pair?

• they act between two objects only

• they act in the same direction

• they are the same type of force

• they are the same size

Answer explanation

Newton’s Third Law can be written: ‘If object A exerts a force on object B, then object B exerts an equal sized force in the opposite direction on object A.’ The two forces are the same type.

A block rests on a table. Which of the following is the correct free body diagram for the block?

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Answer explanation

The forces acting on the block are its weight pulling downwards and the normal reaction force from the table pushing upwards. The normal reaction force of the block on the table acts on the table, not the block, so it is not included.

The diagram shows a ball hanging from a string without moving. Two forces are shown. Which of the following statements are correct?

• this is a free body diagram of the ball

• this is not a free body diagram of the ball

• the two forces are a third law pair

• the two forces are not a third law pair

Answer explanation

This is a free body diagram showing the forces acting on the ball. They have same size and opposite directions but different types: tension and gravitational force. Force of string on ball and force of Earth on ball involve three objects, not two.