9702_s22_qp_13 Question 9

A ball is dropped onto horizontal ground and bounces vertically upwards. When the ball is in contact with the ground, the following forces act: 

● the weight W of the ball 

● the contact force P exerted on the ground by the ball 

● the contact force N exerted on the ball by the ground. 

When the ball is in contact with the ground, the ball is momentarily stationary. At this instant, which relationship is correct? 

A N = P + W 

B N > P + W 

C N = W 

D N > W

ANSWER: D

The Physics behind the answer:

• For the ball moving downwards to eventually move upwards, a resultant force is needed to slow it down to a stop and speeds it up again in the opposite direction.
• In the case of the ball above, the resultant force needed is met when it is in contact with the ground. When it was not in contact yet, the only force acting is gravity or force W.
• When it's already in contact, there are 3 forces given. However, for the resultant force, there are only 2 of those that act on the ball, W and N. The other force P is acting on the ground.
• With only W and N acting, the needed resultant force is the sum of a stronger N and a weaker W.