Out in the street. Chris and Pat are playing hockey. Chris swings the hockey sti

Question

Out in the street. Chris and Pat are playing hockey. Chris swings the hockey stick at the puck and WHACK! it flies towards the goal. During the collision the hockey stick exerts a force on the puck in the direction shown at right. Does the puck exert a force on the hockey stick during the collision ? If so, In the box at right, draw an arrow to indicate the direction of this force, and In the space below, compare the magnitude of the force on the puck by the hockey stick to the magnitude of the force on the hockey stick by the puck. If the puck does not exert a force on the hockey stick, state so explicitly. The refrigerator in Jan's kitchen was moved to clean the floor. Jan is now trying to push it back into place. For each of the two situations below, is the magnitude of the force that Jan exerts on the fridge greater than. less than. or equal to the magnitude of the force exerted on Jan by the fridge? Explain y our answer. Jan is pushing on the fridge but it just won't budge. Jan pushes with all her might and the fridge starts moving at a slow constant speed. Jan's friend Sue is sitting on a stool watching Jan struggle with the fridge and says: Hmmm, jan...unlike living things, non-living things don't always exert forces on things they touch. Take my stool here, for instance If I poke you with my stool, it would exert a force on you. When i am just sitting on it. it doesn't exert any force on me. Instead it acts like the floor and keeps me from falling. " Do you agree or disagree with Sue's reasoning about her stool? Explain.

Explanation / Answer

I. Yes the puck exert a force on the hockey stick and the direction of that force is just opposite to that of the box i.e. to left. The puck exert same amount of force on the stick. Here we have two forces. Suppose the stick exert force F on the puck. This force is much greater than the friction and the puck start to accelerate and just before that the puck also exert force F on the stick. But as the mass of the stick is much larger than that of the puck we hardly see a movement of the stick.

II. From Newton's third law we can easily say, in both the cases, the magnitude of the force that Jan exerts on the fridge is equal to the magnitude of the force exerted on Jan by the fridge. But what is the difference between these two cases?

Case 1 : To move the fridge Jan must have to exert force to overcome the frictional force for the fridge. Suppose the frictional force is Ffriction . In this case Jan is exerting a force F<Ffriction . So the fridge is not moving but it exert same amount of force F on Jan.

Case 2 : Here Jan exert a force F>Ffriction and the fridge start to move in a constant speed. But again the fridge exert same amount of force F on Jan.

III. No totally disagree. Actually, Sue's weight is acting on the stool downwards. And from Newton's third law the stool exert same amount of reaction force on Sue upwards. This reaction force nullyfies Sue's weight and that is why Sue is not falling. Suppose there is no stool, then no reaction force to balance Sue's weight and Sue will fall.

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