a. An ice hockey puck is tied by a string to a stake in the ice. The puck is the

Question

a. An ice hockey puck is tied by a string to a stake in the ice. The puck is then swung in a circle. What force or forces does the puck feel?

A. Gravity and a normal force B. Tension and a normal force C. Gravity and tension D. Gravity, tension and a normal force E. Gravity, tension, a normal force, and a centripetal force

b. You are driving down Grand River Avenue on a Friday night when a pedestrian steps into the road. You slam on the brakes, and are thrown forward until your seat belt catches you. What is the direction of the net force acting on you, from when you start to brake until the seat belt catches you? (Ignore any friction from the car’s seat.)

A. Forward B. Backward C. Downward D. The net force is zero

Explanation / Answer

For (a) the correct option is (D).

The force due to gravity is balanced by normal force and tension provides the required centripital force needed for radial acceleration.

Note that there is no force as centripital force anywhere. However some of the basic forces like tension friction etc provide the required centripital force.

For (b) also the correct option is (D).

As the brakes are hit the car stops but we have a tendency to move forwad due to inertia. So from the frame of car (decelerating and thus non inertial) we are moving forward with some acceleration but from the frame of earth (assumed inertial frame) we are moving with constant velocity until the seat belt cathches us.

Note that here we ignored friction. If friction would have been in play there would be a frictional force on our butt to stop us from moving forward even before seat belt catches us.

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