(a) Explain what is wrong with this: A tennis player hits the ball. As it flies
ID: 1782706 • Letter: #
Question
(a) Explain what is wrong with this: A tennis player hits the ball. As it flies over the net, the forces acting on it are gravity, air resistance, and the force of the hit.
(b) Make a correct statement about the ball and the hit.
(c) Explain what is wrong with this: An elevator is moving upward at constant speed. Neglecting friction and air resistance, the tension in the cable must be greater than the weight of the elevator.
(d) Make a correct statement about the tension being bigger than the weight.
(e) Explain what is wrong with this: I am pushing a heavy crate across a rough floor at constant speed. If I double the force I exert on the crate, its speed will double.
(f) What forces act on the crate, and what will the crate do if I double my force on it?
(g) You jump into the air. Explain why the normal force the floor exerts on your feet must be greater than your weight, both while taking off and while landing.
Explanation / Answer
a) and b) The scenario describes a situation when ball is in the air. Note that the hit force only acts as long as there is a contact between racquet and ball. After that, the force is converted to some other form. When ball is hit, the force acting on it is the applied force and a reaction force due to newton's law.
c) and d) The tension in the cable is bigger than weight only when elevator is moving down. Because when it moves down , the net acceleration acts upward.. Therefore upward force must be greater.
e) and f) As per netwon's second law, F = m*a. Therefore, if we double the force, the acceleration will get double. Forces acting on crate are normal force from ground whiich is equal to weight and applied force. Under the influence of this applied force, crate will accelerate.
g) When we jump up in the air, we tend to accelerate at some point, that force will add up in the normal force and will make overall force greater than our weight. When we land, we will have some momentum until we touch the ground, then momentum will come down to zero.
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