Question 4
A ball is released
from rest above a horizontal surface and bounces several times.
The graph shows how,
for this ball, a quantity y varies with time.
What is the quantity y?
A acceleration
B displacement
C kinetic
energy
D velocity
Reference: Past Exam Paper – June 2013 Paper 12 Q8
Solution:
Answer:
B.
The ball is thrown from
rest above a horizontal surface and it bounces several times.
We know that the ball
falls due to the acceleration due to gravity ‘g’. However, the value of g is
constant (= 9.81 m s-2) and does not vary when there is no air
resistance. [A is incorrect]
Consider
choice B.
Let the displacement be
taken from the position above the surface and take the positive direction to be
downwards. The ball falls until it reaches the surface (this is the maximum
displacement).
Now, the ball falls under
gravity. It accelerates; that is, its speed increases. In a displacement-time
graph, this is indicated by an increasing gradient. This is indeed indicated by
the graph.
As it hits the surface,
the ball rebounds. However, due to energy loss (in form of heat, sound …), the
ball does not reach its original height. As the ball moves up, its displacement
(taken from its original position above before the release) decreases but does
not come to zero as the ball does not reach its original height. [B is correct]
Under careful
consideration, it can be shown that ‘y’ cannot be kinetic energy nor velocity.
Why cant it be kinetic energy
ReplyDeleteAt the instant where the ball hits the surface, its velocity at that instant is zero. So, its kinetic energy at the instant would be zero. The graph does not reduce to zero, so y cannot be the kinetic energy or velocity.
DeleteI am a physics teacher and its helping a lot and I'm realising I don't even know the basics
ReplyDeleteOur physics teacher is so bad at explaining this is helping me a lot thanks
ReplyDelete