SLS College of Sciences & Arts
Conceptual Questions (Motion and Force)
Q.1) If you are riding on a train that speeds past another train moving in the
same direction on an adjacent track, it appears that the other train is moving
backward. Why?
Answer:
Velocity is relative. The velocity of an object measured by an observer depends on the motion of
the observer itself. If train 1 is moving with a velocity v1, train 2 is moving with a velocity v2 and
both are moving along the same direction, then the velocity of the train 2 with respect to train 1
is:
v21 = v2 − v1
If v1 > v2, the above expression becomes negative. This is the reason why the slower train appears
to be moving backward.
Q.2) Can the velocity of a body reverse the direction when acceleration is
constant? If you think so, give an example.
Answer:
Yes, the velocity of a body can reverse its direction with constant acceleration. When we throw a
ball vertically upward it will move under constant gravitational acceleration in a direction along
positive y-axis. After attaining maximum height it will start falling under same gravitational
acceleration whose values is 9.8 ms-2. During its fall its velocity is along negative y-axis.
So in this case ball reverses the direction of its velocity while having a constant acceleration.
Q.3) When you stand still on the ground, how large a force does the ground
exert on you? Why doesn’t this force make you rise into the air?
Answer:
When we are standing on the ground, we are exerting a force equal to our weight due to gravity
on the ground and it is pulling us down. On the other hand the ground is reacting by pushing us
back; we call this force the reaction force (Newton’s Third law).
Hence the ground exert a reaction force on us equal in magnitude to our weight but opposite in
direction i.e. both the forces cancels each other and we are not risen in the air.
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Q.4) A man standing on the top of a tower throws a ball vertically up with
certain velocity. He also throws another ball vertically down with the same
speed. Neglecting air resistance, which ball will hit the ground with higher
speed?
Answer:
Both balls will strike the ground with same speed.
As balls are given same initial speed by the man but one is thrown upwards and the other is
thrown downwards. Both balls will move with same speed as both are moving under the action
of gravity which is constant for all bodies.
The only difference is that they both will strike the ground at different times due to different
heights. The second ball will take less time compared to first one, while both balls will hit the
ground with the same speed as both of them are experiencing same gravitational force and hence
experience a constant acceleration with the magnitude of 9.8 ms-2.
