A stone S and a foam rubber ball R are identical spheres of equal volume. They are released from rest at time t = 0 and fall vertically through the air. Both reach terminal velocity.
Which graph best shows the variation with time t of the speed v of the stone and of the rubber ball?
ANSWER: B
The Physics Behind
- The terminal velocity is reached by an object that are released from rest is reached when the resultant force has already become zero which means that
- the acceleration is also zero [Newton's 2nd Law of Motion] and
- the velocity remains the same [from definition of acceleration, that is, no more change in velocity].
- The resultant force is the sum of the forces acting on objects.
- One force is the downward gravity [weight]
- The other is air resistance dependent on both the surface area and speed of object
- As the object moves down, it speeds up and therefore the air resistance is getting stronger and stronger. The resultant force then is becoming smaller and smaller. However, there is a difference between the objects S and R as though the air resistance on them is same at same speed, stone R is heavier than rubber R.
- The above means that R can have resultant force of 0 [or zero acceleration or terminal velocity] faster than S.
- Option B is the answer.
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