1. You are driving a motorcycle on a horizontal road. It is moving with a uniform velocity. Is it possible to accelerate the motorcycle without putting higher petrol input rate into the engine?
Solution:
Yes, it is possible to accelerate the motorcycle without increasing the petrol input rate. Velocity is a vector quantity, having both magnitude and direction. Acceleration is defined as the rate of change of velocity. Even if the speed remains constant, the motorcycle can still accelerate by changing its direction. When the motorcycle moves along a curved path, it undergoes centripetal acceleration directed towards the center of the curve. The magnitude of this acceleration is given by v2/r, where v is the speed and r is the radius of the path.
Thus, without increasing petrol input, acceleration can be produced by changing direction, not necessarily speed.
Based on the “QUESTIONS FOR SHORT ANSWER” section in the sources, here are the solutions for Questions 2 and 3:
2. Some washing machines have cloth driers. It contains a drum in which wet clothes are kept. As the drum rotates, the water particles get separated from the cloth. The general description of this action is that “the centrifugal force throws the water particles away from the drum”. Comment on this statement from the viewpoint of an observer rotating with the drum and the observer who is washing the clothes.
Solution:
The explanation depends on the observer’s frame of reference.
Observer rotating with the drum:
This is a non-inertial frame. In this frame, a pseudo force called centrifugal force acts radially outward on the water particles.
At the holes of the drum, there is no contact force to balance this outward force, so the water particles move outward and leave the drum.
Observer on the ground:
This is an inertial frame. The drum provides the necessary centripetal force to keep the clothes moving in a circle.
At the holes, there is no inward force acting on the water particles. Due to inertia, they move along the tangent and leave the drum.
3. A small coin is placed on a record rotating at 33⅓ rev/minute. The coin does not slip on the record. Where does it get the required centripetal force from?
Solution:
For circular motion, a centripetal force must act towards the center.
In this case, the required centripetal force is provided by static friction between the coin and the rotating record.
This frictional force acts towards the center and keeps the coin moving in a circular path without slipping.