Introduction to motion
1. Can a body said to be at rest and in motion at the same time?
Solution: Yes, for example a book lying on the table in room is at rest when compared to other objects in the room e.g. table, fan, chair, etc. However, all the objects in the room are sharing the motion of earth. If you view the room from moon, all the objects of the room including the book are in motion. So you can say that the book is at rest with respect to earth but in motion with respect to moon. Hence rest and motion are relative terms.
2. Can you give any physical situation where displacement is zero but not the distance?
Solution: There are many examples where displacement can be zero but not the distance. Consider a ball thrown upward returns to its initial (starting) point after some time. In this case displacement is zero but the distance travelled is 2h where h is maximum height to which ball rises.
3. What is translation motion?
Solution: Rigid bodies that move without rotation are said to undergo translation motion. When we are interested only in translational motion and the object size is not significant, we can treat object as a particle.
Newton's Cradle
4. Which is generally our frame of reference for studying motion of objects?
Solution: Ordinarily our frame of reference for studying motion of objects is a coordinate system attached to the surface of earth. Unless stated otherwise, this is our frame of reference.
5. An aero-plane is flying in the sky. Is its motion three dimensional?
Solution: Yes, it is because it may be changing its direction as well as the height from ground during its flight.
6. Which is the simplest type of motion?
Solution: The uniform motion in a straight line is the simplest type of motion.
7. Can speed be negative?
Solution: No. Although velocity can be positive or negative, speed is never negative. It is because speed involves the rate at which a distance is travelled and not the rate of change of displacement as in case of velocity.
8. What does a speedometer record – instantaneous speed or instantaneous velocity?
Solution: The speedometer of a car records instantaneous speed because it does not indicate the direction of motion. Remember that velocity is a vector quantity and is associated with magnitude as well as direction.
9. The average velocity of a particle is zero but not its average speed. Is it possible?
Solution: Yes, if the particle starts from one point and comes back to the same point. In this case displacement is zero but not the distance travelled. Therefore, the average velocity is zero but not the average speed.
10.Why the time period is repeated twice in the unit of acceleration?
Solution: The velocity of a particle is the rate at which its velocity changes with time; its acceleration is rate of change of velocity with time. Thus acceleration is the rate of a rate. So the time is repeated twice.
