Mechanics

• Mechanics is the branch of Physics that deals with study of objects at rest or under motion in space and time like distance traveled, displacement, velocity, acceleration, speed etc. It also deals with associated causes of motion like force, torque etc.

1. Statics: Branch of mechanics which deals with the study of objects at rest.
2. Kinematics: Branch of mechanics which deals with the study of motion of objects without taking into account the factors which cause motion.
3. Dynamics: Branch of mechanics which deals with the study of motion of objects taking into account the factors which causes motion.

Motion:

• A body is said to be at rest if it does not change its position with respect to given frame of reference or its surroundings with the passage of time.

• A body is said to be in motion if it changes its position with respect to given frame of reference or its surroundings with the passage of time.

Example:

• In night when the sky is full of stars it is observed that the position of stars and planets change while we remain stationary. In reality the earth is moving too. Thus, an object which appears to be at rest, may actually be in motion. Therefore, motion and rest are relative terms. To describe the motion of an object we have to specify how its position changes with respect to a stationary object. This is called the frame of reference.

• A frame of reference is another object of scene with respect to which we compare an object’ s position.

Example:

• A passenger standing on platform observes that tea-vending machine on a platform is at rest. But when the same passenger sitting in moving train, observes that tea-vending machine in motion.

• Observations are different because in first situation observer stands on a platform, in which reference frame is at rest and in second situation observer is sitting in a moving train, in which reference frame is in motion. So rest and motion are relative terms. It depends upon the frame of references.

• In this picture we can see the position of car changes with time. In this case the tree is a stationary object and the position of car is changing continuously with respect to it.