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b)  What are (i) free fall, (ii) acceleration due to gravity (iii) escape velocity (iv) centripetal force ?
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(i) Free Fall: 

Whenever an object moves under the influence of the force of gravity alone, it is said to be in Free Fall. During free fall initial velocity of an object is zero and force of air also acts on an object. Thus real free fall is possible only in a vacuum because there is no air.

(ii) Acceleration due to gravity:

The earth exerts gravitational force on objects near it. According to Newton’s second law of motion, a force acting on a body results in its acceleration. Thus, the gravitational force due to the earth on a body results in its acceleration. This is called acceleration due to gravity and is denoted by ‘g’. 

(iii) Escape velocity: 

The minimum value of the initial velocity at which an object to escape from the gravitational pull of the earth and never comes back to the earth is called the escape velocity.

Formula \(V_{esc}=\sqrt { \tfrac{2GM}{R}}\)

(iv) Centripetal force: 

The force that acts on an object to keep it moving along the circular path with constant speed is called the centripetal force. 

Its formula is \(F_c=\tfrac{mv^2}{r}\)

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