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Laws of Motion

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Physics Part I

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CBSE Gujarat Board Haryana Board

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Class 10 Class 12
A stone of mass tied to the end of a string revolves in a vertical circle of radius R. The net forces at the lowest and highest points of the circle directed vertically downwards are: [Choose the correct alternative]
  Lowest Point Highest Point
a mg – T1 mg + T2
b mg + T1 mg – T2
c mg + T1 – (mv12/ R mg – T2 + (mv12/ R
d mg – T1 – (mv12/ R mg + T2 + (mv12/ R

T1 and V1 denote the tension and speed at the lowest point.
T2 and v2 denote corresponding values at the highest point.

The free body diagram of the stone at the lowest point is shown in the figure below:


According to Newton’s second law of motion, the net force acting on the stone at this point is equal to the centripetal force.

i.e.,  Fnet = T - mg = mv subscript 1 squared over straight R                    ...(i)

v1 is the velocity at the lowest point. 

The free body diagram of the stone at the highest point is shown in the following figure. 


Using Newton’s second law of motion, 

        T + mg = mv subscript 2 squared over straight R                       ...(ii)

where, v2 is the velocity at the highest point.

From equations (i) and (ii), 

Net force acting at the lowest = (T - mg)

Net force at the highest points = (T + mg) 


When the branches of an apple tree are shaken, the apples fall down. Why?

The apple fall from an apple tree when it shaken because of inertia of rest. Apple is in a state of rest and when the tree is suddenly shaken, apples still tends to remain in it's same state of rest whereas branches move. 

So, the apples fall down. 

State Galileo’s law of motion.

Galileo’s law of motion states that, a body continues to move in the same direction with constant speed, if no force is acting on the body. 

Define inertia.

The property by virtue of which the body cannot change its state of rest or uniform motion in a straight line, unless an external force is acting on the body is called as Inertia. 

Is Aristotle’s law of motion now correct?

No. Aristotl'e law of motion is false. 

What is Aristotle’s law of motion?

Aristotle’s law of motion states that an external force is required to keep the body in motion.