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Laws of Motion

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Physics Part I

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Class 10 Class 12
Two billiard balls each of mass 0.05 kg moving in opposite directions with speed 6 m s–1 collide and rebound with the same speed. What is the impulse imparted to each ball due to the other?

Mass of each ball, m = 0.05 kg 

Initial velocity of the ball = 6 m/s 

Magnitude of the initial momentum of the ball, p1 = 0.05 x 6

    = 0.3 kg m/s

After collision, the balls change their directions of motion without changing the magnitudes of their velocity.

Final momentum of each ball, p= - 0.3 kg m/s

Impulse imparted to each ball = pf - pi = -0.3 -0.3 = -0.6 kg m/s

The negative sign indicates that the impulses imparted to the balls are opposite in direction.


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. 

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. 

Is Aristotle’s law of motion now correct?

No. Aristotl'e law of motion is false. 

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. 

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.