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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

Give the magnitude and direction of the net force acting on a stone of mass 0.1 kg,
(a) just after it is dropped from the window of a stationary train,
(b) just after it is dropped from the window of a train running at a constant velocity of 36 km/h,
(c) just after it is dropped from the window of a train accelerating with 1 m s-2,
(d) lying on the floor of a train which is accelerating with 1 m s-2, the stone being at rest relative to the train.
Neglect air resistance throughout.

a) We have here, 

Mass, m = 0.1 kg 

Acceleration due to gravity, a = +g = 10 m/s2

Net force, F = ma = 0.1 × 10 = 1.0 N, acting in vertically downward direction.

b) When the train is running at a constant velocity, its acceleration = 0.

Due to this motion, no force is acting on the stone.


Force on the stone, F = weight of stone = mg

                                 = 0.1 × 10 = 1.0 N

This force acts in the vertically downward direction. 

c) Acceleration of the train, a = 1 m s-2

Additional force, F' = ma = 0.1 × 1 = 0.1 N, acts on the stone in the horizontal direction.

But once the stone is dropped from the train, F' becomes zero.

Net force on the stone, F = mg = 0.1 × 10

                                       = 1.0 N, acting vertically downwards.

d) When the stone is lying on the train, its acceleration is same as that of the train.

Therefore, the force acting on stone, F = ma 

                                                           = 0.1 × 1 = 0.1 N

This force is along the horizontal direction of motion of the train.

In each case, the weight of the stone is being balanced by the normal reaction.


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. 

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.

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.