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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 pebble of mass 0.05 kg is thrown vertically upwards. Give the direction and magnitude of the net force on the pebble,
(a) during its upward motion,
(b) during its downward motion,
(c) at the highest point where it is momentarily at rest. Do your answers change if the pebble was thrown at an angle of 45° with the horizontal direction?
Ignore air resistance.

Irrespective of the direction of motion, acceleration due to gravity always acts downward.

The gravitational force is the only force that acts on the pebble in all three cases.

Its magnitude is given by Newton’s second law of motion as:

                       F = m x a

F = net force, 

m= mass of the pebble, and 

a is the acceleration.

Here, a = g = 9.8 m/s2


               F = 0.05 x 10 = 0.5 N.

The magnitude of net force = 0.5 N, which is acting vertically in the downward direction.

If the pebble is thrown at an angle of 45° with the horizontal direction, it will have both the horizontal and vertical components of velocity. At the highest point, only the vertical component of velocity becomes zero. However, the pebble will have the horizontal component of velocity throughout its motion. This component of velocity produces no effect on the net force acting on the pebble. 

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. 

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.