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# Force and laws of motion

First law of motion

Tanusri Gururaj, Academic content writer of Physics at Edumarz

• The first law of motion states that a body remains in the state of rest or uniform motion unless acted upon by an external force.

• The tendency of objects to remain in their state of rest or motion is known as inertia.

• Galileo’s experiment:

1. Galileo observed a marble rolling down an incline.

He noticed that the velocity of the marble increased when moving down the inclined plane and decreased while rolling up.

1. He also argued that a marble dropped from the left inclined plane would go up the other end of the incline and climb the same height.

1. Galileo believed that if the right incline was made flat, the marble would travel forever on the flat surface to reach the height from which it was dropped.

The conclusion from the experiment:

An unbalanced external force is necessary to change a body’s state of motion or rest.

But in practical situations, the unbalanced forces on a body can never be zero due to friction. Friction always acts in the opposite direction of motion and, hence, the marble eventually stops.