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According to the first law of motion, a physical object stays at the same velocity (zero in the state of rest) unless an external force acts on it. It is also known as the law of inertia since the property of the physical object that resists any change in its velocity is called inertia. If we slide a box across a floor, it stops after a while. It happens due to the force of friction that is acting upon it. Had there been no external opposing force, the box would have stayed in motion at the same velocity in the same direction.
Aristotle’s Fallacy
Before moving further with the first law of motion, let’s briefly look at Aristotle’s fallacy. If a box is moving across a floor, its velocity will keep decreasing with time and eventually come to rest. To move it again, more force has to be applied. Aristotle hypothesised that staying at rest is the natural tendency of objects and force has to be applied to keep them in motion. That was well before the concept of friction was introduced.
We know that the force of friction that acts in the opposite direction causes the box to slow down and eventually stop. If that force and other external forces were absent, the box would stay in motion. Hence, Aristotle’s observation proved to be false, and it is known as Aristotle’s fallacy.
Galileo’s Experiment
Galileo explained the concept of inertia by performing a series of experiments that proved that bodies in motion would stay in motion unless an external force was applied to them. He made use of a double-ramped surface to explain this. The surface had two inclined ramps connected by a horizontal ramp.
He dropped a ball down the inclined ramp. The ball gains speed as it moves down and then moves on the horizontal ramp and then moves up the second inclined ramp. When the ramp is smooth, the ball reaches as high on the second inclined ramp as it possibly can (almost equivalent to the height of the ramp). However, when the ramp is rough, then the distance travelled by the ball is much shorter. This happens because the force of friction is much higher when the surface is rough as opposed to when the surface is smooth.
This led Galileo to believe that the ball would have kept on moving had there been no frictional force (which is an ideal scenario and cannot exist in real life).
Inertia and Mass
Inertia is directly proportional to mass. An object with high mass would have more inertia than an object with low mass. This explains why more force is required to move a heavy stone when it is at rest as compared to a lighter ball. The heavier an object is, the more force would be required to change its state of motion, and hence, it can be said that it possesses more inertia.
Real-life examples of the First law of motion
- Cleaning of carpet: The carpet is cleaned by beating it with a wooden stick. The dust falls off the carpet as the carpet moves from its position of rest. This happens due to inertia of rest, as per which, the dust on the carpet tends to stay at rest. So, the dust particles remain at rest even when the carpet moves, due to which they get separated from the carpet and fall.
- Cardboard coin experiment: This experiment can be easily performed at home. Place a piece of cardboard on a glass of water and place a coin on the top of the cardboard. All objects are in a state of rest. Now, grasp and pull the cardboard so as to remove it entirely from above the glass. The coin would fall into the glass of water. This happens because the coin is still in a state of rest by virtue of inertia.
Conclusion
The first law of motion is vital in explaining numerous real-life incidents. It explains why our torso (upper body) moves backward when the bus we are sitting in starts suddenly. Our body tends to stay at rest even as the bus has started moving. Then, our body tilts in the direction opposite the bus’s direction at a sharp turn. It happens because our body tends to stay in motion in a particular direction. Then, our upper body moves forward as the bus brakes are applied because the body tends to stay in motion even as the bus comes to rest. In just one bus journey, we understand how crucial the first law of motion is.
