Newton’s First Law

Newton’s first law states that a body tends to remain at rest or in uniform rectilinear motion if the resultant of the forces acting on it is zero.

Driving trucks with very heavy loads is dangerous: moving loads have very high inertia and therefore tend to keep moving.

Newton ‘s first law , also known as the law of inertia, states that if the resultant force on a body is zero (equal to zero), that body will be at rest or in uniform rectilinear motion. Check out the translation of the original statement of Newton’s first law:

“ Every body continues in its state of rest or of uniform motion in a straight line unless it is forced to change that state by forces applied to it.”

Inertia is a property of matter that measures the resistance that a body offers to being accelerated. The greater the inertia of a body, the greater the force required to change its state of motion. Furthermore, the inertia of a body is quantitatively equal to its mass. Therefore, according to the International System of Units (SI), it can be measured in kilograms (kg).

inertial frame of reference

Like Newton’s other laws, the law of inertia is only valid for inertial frames. Thus, the point where the observer of the movements is located cannot be accelerated , therefore, he must move with constant speed or be stationary.

The schematic table below will facilitate your understanding of Newton’s first law:

Every body in equilibrium of forces is at rest or in uniform rectilinear motion.

Understanding Newton’s First Law

We can test the behavior of inertia with simple experiments. One of them consists of supporting a small piece of cardboard on top of a glass. Then place a coin on top of the card. When we quickly pull or push the card, the coin will fall straight into the cup. This is thanks to the currency’s inertia, that is, its tendency to remain at rest.

Pulling the cardboard quickly, the coin remains at rest, as the friction force was not enough to move it.

Another interesting fact about inertia is that it is only possible to perceive it if we are in an accelerated frame of reference. For example: if you are inside a train with closed windows, equipped with a good damping and soundproofing system, you will not be able to tell if the train is stopped or moving.

However, if that train turns, accelerates, or decelerates, you will be able to see the change in its motion. A proof of this is that, although we are moving at the same speed as the Earth moves around the Sun, we are not able to feel such speed.

Despite being accepted and well understood today, the more recent notion of inertia took a long time to establish. Before the work of Galileo Galilei , it was believed that, for there to be movement, the constant action of a force capable of impelling bodies to move was necessary. It was Galileo who realized that, in fact, for a body initially at rest to be set in motion , it was enough for the action of a force.

Galileo did not have the technological resources capable of demonstrating the inertia of a body, so he resorted to some thought experiments. In one of these experiments, the physicist proposed the existence of a perfectly polished surface, which would not exert any friction with a body that was supported on it. According to his reasoning, a body set in motion on this perfectly smooth surface would never stop:

After starting its movement, the body would slide eternally, with constant speed, along the perfectly smooth surface.

Galileo’s conclusion from the experiment proposed above was that if there were no forces contrary to the motion of the body, such as the force of friction , the body should move in a straight line indefinitely.

Later, Isaac Newton determined the law of inertia, giving it a precise mathematical definition through Newton’s second law. In this way, Newton was able to relate the inertia of a body with the resultant force on it and with the acquired acceleration.

Examples of Newton’s first law

On a daily basis, we can do several activities based on Newton’s first law. Check out some of these situations:

  • If a person is inside a train or bus moving at constant speed and throws a ball of paper in the air, the ball should fall back into his hand. This indicates that the person, the means of locomotion and the ball move at the same speed. In addition, the ball has an inertia capable of keeping it moving at the same speed.
  • When braking or accelerating a car, we are “thrown” forward and backward, respectively. Although it appears that there is a force moving us, what we feel refers to the tendency to remain at rest or in uniform motion.
  • In a very sharp turn, we are “squeezed” against the car door. This is because a centripetal force , which points towards the center of the turn, causes the car to turn. In this way, our inertia acts in the same direction as this force, but in the opposite direction.
  • One way to fix the head of a hammer is by tapping the base of its handle against a surface. As the hammer head has a relatively large inertia, it will be at rest while the hammer handle will rise, fixing it.

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