Which law is inertia

Experience suggests that an object at rest will remain at rest if left alone, and that an object in motion tends to slow down and stop unless some effort is made to keep it moving.

We will define net external force in the next section. An object sliding across a table or floor slows down due to the net force of friction acting on the object. If friction disappeared, would the object still slow down? The idea of cause and effect is crucial in accurately describing what happens in various situations.

For example, consider what happens to an object sliding along a rough horizontal surface. The object quickly grinds to a halt. If we spray the surface with talcum powder to make the surface smoother, the object slides farther. If we make the surface even smoother by rubbing lubricating oil on it, the object slides farther yet. Extrapolating to a frictionless surface, we can imagine the object sliding in a straight line indefinitely.

You experience inertia in a moving car all the time. In fact, seatbelts exist in cars specifically to counteract the effects of inertia.

Imagine for a moment that a car at a test track is traveling at a speed of 55 mph. Now imagine that a crash test dummy is inside that car, riding in the front seat. If the car slams into a wall, the dummy flies forward into the dashboard. Because, according to Newton's first law, an object in motion will remain in motion until an outside force acts on it. When the car hits the wall, the dummy keeps moving in a straight line and at a constant speed until the dashboard applies a force.

Seatbelts hold dummies and passengers down, protecting them from their own inertia. Interestingly, Newton wasn't the first scientist to come up with the law of inertia. In fact, the marble-and-ramp thought experiment described previously is credited to Galileo. Newton owed much to events and people who preceded him. Before we continue with his other two laws, let's review some of the important history that informed them.

The F , the m and the a in Newton's formula are very important concepts in mechanics. The F is force , a push or pull exerted on an object. The m is mass , a measure of how much matter is in an object. The behavior of the water during the lap around the track can be explained by Newton's first law of motion.

There are many applications of Newton's first law of motion. Consider some of your experiences in an automobile. Have you ever observed the behavior of coffee in a coffee cup filled to the rim while starting a car from rest or while bringing a car to rest from a state of motion? Coffee "keeps on doing what it is doing. While the car accelerates forward, the coffee remains in the same position; subsequently, the car accelerates out from under the coffee and the coffee spills in your lap.

On the other hand, when braking from a state of motion the coffee continues forward with the same speed and in the same direction , ultimately hitting the windshield or the dash. Coffee in motion stays in motion. Have you ever experienced inertia resisting changes in your state of motion in an automobile while it is braking to a stop? The force of the road on the locked wheels provides the unbalanced force to change the car's state of motion, yet there is no unbalanced force to change your own state of motion.

Thus, you continue in motion, sliding along the seat in forward motion. A person in motion stays in motion with the same speed and in the same direction Seat belts are used to provide safety for passengers whose motion is governed by Newton's laws.

The change in velocity divided by the change in time is the definition of the acceleration a. The second law then reduces to the more familiar product of a mass and an acceleration:.

Remember that this relation is only good for objects that have a constant mass. This equation tells us that an object subjected to an external force will accelerate and that the amount of the acceleration is proportional to the size of the force.

The amount of acceleration is also inversely proportional to the mass of the object; for equal forces, a heavier object will experience less acceleration than a lighter object.

Considering the momentum equation, a force causes a change in velocity; and likewise, a change in velocity generates a force. The equation works both ways. The velocity, force, acceleration, and momentum have both a magnitude and a direction associated with them. Scientists and mathematicians call this a vector quantity.

The equations shown here are actually vector equations and can be applied in each of the component directions. We have only looked at one direction, and, in general, an object moves in all three directions up-down, left-right, forward-back.