How Does Inertia Work? How does inertia act and how is inertia overcome?


What is inertia? How does inertia act and how is inertia overcome? What is centrifugal force and teh relationship between inertia.


Energy is used in overcoming three different kinds of resistance: (1) the resistance of gravity, (2) the resistance of friction, and (3) the resistance of molecular attraction. Work must also be done when another kind of resistance is overcome.

When you get on a bicycle, you must exert a force to make it move. After it starts to move, much less force is needed. In fact, once the bicycle is moving along fairly fast, it will coast for some distance without needing any force at all. But to slow it down or stop it, a force must be exerted. Scientists know that all objects act like this because of a certain property that matter has. This property is called inertia.




  1. Tie a cord at least 6 feet long around a brick and hang the brick where it can swing freely. Pull the brick slowly toward you. Does it take much force to start it moving?
  2. Stop the brick from swinging. Does it take much force to do this?
  3. Now start the brick with a quick jerk. Is more force needed this time?
  4. As the brick is swinging rapidly back and forth, stop its motion. Be careful not to injure your hand when you do this. Is more force needed than before?
  5. If you have a spring balance, you can measure the force needed to overcome the resistance of inertia. Fasten the spring balance to the brick with a cord and then repeat each part of the experiment.

All objects have inertia. They will stay in the same place unless a force starts them moving. And they will keep on moving in the same direction unless a force stops them or turns them aside. So we can say that inertia is the tendency of all objects to stay still if still or, if moving, to go on moving in the same direction unless acted on by a force. If an object is standing still, we say that it has the inertia of rest. But if it is moving, we say it has the inertia of motion.

You have noticed these two forms of inertia many times. If you lay a book on a table, it stays where you put it because of the inertia of rest. When you throw a ball, it leaves your hand and keeps on going because of the inertia of motion. Did you ever bump into a chair in the dark or stub your toe on a stone when you were barefooted? You had the inertia of motion, while the chair or the stone had the inertia of rest.

Probably you have been in a car that started up suddenly. As it jumped forward, you felt your body pushed backward against the seat. Actually, because of the inertia of rest, you stood still for an instant after the car started. The seat really moved forward and pushed against you. If the car stopped quickly, you kept on going forward because of the inertia of motion. Perhaps you even slid off the seat. Do you remember what happened when the car turned a corner or went around a sharp curve? If it was moving fairly fast, you felt as though you were being thrown outward against the side of the car. The car changed its direction as it turned, but you were moving straight ahead. The outward force that you felt was caused by the inertia of motion. This outward force has a special name. It is called centrifugal force.

centrifugal force

centrifugal force

When a ball tied to a string is whirled around in a circle, the ball pulls outward on the string. The faster the ball whirls, the harder it pulls. If it is whirled fast enough, it may even break the string and go flying away. Centrifugal force makes the ball pull away from the center around which it is whirling. The ball tends to move in a straight line because of inertia, but the string pulls inward and makes the ball move in a circle.

Centrifugal force is caused by the inertia of objects that are made to move in a curve instead of a straight line. This force is well named. Centrifugal means fleeing, or flying away, from the center. Ali whirling objects have centrifugal force, because their direction is constantly being changed. Centrifugal force throws your arms out from your sides when you spin around on skates. It throws water and mud off the wheels of bicycles and cars. And sometimes centrifugal force even bursts grinding wheels that are turned too fast.

Stationary objects resist being moved, and moving objects resist being stopped or having their direction changed. No object starts, stops, or changes direction by itself. A force is always needed to overcome the resistance of inertia. Without energy to exert forces, we could not move stationary objects. Nor could we stop or change the direction of moving objects. So, by using energy to exert a force greater than the inertia, we can overcome the resistance of inertia. inertia of rest is overcome when a piano is moved, while inertia of motion is overcome when a bicycle is stopped or when it has its direction changed.


On earth inertia does not keep objects moving forever. Some resistance, such as gravity or friction, finally stops them. A car moving along a level road is soon stopped if the engine is shut off. Friction acts on many parts of the car. The tires and the pavement rub against each other. There is even friction between the car and the air! Because of friction in ali these places, the car is slowed down and finally stopped. Little by little, friction overcomes the inertia of the moving car. To keep the car going fonvard, enough force must be exerted to overcome the resistance of friction. If the car is climbing a hill, extra force is needed to overcome the resistance of gravity. Energy from the engine is used to exert the force that overcomes the resistance and keeps the car moving forward.

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