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Inertia is the tendency for a moving object to stay in constant motion. Until an unbalanced force acts on the object, it will keep moving at the same speed and in the same direction.
If there was no air resistance or gravity, you could throw a ball horizontally from the top of a mountain and it would fly off into space. The ball would continue travelling in a straight line at constant speed until it hit a satellite or planet. In reality, however, moving objects change speed or change direction fairly quickly because of the many opposing forces acting on them. These opposing forces can include friction, air resistance, and gravity.
Humans experience the Law of Inertia every time they drive in a car. When the driver makes a quick turn or stops suddenly, the passengers' bodies jolt and try to keep moving in the direction the car was just going. Because of inertia, it is very important that passengers in a car wear their seat belts while driving. In the case of an accident, seat belts provide an opposing force that can prevent a passenger from being thrown from the vehicle.
Consider a brick placed in a wagon. If the wagon was travelling at constant speed in a straight line, the brick would stay in the same position in the wagon. If the wagon hit a wall, however, the brick would slide to the front of the wagon until it hit the wall or the edge of the wagon. This is because it needs to experience an opposing force before it will stop moving.
Jan's car broke down on the Trans-Canada Highway in the middle of the prairies. If the highway was straight and level, couldn't Jan just put the car in neutral and keep going until she applied the brakes?
Answer: She could if there was a way to overcome the opposing forces of air resistance and friction.
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