One reason why the law of conservation of energy onlys works in theory is because of friction. When you push an object over a horizontal plane it should, theoretically, never stop. But friction keeps slowing it down until it's at rest.
The force of friction between two bodies depends on the magnitude of the perpendicular forces of the surfaces in contact. This perpendicular force is known as the normal force and the ratio of the frictional force to the normal force is known as the coefficient of friction. The normal force of an object on a horizontal plane is the weight of the object. (See Figure 1)
The coefficient of friction varys on the materials in contact. If you went down on a waterslide with no water it probably wouldn't be much fun because you would be going so slowly, that is if you were moving at all. When there's water there is much less friction between you and the slide so you end up sliding down very fast and smoothly. Friction is used to slow and stop many amusement rides including rollercoasters and the free fall.
There are many kinds of friction. One type is static friction. Static friction is between two solid bodies with respect to each other. Static friction prevents the motion of one object to the other until the applied force is greater than the static friction between them. To understand this we'll use a heavy box on the floor as an example. You try to push the box with say a force of 50 newtons but the box doesn't budge one bit. This means that the force you used, 50 newtons, was not enough to overcome the static friction. The minimum force needed to start moving the box is known as the starting friction. Once the box is moving the friction between the box and the floor decreases. The friction of an object in motion is called, kinetic or sliding friction.
When a ball rolls over a surface the frictional force is usually less than if it were sliding over the surface. The friction of any rolling object is called rolling friction. The frictional force of an object moving through a fluid like water or air, is called fluid friction.
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