1.2 Newton's first law of motion: inertia  (Page 18/9)

Newton's first law and friction

Newton's first law of motion    states that:

1. A body at rest tends to stay at rest $a\propto \frac{1}{m}$
2. A body in motion tends to remain in motion at constant velocity unless acted on by a net external force (recall that “constant velocity” means in a straight line and at constant speed). $F_{net}=0$ or ( $\sum F=0$ )

Newton's 1 ^st Law: $F_{net}=0$ Or ( $\sum F=0$ )

At first glance, this law may seem to contradict your everyday experience. You have probably noticed that a moving object will usually slow down and stop unless some effort is made to keep it moving. The key to understanding why, for example, a sliding box slows down (seemingly on its own) is that a net external force acts on it to make it slow down. Without this net external force, the box would continue to slide at a constant velocity (as stated in Newton’s first law of motion). What force acts on the box to slow it down? It is called friction    . Friction is an external force that acts opposite to the direction of motion (see [link] ). Think of it as a resistance to motion that slows things down.

Friction:

1. Goes in the opposite direction

   

2. Causes an object to slow down ( [link] )

Consider an air hockey table. When the air is turned off, the puck slides only a short distance before friction slows it to a stop. However, when the air is turned on, it lifts the puck slightly so that the puck experiences very little friction as it moves over the surface. With friction almost eliminated, the puck glides along with very little change in speed. On a frictionless surface, the puck would experience no net external force (ignoring air resistance, which is also a form of friction). Additionally, if we know enough about friction, we can accurately predict how quickly objects will slow down.