If we add this force to the weight of the box, we find that the normal force is 15 + 5.0 = 20 N. Thus, the maximum force of static friction is:
The force pushing the box forward is the horizontal component of
, which is:
As we can see, this force is just slightly greater than the maximum force of static friction opposing the push, so the box will slide forward.
Tension
Consider a box being pulled by a rope. The person pulling one end of the rope is not in contact with the box, yet we know from experience that the box will move in the direction that the rope is pulled. This occurs because the force the person exerts on the rope is transmitted to the box.
The force exerted on the box from the rope is called the
tension force, and comes into play whenever a force is transmitted across a rope or a cable. The free-body diagram below shows us a box being pulled by a rope, where
W is the weight of the box,
N is the normal force,
T is the tension force, and
is the frictional force.
In cases like the diagram above, it’s very easy to deal with the force of tension by treating the situation just as if there were somebody behind the box pushing on it. We’ll find the force of tension coming up quite a bit in the chapter on special problems in mechanics, particularly when we deal with pulleys.
