Now that we understand how gravity and the normal force work together
to work on a slope problem like the one we just solved,
we can introduce one more force which is the tension.
If I have an object like this one hanging from a bar
or a ceiling or something like this from a string or a rope
and the gravity is pulling the object downwards
and it's not moving, there's certainly a force
trying to stop the object from moving and that's the force from the string or the rope.
This force comes because when we try to stretch a string
or a rope or something that doesn't wanna be stretched,
it will fight back and have these chemical bonds pulling back inwards.
So this tension force is existing throughout the rope as we see it here.
So this tension force is pulling upwards on the object
and will react to whatever forces are pulling downwards on the object.
So the tension force is something that we can find from a problem
based on the other forces and the fact that the object is not accelerating,
which we'll see below.
But first something important to point out about the tension force
that can be really confusing which is suppose you have a pulley
and two masses are suspended from the pulley as I've shown them here.
Then they're both causing a tension in this rope
and the tension is pulling upwards on these two masses.
One tricky thing is that people often don't realize that the tension
in both of these ropes is the same, it's the same tension force.
So you could, for example, write Newton's second law for the mass on the left,
write it again for the mass on the right,
and then you could equate the tension forces in both equations
that you've just written because the tension force is the same
everywhere throughout a given rope or string.
The second possibly confusing thing is that
if we were writing Newton's second law for the pulley,
the forces of tension will be acting downwards on the pulley
and there wouldn't just be one force of tension
even though there's one rope,
we would actually say that there's two forces of tension,
one on either side acting on the pulley.