Hey guys!
In this lesson, we’re going to learn about tension forces.
That’s the kind of force we see when we pull on a rope, for example.
Alright, so let’s get started!
Suppose we’ve got a ball hanging from the ceiling by a rope.
What is keeping the ball from falling to the floor anyways?
After all, gravity is acting on the ball right?
Well, it’s the rope that’s keeping the ball up!
So the rope must be applying a force on the ball to prevent it from falling.
We call that force the tension force, because the rope is being put under tension by the
ball.
So any force that appears as a result of pulling on a rope, a string, or anything similar to
that is called tension.
Actually, we can think of tension forces as being, in a limited sense, the opposite of
normal forces.
Now if you don’t remember what a normal force is, we encourage you to watch our videos
on normal forces.
Very briefly, we can say that a normal force occurs when a surface, like a table, resists
being compressed by a force.
The surface applies a force which prevents an object from moving into the surface, and
we call that the normal force.
Tension forces do something quite similar, except that instead of resisting compression,
they resist forces that cause things to stretch.
For example, when we hang a ball on a rope, the rope applies a force to the ball which
makes it resist being stretched out.
The result is tension.
Looking back at normal forces, it seems obvious that when an object is not pushing against
a surface, the normal force must be zero.
Tension forces are similar to this, but in an opposite sense.
If an object is not pulling on a rope, the tension force is zero.
Alright, I think we’ve got enough knowledge to look at a very simple example.
Let’s look at the ball on a rope again.
If the ball has a mass of 10 kilograms, what is the tension force?
Hmm, how would we solve a question like that?
Well, the answer would be 98.1 newtons upward.
How did we get that answer?
First of all, we have to look at the forces that are acting on the ball.
One of them is obviously gravity.
What other forces are there?
Well, there’s really only one other force: the force of tension!
More specifically, it’s the tension force being applied by the rope onto the ball.
And since the ball isn’t moving, the forces must cancel out.
So our next step will be to find the force of gravity on the ball.
Cool!
To find the force of gravity, in other words the weight, we’ll need to use Newton’s
Second Law.
We already know the mass of the ball, which is 10 kilograms.
As for the acceleration, that’s constant near the Earth’s surface which is 9.81 meters
per second squared downward.
And once we substitute those values into the equation, we’d get 98.1 newtons downward
for the weight of the ball.
And since the gravitational force must be cancelled by the tension of the rope, we find
that the tension force being applied by the rope on the ball is 98.1 newtons upward.
Awesome!
Alright, that brings us to the end of this video on the Introduction to Tension!
In our next video, we’re going to talk about idealized ropes with properties like zero
mass.
And we’re going to also see how that might simplify our calculations.
So, we hope to see you in the next video, and until then, have a good one!