# Physics 21 Balancing Forces

Many Projects ago we defined a force as a push or a pull. We found forces could add to or subtract from each other. This Project we will try balancing forces so an object does not move when pulled by three forces at the same time in different directions. Question: How do forces balance?

Materials:

Metal or other rigid ring

3 Spring scales [You can use three identical rubber bands but will not be able to measure the forces instead measure the rubber bands]

Protractor

Procedure:

This Project works well with friends to help. If you are working alone like I do, you will need tape to fasten the scales in place. A simple spring scale has a pull strip to zero the scale and two scales. One is in grams for obtaining mass. The other is in Newtons, a unit of force.

Secure one scale to the table

Attach the secured scale and another scale to the ring

Pull on the second scale until it reads the same force as the first scale and  stops moving around the ring balancing forces

Record the measurements on the two scales

Note: I have three spring scales that measure forces in three levels of magnitude. If this is how your scales are, check your measurements carefully as each scale will look different.

Secure the second scale to the table

Attach the third scale to the ring

Pull on the third scale until it has the same force reading as the other two scales again balancing forces

Pull the third scale a little more and record the forces shown on the three scales

Secure the scale to the table

Use the protractor to measure the angles between the scales

Observations:

Record the force you use on your scales (or length of rubber bands):

Draw out where the two scales are on the ring

Draw out where the three scales are on the ring

Measurement of angles between the three scales

Record what happens to the three scales when you pull harder on one

Conclusions:

Do you think you could move one of the two scales pulling on the ring so they were not opposite and still balance the forces? Why do you think this? Even if the second scale is next to the first one when hooked to the ring, it will shift to opposite the scale balancing forces with equal and opposite vectors.

If you used vectors to show the forces of the two scales, would they be the same length? Would they point the same direction?

What happened when you started pulling with the third scale?

What happened when you pulled harder on one scale?

What do you think would happen if you pulled on the ring with a fourth scale? Try it and find out.

What I Found Out:

All three of my scales used a gram and a Newton scale. All three would register .5 Newtons so I decided to use this amount of force.

I attached one scale to the table, put the ring on the hook and put the hook of another scale on the ring. As soon as I started pulling with the second scale, the hook slid until the two scales were exactly opposite of each other. The scales had to be opposite each other for the forces to balance each other so I couldn’t move one scale.

If I drew vectors to show the forces, the arrows would be the same length because both scales pulled with the same force. The arrows would point in opposite directions. Adding a third scale causes the ring to shift. Having the same force with each scale balances the forces. Putting a ruler from the end of one scale to the next will create an equilateral triangle.

The second scale was attached to the table. I put the third scale’s hook on the ring and started to pull it. The hook slid around the ring until the three scales were the same distance apart around the ring.

Whatever force I pulled with on the third scale, the other two scales showed the same force.