# Physics 11 Circular Motion

Things move in different paths. So far we’ve looked at straight motion and pendulum motion. What if a pendulum didn’t swing back and forth but went all the way around? This is circular motion. How is circular motion different from pendulum motion?

Question: How does circular motion work?

Materials:

String

Nut

Procedure:

Cut a piece of string 1.5 m long

Measure off 1.5 m of string. My string unravels easily so I put a piece of tape over the end to hold it together.

Put a loop in one end big enough to fit on your wrist

The loop at the end of the string needs to be big enough to slide over a hand but small enough to not slip off the wrist easily.

Tie the nut to the other end of the string [I taped the knot as my string doesn’t hold a knot well.]

The nut is tied to the end of the string. Be sure to secure the knot so it will not come loose while you are swinging the nut around. I used tape.

Measure up the string 0.5 m and make a small knot

The first knot is tied 0.5 meter from the nut.

Measure up the string 1 m and make a small knot

The second knot is tied at 1 meter from the nut.

!Warning!: Getting hit by the nut can hurt. Hitting something else with the nut can get you into a lot of trouble picking up broken things off the floor.

Put the loop around your wrist

Hold the string at the first knot

Swing the nut back and forth like a pendulum but keep adding force until the nut goes all the way around

Swing the nut around in a circular path several times

Stop the string

Hold the string at the second knot

Swing the nut back and forth like a pendulum but keep adding force until the nut goes all the way around

Swing the nut around in a circular path several times

Stop the string

Observations:

How did you have to move your hand to add force to increase the swing of the nut?

The nut swings at the end of the string. The hand holding the string keeps the nut moving at a fixed distance so it travels in a circular path.

Describe any differences for the longer string

Describe how it felt as the nut moved in a circular path

Describe any differences for the longer string

Conclusions:

Why do you loop the string around your wrist?

If you put a little bit of force into making the string swing, does the nut go all the way around?

Does the nut want to continue in a circular path or does it try to leave that path? Why do you think so?

What will the nut do if you let go of the string? If you decide to test this, be sure you are outside and not swinging the nut toward anything like a window. Take the loop off your wrist, swing the nut so it is going in a circle and let go of the string as it tops the circle. You can get a little idea of what it does by leaving the loop around your wrist, swing the nut by the first know and letting it go at the top of the circle. Be aware the nut could hit you when you do it this way.

Compare the speed of using a short string and using a long string. If you decide to time the swings, have a friend use the stopwatch. It would be easier to get an accurate time if your friend times three to five revolutions instead of one.

Try drawing the vectors to show how the nut travels in a circular motion. Remember one vector will follow the string as it holds the nut in the pathway. Which way will the nut’s forward vector point? Will it be curved or straight? Does gravity have much of an effect on this motion?

What I Found Out:

The nut was easy to put on the string. If it hits something breakable like a window, this is bad news. Keeping the string attached to my wrist and taping the knot holding the nut on the string made sure it couldn’t fly off and hit anything or anyone.

My hand swung back and forth to make the nut swing. This hand movement could turn the nut into a pendulum, even one that went very high. It did not make the nut go around in a circular pathway.

I had to move my hand in a circular path to get the nut to go around. With the short string, the nut went around very easily. It was very hard to slow down enough for the nut to not go around.

The longer string took more and bigger movements of my hand to get it started going around. If I slowed down at all, the nut would make only a partial circle and fall down toward the ground.

Once the nut was going around on the long string, I could make the same small movements with my hand to keep it going as long as I kept it going fast enough to go around.

I think gravity pulls on the nut. When the nut is going fast enough, gravity can’t pull hard enough to make it fall. If the nut slow down, gravity takes over and pulls it down.

I could feel the nut pulling on my hand as it went around. There was a bigger pull with the longer string.

When I let go of the string, the nut flew out away from the circular path. I had to keep the loop around my wrist doing this so the nut hit the end of the string and fell to the floor.

The nut was traveling fairly fast around the path making timing challenging. The short string gave me 3.09 sec and 3.06 sec for five times around. The long string times to 3.62 sec and 3.56 sec. The longer string seemed to give a longer time for each revolution. I would wonder how accurate this is because I could not measure the force used to make the nut go around so this may have been very different for the long and short strings.

There are three vectors interacting in circular motion. One points in to the center of the circle holding the object in its circular pathway. One is the pull of gravity. One is the straight line motion path the nut would take if the other two forces did not exist.

Drawing the vectors depends a little on where the nut is on the circular pathway. One vector arrow must point down toward my hand. I know this because I had to hold onto the string and felt the nut trying to pull free.

One vector arrow will point down toward the ground. This is gravity. It is a smaller arrow as the nut is going around, not falling straight to the ground.

The last vector arrow goes straight out from wherever the nut is. The nut is trying to go in a straight path. The vector arrow pointing to the hand keeps it from flying off so the straight vector is bigger than the gravity arrow and smaller than the one going to the hand.