# Physics 18 Momentum

We met Newton’s First Law of Motion some time ago when we ran the little cars down a ramp with pennies on their roofs. The car stopped. The penny didn’t. Inertia was the reason but the penny also had momentum. Where did that momentum come from?

Recently we dropped a ball while walking. Even though we didn’t try to push the ball, it still moved forward as it fell. The ball had momentum. Where did it come from?

When a moving object hits another motionless object, some of the momentum must transfer to the other object. Think about what happens when a bowling ball hits the pins. Think about catching a ball and how your hand feels especially the difference between a fast ball and a slow ball. Question: How do mass and velocity affect momentum?

Materials:

Ramps [1 with marked meter]

2 identical balls [Ball 1 and Ball S]]

1 smaller, lighter ball [Ball L]

1 heavier ball [Ball H]

Stopwatch

Meter stick

Scale

Procedure:

Mass the balls Ball 1 has a mass of 18 g. It’s momentum will change with its velocity.

Set up the ramps so one ends at the edge of a table [like in Project 17]

Mark 2 starting lines so one is twice the height of the other The sloped ramp accelerates the ball. The flat ramp sets up the collision.

Use the stopwatch to find the velocity of Ball 1 released from each of the starting lines

Remember to do several times for each Timing is important for determining velocity of Ball 1.

Set Ball L at the end of the ramp

Release Ball 1 at the lower starting line

Mark where Ball L lands [You may want to use a long box with towels in it to catch the balls.]

Observe what Ball 1 does after the collision

Measure the distance

Repeat this several times

Put Ball L back at the end of the ramp

Release Ball 1 at the higher starting line

Mark where Ball L lands

Observe what Ball 1 does after the collision

Measure the distance

Repeat this several times

Repeat this using Ball S and then Ball H

Observations:

Mass of balls

Ball 1

Ball L

Ball S

Ball H

Velocity (m/s):

Low starting line

High starting line

Distances:

Ball L

Low starting line:

High starting line

Ball S

Low starting line

High starting line

Ball H

Low starting line

High starting line

What Ball 1 does after the collision

Ball L

Low starting line:

High starting line

Ball S

Low starting line

High starting line

Ball H

Low starting line

High starting line

Analysis:

Calculate the average time for the low and the high starting line to get the velocities

Momentum is the mass (in grams) of an object times its velocity (m/s). Any moving object has momentum because it is moving. Ball 1 accelerates down the ramp. Increased velocity turns into momentum transferred to the ball at the end of the ramp when the two collide.

Calculate the momentum of Ball 1 for each starting line.

Make a graph of the distances for the balls [ball vs. distance]. Do the three low starting line distances in a group and the three high starting line distances in a group.

Conclusions:

Which starting line provides the most velocity to Ball 1? Why?

Which starting line provides the most momentum to Ball 1?

Which starting line transfers the most momentum to the other ball? Why do you think so?

Which ball, L, S or H, had the most momentum? Why do you think so?

How does mass affect momentum?

How does velocity affect momentum?

If you are riding in a car going 60 MPH [2.5 m/s], what is your momentum? [1 pound is about 2.2 kg]

If you divide the momentum by 2.2, you get your relative mass if the car suddenly stops as in a crash and you continue on like the pennies did. Why are seat belts useful in a car crash?

What I Found Out:

I set up my ramps a little longer than for Project 17 so I would have a little more time to do the timing for the velocity. This helped a lot since I work alone.

My ball masses were 18 g for Balls 1 and S, 2.08 g for Ball L and 53.78 g for Ball H.

From my low starting line my velocity was 1 m/.68 s. This gave me a momentum of 26.4 g-m/s. The high starting line had an average velocity of 1 m/.47s giving a momentum of 36.1 g-m/s.

Gravity pulls the ball down the ramp. The more time and height. The more gravity pulls making the ball go faster. Since the mass remains the same from either line, the velocity affects the momentum. The greater the velocity, the greater the momentum. The lightest ball was one ninth the mass of Ball 1. When hit by Ball 1, it almost flew out of the ramp.

When Ball 1 hit Ball L from the low line, Ball 1 bounced out of the ramp and Ball L went an average of 68.5 cm. When Ball 1 started at the high line, both balls went off the ramp but Ball L went an average of 106 cm and Ball 1 mostly fell straight down.

When Ball 1 hit Ball S from the low line, it bounced back inside the ramp. Ball S went an average of 17.8 cm. From the high line Ball 1 bounced back less and Ball S went an average of 29.9 cm.

When Ball 1 hit Ball H from either line, it stopped. Ball H went an average of 10 cm from the low line and 22.4 cm from the high line.

The higher starting line gave Ball 1 more momentum so it could give more to the other balls. This showed as the average distances for the high line were greater for all the balls than the average distances for the low line.

Ball L went the farthest both times so it got the most momentum from Ball 1. I think this was because the ball had the least mass so more force was used to create velocity than to make the ball move. The greater the velocity, the greater the momentum.

Once a ball is moving, if all the balls moved at the same velocity, the heaviest ball would have the most momentum.

If I were in a car moving 60 MPH, I would have a momentum of 605 kg-km/s. this would give me a relative mass of 275 pounds. A seat belt keeps me from hitting the front of the car that hard.