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Conservation of Momentum: Physics Lab

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  • 0:03 What is Conservation…
  • 0:43 Physics Lab Steps
  • 2:55 Data Analysis
  • 4:16 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lab, you will be able to explain what conservation of momentum is and conduct an experiment to demonstrate conservation of momentum. You can also take the short quiz to test your knowledge.

What Is Conservation of Momentum?

Momentum is a quantity of motion equal to the product of the mass and the velocity of the object. An object with more mass has more momentum, and an object with more velocity also has more momentum.

Conservation of momentum says that momentum is neither created nor destroyed, it only moves from one place to another. So, if two cars hit each other in a crash, all that momentum goes somewhere. The reason it seems like it disappears is that momentum can be absorbed into the earth.

But we can minimize that effect for a lab by using low-friction tracks or billiard balls. Today, we're going to use one of those low-friction tracks, called an air track, to investigate conservation of momentum.

Physics Lab Steps

For this physics lab, you will need:

  • A metal track or air track
  • Two carts for the track
  • Masses that attach to the carts
  • A stopwatch with a lap feature
  • A tape measure or ruler
  • Duct tape
  • A weighing scale
  • And, a cart starter (something that pushes a cart on the track at the press of a button)

Step 1: Add some mass to one of the carts so that the two carts have different overall masses. Measure the masses of each (including the cart) on a scale and write it down.

Step 2: Set up the air track on a level surface. Position the lighter cart in the middle of the air track, and the heavier cart near one of the edges next to the cart starter. (The heavier cart will be propelled by the starter until it hits the lighter cart.)

Step 3: Attach pieces of duct tape to the track. Mark an ending point for the heavier cart a little way down the track. This point should be before the heavier carts hits the lighter cart. Also mark an ending point for the lighter cart. This ending point should be close enough to the starting point that the cart won't have lost too much speed.

Step 4: Measure the distance from the starting position of the heavier cart to the ending position you marked. Then, measure the distance from the lighter cart's starting position to its ending position. Write down those distances.

Step 5: Press the starter, causing the heavier cart to hit the lighter cart. Use the stopwatch to measure the time it takes for the heavier cart to travel to the end point you marked. Write it down and repeat five times.

Step 6: Press the starter again. This time, use the stopwatch to measure the time it takes for the lighter cart to travel the marked distance. Write it down and repeat five times.

Step 7: If the heavier cart rebounds or otherwise continues to move after impact, mark a rebound distance using the duct tape, and measure the time it takes to travel that distance, too. Write it down and repeat five times.

Average your sets of five trials to find the average time in each case. Use the speed = distance / time equation, to calculate how fast each cart was traveling in the period you timed it for.

You can collect your data in a table. If you haven't already, now it's time to pause the video and get started. Good luck!

Data Analysis

After completing the experiment, you should now have the speed of the heavier cart before the collision, the speed of the lighter cart after the collision, and in the case of rebound, the speed of the heavier cart after the collision. If there was no rebound, then that value is zero. You also have the total mass of each cart (including the attached weights).

To analyze this data, we want to check to see if conservation of momentum holds. Clearly, it won't be perfect, because nobody is perfect at using a stopwatch and measuring distances. But we can still have a go at seeing how close we can get our data.

This equation is the equation for conservation of momentum:

p = mv

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