# Physics lab – momentum | Physics homework help

Name ________________________________________ Date __________________________ Period _________

The Conservation of Momentum

Find the Lab

Conservation of Momentum.

 If someone else used the computer for this lab before you, click New Experiment. This will

ensure that you have your own unique cart data when you do the experiment.

Part I: Measure the Carts

 To find the length of the purple cart, use your mouse to drag the cart over the caliper in the

upper left corner of the lab. Convert the length to the SI unit of meters, then record your result

in the table below. Repeat for the green cart.

 Find the masses of the carts by dragging each one in turn over the electronic balance in the

upper right corner. The balance reads in grams, so convert each mass to the SI unit of

 These measurements will stay the same as long as you don’t refresh the screen or click the button to
start a new experiment. If you don’t complete the lab if one sitting and have to load the lab page again,
the lengths and masses will change. If this happens, you will need to measure them again and use the
new values for the remainder of the lab.

Part II: Determine the Carts’ Velocities

 Select “same direction” from the Carts’ Direction menu and “inelastic” from the Collision

 Click Start Carts to put the carts in motion. The red numbers you will soon see tell you how

many seconds it took each cart to pass through that photogate. If you lose track of which

photogate is measuring which cart, notice the purple and green arrows labelling each; a half

purple/half green arrow is used when both carts were stuck together as they passed through.

You can also click Start Carts if you want to watch the collision again.

 Record your times in the data table at the top of the next page. Also copy the lengths from

part I. Be sure to add the lengths of the two carts when the carts are stuck together.

 Calculate each cart’s velocity and enter it in the table as well.

Mass of purple cart Length of purple cart Mass of green cart Length of green cart

Part III: Calculating Momentum

 Use the fact that momentum equals mass times velocity to calculate the momentum of each

cart. Remember to add the masses when the carts are stuck together.

 Calculate the total momentum of the two carts before and after the collision.

 You should find that the total momentum before and after the collision is identical (at least to

within rounding errors.) If you don’t, you should find out what went wrong and correct it before

you complete the next part.

Part IV: The Elastic Collision

 This time, set the Carts’ Direction to opposite and the Collision Behavior to elastic. Repeat the

same steps as in part II and III. (The data table is at the top of the next page.)

 When you calculate the velocities and momenta, signs matter.

Make sure that carts that are moving to the left have negative

velocities. If you lose track of which direction the carts were

going for each photogate, you have the arrows to help you, and

you can click Start Carts to watch the collision again.

Elapsed time Length Velocity

Purple cart before collision

Green cart before collision

Carts stuck together after collision

Mass Velocity (from part II) Momentum

Purple cart before collision

Green cart before collision

Carts stuck together after collision

Purple cart’s momentum Green cart’s momentum Total momentum

Before collision

After collision ——————– ———————-

Part V: One More Case

 Repeat the experiment once more, this time with any combination of Carts’ Direction and

Collision Behavior you have not used already. Record which settings you use, then complete the

calculations as before.

Carts’ Direction ___________________________ Collision Behavior _________________________

Elapsed time Length Velocity (with sign!)

Purple cart before collision

Green cart before collision

Purple cart after collision

Purple cart before collision

Mass Velocity Momentum

Purple cart before collision

Green cart before collision

Purple cart after collision

Purple cart before collision

Purple cart’s momentum Green cart’s momentum Total momentum

Before collision

After collision

Elapsed time Length Velocity (with sign!)

Purple cart before collision

Green cart before collision

Purple cart after collision

Purple cart before collision

Part VI: Conclusions

What did you notice about the total momentum before the collision and the total momentum after

the collision in each of the above cases?

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The principle you should have noted in the previous question is called conservation of momentum.

What do you think it means to say something is conserved in the context of physics?

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Do you think there is any combination of conditions in this lab under which momentum would not

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Learning physics? Teaching physics? Check out www.gigaphysics.com.

Mass Velocity Momentum

Purple cart before collision

Green cart before collision

Purple cart after collision

Purple cart before collision

Purple cart’s momentum Green cart’s momentum Total momentum

Before collision

After collision

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