Lab

1 PHY 241 Fall 2018

PHY 241 Lab 7- Momentum is Conserved

Introduction:

Momentum is a vector quantity which is measured by taking the product of an objects mass and

velocity,

𝑝 = 𝑚�⃗�. (1)

Much like energy, the concept of momentum is useful because we have a law which guarantees that the

momentum of an appropriate system is conserved.

“The total amount of momentum in a system is a constant unless momentum is transferred

through the system boundary by an Impulse.”

Where an impulse is an external force which acts on a system over time,

𝐼 = ∫ 𝐹𝑒𝑥𝑡⃗⃗ ⃗⃗ ⃗⃗ ⃗⃗ 𝑑𝑡.

Equipment:

Two CBR 2- connected directly to a computer using USB cables

Various collision carts

Mass blocks for carts

2 m track

Bubble level

Computer with Logger Pro or Logger Lite and Excel.

Triple beam balance scale.

Procedure:

1) Design a procedure to collect the information you need to measure the momentum of two carts simultaneously. WARNING: Occasionally, the clicks from your two different CBRs will

interfere with each other and give incorrect data. Your group should develop criteria to

determine when data is invalid and a response.

2) Generate a plot of the momentum of each cart as well as the total momentum similar to “Carts’ Momenta.” Notice you must correct for the fact that the two different CBRs are

using different coordinate systems.

2 PHY 241 Fall 2018

3) Similarly, generate a plot of the kinetic energy of each cart as well as the total kinetic energy.

4) This should allow you to make a single plot containing both the Kinetic Energy and the Momenta for the same collision. Notice you will need to let Excel know that Energy needs

to be plotted on a “Secondary Axis” because these two quantities have different units.

1 1.2 1.4 1.6 1.8 2

E n

e rg

y (

J)

M o

m e

n tu

m (

k g

m /s

)

Time (s)

Energy and Momentum

Total Momentum Total Kinetic Energy

1 1.2 1.4 1.6 1.8 2

M o

m e

n tu

m (

k g

m /s

)

Time (s)

Carts' Momenta

Cart 1 Cart 2 Total Momentum

1 1.2 1.4 1.6 1.8 2

E n

e rg

y (

J)

Time (s)

Carts' Energies

Cart 1 Cart 2 Total Kinetic Energy

3 PHY 241 Fall 2018

5) At this point there are a few questions that that arise from the Energy and Momentum graph above. To

A) DA- Is the behavior of the Energy and Momentum graph unique to the specific details of

the collision. Collect energy and momentum data for at least four different collisions

(magnet/spring/Velcro, different mass carts, etc.) and find a way to visualize all this data

so you can qualitatively compare and contrast features you see in the data.

B) Researcher- Choose a single trial to investigate momentum carefully. Is momentum

conserved? Measure the Impulse generated by force(s) on your system and see if you

can account for any changes in momentum you observed. Be as quantitative as

possible.

C) PI- Choose a single trial to investigate energy carefully. Energy appears to NOT be

conserved in the “Energy and Momentum” graph in the manual. Where did the lost

energy go for your trial? Why/how did some energy come back into the system? Be as

quantitative as possible.

6) Before leaving the classroom, make sure you email the data out to the entire group and clean up your work station, returning small equipment to the appropriate storage.