AFTER?

6 m/s

1 kg 1 kg

BEFORE

AFTER ?

6 m/s

1 kg 3 kg

BEFORE

stopped

PhysicsUnit 5 Week 3

DiscussionConservation of Momentum

Before You Begin!Directions: State the role each member of your group will take.

Manager (Keeps the group on-time/on-task):Explainer (Clarifies and restates any comments):Record-Keeper (Write down + turns in work):

Part I: Using Your IntuitionAbove, your intuitions about oomph led to a formula for momentum. Now let’s see if your intuitions about collisions lead to similar progress.

1. A 1 kg cart, rolling at 6 m/s, collides with and stick to an identical cart. After colliding, both carts roll together as a single unit.

a. Using your intuition, predict guess the post-collision speed of the two carts. Briefly explain your reasoning.

b. According to the intuitive guess you just made, predict if the overall momentum of the two-cart system after the collision greater than, less than, or equal to the overall momentum before the collision? Calculate this out using the momentum formula you figured out above and plugging in the relevant numbers.

2. In a similar experiment, the 1 kg cart collides with a 3 kg cart but doesn’t stick to it.

Instead, the 3 kg cart gets knocked forward by the 1 kg cart which comes to rest after the collision.

a. Again, using intuition, predict the post-collision speed of the 3 kg cart. Briefly explain your reasoning.

b. According to the guess you just made, is the overall momentum of the two-cart system greater than, less than, or equal to the overall momentum before the collision.Calculate this out using the momentum formula you figured out above and plugging in the relevant numbers.

Part II: More Applications

3. SUV vs. CarOn a test track, an SUV of mass 2000 kg, traveling at 12 m/s, hits a stationary car of mass 1000 kg. The car crumples and stays in contact with the SUV; they move together as a unit.

a. Answer this without doing calculations: Predict which vehicle, if either, undergoes a greater change in velocity during the collision? How do you know?

b. Without using formulas, see if you can intuitively guess the post-collision speed of the two vehicles. Briefly explain your common-sense reasoning.

c. Calculate the post-collision speed using momentum.

4. Bowling Ball vs. Bowling PinA bowling ball rolls into a stationary bowling pin, which is much lighter than

the ball.

a. During the collision, predict if the force exerted by the ball on the pin greater than, less than, or equal to the force exerted by the pin on the ball? Briefly explain.

b. During the collision, predict if the bowling ball’s change in speed greater than, less than, or equal to the pin’s change in speed? Briefly explain.

c. During the collision, predict if the bowling ball’s change in momentum greater than, less than, or equal to the pin’s change in momentum? Briefly explain.

d. A student makes the following argument about part C:

In the collision, the pin’s momentum changes more than the ball’s momentum, because the ball’s motion hardly changes, while the pin’s motion changes a lot; it bounces off the ball really fast.

How could you help this student reconcile his intuitive ideas about the “changes in motion” with the fact that the ball’s momentum and the pin’s momentum change by the same amount?