Momentum - how to get off a frozen lake

You're on ice with only your backpack...

You're in the middle of a frozen lake...no idea how you got there.

The ice is perfectly frictionless, and all you have with you is your backpack.

Multiple Choice

How can you get off the ice?

• Gently push off the ice.

• Wave my arms frantically.

• Lean forward or backward to start moving.

• Throw my backpack.

Multiple Choice

If you throw your backpack to get off of the ice, which statement is true for the system that includes both you and the backpack?

• The momentum of the system before the throw EQUALS the momentum of the system after the throw.

• The momentum of the system before the throw IS LESS THAN the momentum of the system after the throw.

• The momentum of the system before the throw IS GREATER THAN the momentum of the system after the throw.

Before you throw your backpack, both you and your backpack are at rest, so the momentum of the system -- you and your backpack -- is zero.

​remember, momentum is:

p = mv

Multiple Choice

As you throw your backpack, which of the following statements is true?

• The force you exert on your backpack is GREATER THAN the force the backpack exerts on you.

• The force you exert on your backpack is EQUAL TO the force the backpack exerts on you.

• The force you exert on your backpack is LESS THAN the force the backpack exerts on you.

Multiple Choice

As you throw your backpack, the force you exert on the backpack is EQUAL to the force the backpack exerts on you. Which law?

• Newton's 1st Law

• Newton's 2nd Law

• Newton's 3rd Law

Multiple Choice

Which statement is true about the momentum of the you-backpack system AFTER you have thrown the backpack?

• The momentum of the system is GREATER than zero.

• The momentum of the system is EQUAL to zero.

• The momentum of the system is LESS than zero.

Throwing a backpack...

The momentum of the system is still zero because there were no external forces.

After the throw

You and the backpack are at rest, so you the momentum of the system is zero.

Before the throw

Multiple Choice

Let's say your mass is 50 kg and the mass of the backpack is 5 kg. You throw the backpack at 1 m/s. What is your velocity after the throw?

• 1 m/s

• -1 m/s

• 0.1 m/s

• -0.1 m/s

Poll

Which statement best matches how you feel about this?

• I totally get this. I could tutor someone.

• I feel pretty confident, but not well enough to explain it.

• I sort of get it.

• I'm lost.