3/4 PotU: Last class build day for mousetrap cars.

Warm up

A 10 kg box is sliding on friction-less ice at 6 m/s. It hits and sticks to a stationary 10 kg box. What is the speed of the two blocks as they move away stuck together?

Answer

Conceptually

Since the combined mass is twice the original mass, the speed must be 1/2 of the original speed, so the two blocks are going 1/2 of 6, or 3 m/s.

Mathematically

• Momentum initial must equal the final momentum
• Initial momentum is 10 kg * 6 m/s = 60 kgm/s
• Final momentum must be 60 kgm/s
• p = m*v so…
  • 60 kgm/s = 20 kg * speed final
  • (60 kgm/s) / (20 kg) = 3 m/s.

Today

• Last in-class build day.
  • If you do not finish today, you must come in at lunch or 7th period to finish your car.
  • Cars are due by Friday, when we will test them out.
• Don’t forget the photo assignment due next Wednesday.