Wednesday, October 3, 2001

• Due dates for chapter 6 assignments have been changed (see below)

Newton's Second Law (as Newton defined it):  F = dp/dt

Momentum (linear) is defined as:  p = mv

If no external forces act on a system, momentum is conserved (constant): pi = pf
A 2-dimensional collision can be analyzed along x and y axes.

Impulse = F*t = change in momentum

In an inelastic collision, momentum is conserved, but kinetic energy is not.
In an elastic collision, both momentum and kinetic energy are conserved.
Most collisions are partially elastic/inelastic.

Demos:

• Skateboard momentum
• Egg on your face?

Solution:  You need to know the distance your body moves as you bend your knees:  x ~ 0.5 m
From conservation of energy, your speed just before touching the ground is:  v = sqrt(2gh).
From here, you can take two different approaches to solve this problem:

Conservation of energy:  work done by ground = change in kinetic energy
    F*x = 0.5mv²
    F = mv²/(2x)

Conservation of momentum:  impulse = change in momentum
    F*t = mv
    x = 0.5at²  and v² = 2ax, so t = 2x/v
    so  F = mv²/(2x)   - same as above for conserv. of energy

•     What did you learn today?
•     What remaining questions do you have?
•     Who would you like to work with on the Web Project?
•     Do you want WebAssign to count for 15% of your course grade?

• RWP6 - Stranded Astronaut (due Friday)
• HW6b (due Monday, October 8)
• Begin thinking about topic ideas for your group's physics Web Project