# Physics Lect 4

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1. Momentum
• p = mv
• in an isolated system momentum is always conserved
• a vector
2. Collisions are either:
• 1) Elastic - mechanical energy is conserved, all E is transferred between objects
• 2) Inelastic - momentum is conserved, some E is transferred to internal E
3. Completely Inelastic Collisions
Stick together and pi = pf
4. Solving Inelastic Collisions
draw 2 systems: 1 right before the collision and one right after
5. Reverse Collisions
• Start from rest and then burst apart
• pi = pf = 0
• in order to conserve momentum they must sep. in exactly opp. directions
• ineslastic
6. What is nec. in order for an object to "bounce back" after it has struck another object?
It must have less mass than the object it has hit
7. Impulse
• Change in Momentum
• J = Δp = Δmv or J = Fave Δt
• this states that if the time over which a force acts is increased, the same Δv can be achieved (how airbags reduce force on a passenger)
8. Machines
• Reduce the amount of F needed to do work but does not change the amount of W
• Ramp, Lever, Pulley: all F proportional to 1 / d