Bouncing and Momentum Change

• Bouncing Results in greater forces…..Ball

p -p

Δp=FΔt

Bouncing and Momentum Change

• Bouncing Results in greater forces…..Ball

p -p

Δp=FΔt

Pelton Wheel (1888, Lester Pelton)

• Pelton wheel was a modified water wheel that increased the previous efficiency (90.2% vs previous 76.5%)

Impulse - Momentum Example

A 1.3 kg ball is coming straight at a 75 kg soccer player at 13 m/s who kicks it in the exact opposite direction at 22 m/s with an average force of 1200 N. How long are his foot and the ball in contact?

answer: We’ll use Fnet t = p. Since the ball changes direction, p = m v = m (vf - v0) = 1.3 [22 - (-13)]

= (1.3 kg) (35 m/s)

= 45.5 kg · m /s.

Thus, t = 45.5 / 1200 = 0.0379 s, which is just under 40 ms.

Angular Momentum

• Defined as the quantity of motion of objects that are rotating about a fixed axis.

• Depends upon the • Mass of the object (greater mass, greater L)• Distribution of that mass (at large radius, greater L)

• Tangential velocity (greater VT, greater L)

Angular Momentum

Moment of Inertia

I = mr2

Moment of inertia is dependent upon the shape of the object rotating and the distribution of its massMoment of inertia is like a “rotational inertia”

Moment of inertia for some objects Page 304

Moment of Inertia

Conservation of Momentum

• “For a closed, isolated system, the total momentum remains constant”

Closed Isolated SystemA system is a collection of two or more objects. An isolated system is a system which is free from the influence of a net external force which alters the momentum of the system . A closed system is one that doesn’t gain or lose mass.

Why does cannon move backward?????

Johnny Hart—died April 2007

The Sport of Curling

If curling stones have same mass….what happens when one strikes a stationary one head on?

What do rockets “push against?”

Spacecraft in outer space can accelerate by firing attached rockets. Since there is nothing in space for the escaping gases

to push against, how can the hot gases escaping from these rockets cause direction change?

Because momentum of “system” was zero before he pulls trigger, then momentum is also zero after he pulls trigger. This then means that the forward momentum of man and extinguisher must be equal, but opposite, the momentum of the gases exiting the extinguisher.

Conservation of Angular Momentum in Spin of Ice Skater

Why does this skater spin faster??Why does pulling the arms in cause the spin rate to increase?

Pulling the arms in causes her moment of inertia to decrease. If the angular momentum remains constant, then the angular velocity must increase for that to happen.

What happens to wind speeds if tornado “shrinks” in diameter?????

Collisions

• Elastic Collisions -- collide without permanently deforming (bounce)

• Inelastic Collisions -- collide, may be deformed permanently, or stick (generate heat in the process)

Types of Collisions

Elastic Collision Inelastic collision

Remember, momentum remains constant……

Conservation of Momentum