08 slide handout 1(3) momentum and collision

*Property of STI H0055

Momentum and Collision

Momentum

Collision

H0055

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The product of the mass of an object and its

velocity

p = mv

A vector quantity

SI unit of momentum is kg m/s

Can be increased by

Increasing the mass

Increasing the velocity

Increasing both the mass and velocity

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300 kg

10 m/s to the east

Find the momentum.

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2.0 kg

4.0 m/s

Find the momentum.

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Change in the momentum of an object

The product of impact force (or average

force) and the impact time

I = Ft = m∆v

SI unit is N s, where 1 N s = 1 kg m/s

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Impulse on an object results in a

corresponding change in its momentum

Same impulse can be had by reducing the

force of impact and lengthening the impact

time

Force that acts over a very short period of

time during collisions

I = t = pf – pi = mvf – mvi

F

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A 65 kg clown is crossing a tightrope as part of his

circus act when he slips and falls on the safety net

below. The safety net stretched for 2.0 seconds until

it finally stops the clown’s downward motion. If the

rope bridge is 20.4 m above the safety net, what is

the average force exerted by the safety net on the

clown?

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“In the absence of an external force, the

momentum of a system remains

unchanged.”

Spbefore = Spafter

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A 50.0 kg siege cannon at rest on a frictionless surface

was accidentally fired without anything supporting it.

If the cannon ball has the momentum of 200.0 kg m/s,

what is the recoil velocity of the siege cannon?

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Demonstrates conservation of momentum

The net momentum before and after collision does not change

Σpbefore = Σpafter

May be written as an equation, for two body system:

p1before + p2before = p1after + p2after

m1v1before + m2v2before = m1v1after + m2v2after

2 Kinds:

Elastic collision

Inelastic collision

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Happens when two colliding objects don’t

stick together after collision.

A special case of elastic collision, called

perfectly elastic collision happens when

momentum and kinetic energy are both

conserved:

Σpbefore = Σpafter

ΣKEbefore = ΣKEafter

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A cue ball travels across the table

with a speed of 1.50 m/s. It hits

an 8-ball at rest. If the cue ball

stops, with what velocity will the

8-ball move?

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A 3000 kg truck is moving to the right at 10

km/hr towards a slower moving 1000 kg car which

is also traveling to the right but at 5 km/hr. If the

truck loses half of its speed as a consequence of

the collision, then how fast in meters per second

will the car be moving after the collision?

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Happens when two colliding objects stick together after collision

The equation may be modified as:

m1v1before + m2v2before = (m1 + m2)vafter

Perfectly inelastic collision happens when both objects stop after the collision reducing all kinetic energy to zero:

m1v1before + m2v2before = 0

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A 0.010 kg pebble with a speed of 5.00 m/s due

north hits head-on a 0.250 kg clay moving at 0.20

m/s due south. Where and how fast will the clay-

pebble combination move? Is the collision perfectly

inelastic or not?

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A 15 kg ball is thrown at a velocity of 25 km/hr

to a 60 kg girl who is at rest on ice. The girl

catches the ball and subsequently slides with the

ball across the ice. Solve for the velocity of the

person and the ball after the collision.

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A 0.153 kg baseball moving at a speed of 48.0 m/s

crosses the plate and strikes the 0.250 kg catcher's

mitt, which is originally at rest. The catcher's mitt

immediately recoils backwards at the same speed

as the ball, before the catcher applies an external

force to stop its momentum. If the catcher's hand

is in a relaxed state at the time of the collision, it

can be assumed that no net external force exists

and the law of momentum conservation applies to

the baseball-catcher's mitt collision. Determine

the post-collision velocity of the mitt and ball.

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08 slide handout 1(3) momentum and collision

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