Bell Ringer Dec 19, 2012Bell Ringer Dec 19, 20121. If you push an object twice as far while

applying the same force, you doa) Four times as much workb) The same amount of workc) Twice as much workd) ½ as much work

2. A rifle recoils from firing a bullet. The speed of the rifle’s recoil is small because thea) Momentum is mainly concentrated in the bulletb) Momentum of the rifle is smallerc) Rifle has more mass than the bullet

Impulse Day 3Impulse Day 3Wednesday Dec 19, 2012Wednesday Dec 19, 2012

Question……Question……

• According to the Conservation of According to the Conservation of Momentum Law, when is momentum Momentum Law, when is momentum notnot conserved?? conserved??

• What about the object is changing?? What about the object is changing?? – Its mass??Its mass??– Its velocity??Its velocity??

Change in MomentumChange in Momentum

• When a force is applied, an object’s momentum changes.

Δp = = m ∙ Δv

Δp = = m m ∙ (v∙ (vff – v – vii))

Beaker vs. Floor!!!Beaker vs. Floor!!!

• What happens when you drop a What happens when you drop a glass beaker on the floor?glass beaker on the floor?

• On the carpet?? On the carpet??

ImpulseImpulse

• In order to change an objects momentum, it is necessary to apply a…..– Force– Against the objects motion– For a given time

Impulse = F ∙ tImpulse

(N ∙ s)

Force (N)

Time (s)

What could happen to What could happen to cause a change in cause a change in momentum while you are momentum while you are driving??driving??• If you are out of control in a car……If you are out of control in a car……

– Would you rather hit……Would you rather hit……• a a brick wall brick wall or a or a haystackhaystack??

– If the car comes to a If the car comes to a stopstop, which has a , which has a greater change in momentum?greater change in momentum?•They have the They have the same change in same change in momentummomentum. Why?. Why?

– Because the result in both is a momentum of “0”Because the result in both is a momentum of “0”

Haystack vs. WallHaystack vs. Wall• By hitting the haystack instead of By hitting the haystack instead of

the wall, youthe wall, you– Extend the time of impactExtend the time of impact

• The time it takes you to change your The time it takes you to change your momentum to zeromomentum to zero

mV Ft

mV Ft

Class DiscussionClass Discussion1.1. Egg toss & Egg into sheet Egg toss & Egg into sheet

2.2. Explain why the goal posts have padding around their base.Explain why the goal posts have padding around their base.

– The padding will increase the time required to stop and The padding will increase the time required to stop and decrease the force. decrease the force.

3.3. What safety features in cars lengthen the time during a What safety features in cars lengthen the time during a collision in order to decrease the maximum force during collision in order to decrease the maximum force during that collision.that collision.

4.4. Examine the inside of a football helmet. Examine the inside of a football helmet.

– How is the helmet constructed so that the player’s head How is the helmet constructed so that the player’s head will have more time to stop during a collision? will have more time to stop during a collision? • There is space in the interior of the helmet for the head to move, thereby There is space in the interior of the helmet for the head to move, thereby

increasing the time. increasing the time.

Change in Momentum Change in Momentum for an objectfor an object

Change in Momentum #1Change in Momentum #1

A 1800 kg police car at rest increases A 1800 kg police car at rest increases his velocity to 45 m/s in order to catch up his velocity to 45 m/s in order to catch up to a speeding car. What is the police to a speeding car. What is the police car’s change in momentum?car’s change in momentum?

Where did the momentum get transferred?Where did the momentum get transferred?

G U E S S

Change in Momentum #2Change in Momentum #2

A 75 kg quarterback running at 11m/s east A 75 kg quarterback running at 11m/s east gets tackled to a stop. What is the gets tackled to a stop. What is the quarterback’s change in momentum?quarterback’s change in momentum?

Where did the momentum get transferred?Where did the momentum get transferred?

G U E S S

ImpulseImpulse• Impulse is a change in momentum

Impulse = Δp

F F ∙ t = ∙ t = m m ∙ (v∙ (vff – v – vii))

F ∙ t = m ∙ (vf – vi)

Impulse-Momentum Practice Impulse-Momentum Practice ProblemProblem

• A 90 kg football player is moving at 7 m/s. Calculate the force required to stop him in 0.5 seconds.

G U E S SFind the change in momentum here:

Find the force here: