Glencoe Physics Chapter 9 Momentum and Its Conservation.

Glencoe Physics Chapter 9

Momentum and Its Conservation

Air Bags

• How exactly does an airbag prevent serious injury in an auto accident?

Baseball

• Why are baseball players encouraged to “follow through” when hitting the ball with a bat?

Why does a cannon move backward when firing?

Highway safety

• What can we do to make highways safer for people?

Common usage of the term “Momentum”

• A team that has momentum is on the move and is going to take some effort to stop. – “Going into the final weeks, the St. Louis Cardinals

have the momentum." – The headlines declare “Pittsburgh Steelers Gaining

Momentum."

• A team that has a lot of momentum is really on the move and is going to be hard to stop.

• Momentum is a physics term; it refers to the quantity of motion that an object has.

• Refers to how hard something is to stop.

• 1330 Jean Buridan “the true measure of an object’s motion”

• 1600’s Rene Descartes “quantity of motion

• Galileo “Momento”• Newton “Momentum”

Momentum (from latin, meaning movement)

Which is probably harder to stop????? Why?

Momentum Defined

p = m vp = momentum

m = mass

v = velocity

Yep…….that’s p for Momentum!!!

Unit must be kg m/s

• So momentum of an object depends on the amount of stuff moving (mass), and the velocity at which the stuff is moving.

• Question? Under what conditions of motion would a semi-truck and a roller skate have the same momentum?

Road Train in the Australian Outback

Equivalent Momenta

Bus: m = 9000 kg; v = 16 m /s p = 1.44 ·105 kg · m /s

Train: m = 3.6 ·104 kg; v = 4 m /s p = 1.44 ·105 kg · m /s

Car: m = 1800 kg; v = 80 m /s p = 1.44 ·105 kg · m /s

Momentum Facts

• Momentum is a vector quantity

• Velocity and momentum vectors point in the same direction

• SI unit is the kg·m/s ( no special name)

• Momentum is a conserved quantity

• Momentum is directly proportional to mass and velocity of object

Momentum Facts

• A net force is needed to change a particular object’s momentum (a change in velocity)

• Something big and slow can have the same momentum of something small and fast

Momentum Examples

10 kg 3 m /s

10 kg30 kg · m /s

p = 45 kg · m /s at 26º N of E

5 g

9 km /s26º

Now, find the vertical and horizontal components of the momentum>>>>>

Sample problems

• Find the magnitude of the momentum of a car of mass 725 kg moving at a velocity of 100. km/h.

Sample problems

• Calculate the velocity of a car that has a mass of 2175 kg and the same momentum as the previous car.

Impulse• In order to change the momentum of an

object, a force must be applied– Gives an alternative statement of

Newton’s second law…..– Watch as a little mathematical magic

is performed!Magic???

When an impulse (force applied over a period of time) is applied, a change in momentum will occur

“Big Bertha” 400-ton barrel

Shells weighed 7 tons!!!