Bell Work: Types of MachinesAnswer with: lever, inclined

plane, pulley, or wheel and axel1. A pencil sharpener is a _____ and _____.2. A crow bar is a ____ and _____.3. A can opener is a _____, _____ and _____.4. An axe is a _____ and _____.5. A block and tackle is a ______.

Impulse and Momentum

Momentum

Inertia in motion Combination of mass and velocity Symbol for momentum is p v Momentum units are m/s

Big momentum created by big mass, big velocity, or both.

Examples: cruise ship moving 2 m/s pebble in orbit around the earth cruise ship in orbit around the earth

NO VELOCITY = NO MOMENTUM

Impulse change in momentum because of a

change in speed (acceleration occurs)

it takes force to change the speed (2nd law)

to change momentum, exert an impulse Symbol for impulse is Δp

Impulse units are Momentum and impulse can be

combined: t

Δv = change in velocity To increase momentum: exert more

force or exert force for a longer period of time

Ex: follow through in sports – tennis, hockey, etc.

To decrease momentum: force must be in opposite direction of motion

Ex: friction, air resistance, tree slowing down a car

Changes in momentum caused by cushy objects have less force and more time than those caused by hard items.

Double time, cut force in half (inversely related)

Ex: Jump off a 3 meter platform onto concrete – time to slow down your motion is very small once you contact the cement so the force is great.

Jump off a 3 meter platform onto a thick mat – time to slow down your motion is extended because the matt gives so the force is lessened.

Ex: Car crumple zones increase the time of impact so that less force is exerted causing fewer injuries to passengers.

More Examples of increasing time to decrease force: bending legs when landing moving away from a punch stretchy cord when bungee jumping

Practice Problem:What is momentum of a 1500 kg car moving at 30 m/s?

What is momentum of the car at rest?0 kg·m/sIf the car (going 30 m/s) slams on its brakes and stops in 10 seconds, how much force was exerted by the brakes?

Given Unknown Equation Substitution Solution  1500kg = m 30 m/s = v p=? 

p=mv p=1500x30 45,000 kg·m/s

Given Unknown Equation Substitution Solution  10 s = t 45,000 = Δp F=? 

Δp=Ft 45,000=F(10) 4,500 N

An air bag increases the time of impact to 0.9 seconds in a car crash. If the above car crashed when moving 30 m/s, how much force acts on the 75 kg driver with the air bag?

Given Unknown Equation Substitution Solution

  0.9 s = t 75 kg = m 30 m/s = v F=? 

mΔv=Ft 75(30)=F(0.9) 2,500 N

Conservation of Momentum For a closed system (no external forces),

total momentum remains the same. Momentum cannot be created or

destroyed, but can be transferred from one object to another.

Firing a Cannon

Ex: Cannon with cannon ball ready to fire has

0 kg·m/s of momentum. Cannon firing cannon exerts force on

cannon ball, cannon ball exerts same force back on the cannon.

These forces occur over the same time period.

Cannon ball and cannon have equal and opposite impulses.

Because the cannon ball has less mass, its speed changes more than the cannons.

The acceleration of the cannon is called recoil – it moves backwards because the cannon ball is shot out forward.

Collisions Total momentum before a collision equals the total

momentum after a collision. Momentum transferred from one object to another.

= +

Elastic – the objects hit and bounce off one another The velocities change for each object The mass will stay the same for each object Momentum and kinetic energy are conserved.

Two Kinds of Collisions

Inelastic – the object collide and stick together The mass becomes a combination of the two objects There is only one velocity for the two objects stuck

together The kinetic energy isn’t conserved. Momentum is conserved.

Ex: Trains: 1st boxcar has a mass of 65000 kg. The 2nd boxcar has a mass of 92000kg After the collision, the boxcars move forward together.

Before After

=1.01m/s

10 m/s1.3m/s 0.8m/s ? m/s