Applied Science III - Finau

What is inertia?  An object’s ability to change its motion  A measure of the amount of matter within the object

Which has more inertia? A car traveling at 20 mph or a bus standing still? Why?  The bus has more inertia  It has more mass, thus harder to change its motion

Which would have more momentum?  The car would have more momentum because it has more velocity (speed)

If a bus and a car are both traveling at 20 mph, which would have more momentum?  The bus would have more momentum because it has more mass

If two Mustangs w/the same mass move at different speeds (1 st at 20mph & 2 nd at 40 mph), which would have more momentum?  The 2 nd Mustang would have more momentum because it is moving with a faster velocity

What does momentum depend on? What are the Relationships?  Momentum depends on both mass & velocity of an object  Both are directly proportional to Momentum

Can you define momentum? Equation? Formulated by French Scientists/Philosopher René Descartes ( ):  The amount of motion of an object that determines the amount of time needed to change its motion when acted on by a force  Momentum = mass X velocity

How much momentum does a 1000 kg car have traveling at 5 m/s?  p = mv  Mass = 1000 kg  Velocity = 5 m/s  p = (1000)(5) = 5000 kg  m/s

Differences in Momentum: Sumo vs Punch

In order to change an object’s momentum, what must happen?  Change its velocity  Create an acceleration  Apply a force

What is the difference between applying a force for 2 seconds and applying a force for 4 seconds?  Applying a force for more time changes an object’s speed more  ie. - The longer you push, the faster it goes

What is impulse? Equation?  Impulse is a force applied over a time interval that changes the momentum of an object  Impulse = Force X time

How big of an impulse would pushing on a couch with 50 N of force over 2 seconds create?  I = Ft  Force = 50 N  Time = 2 s  I = (50)(2) = 100 Ns

How are Impulse and change in momentum related?  Directly Proportional – increase Impulse, increase change in momentum

What is the Impulse-momentum theorem?  The change in an object’s momentum is equal to the impulse acting on the object  I = Δp  *note – Δ means “change in”  If we break into variables  Ft = mΔv

The boxer throws a punch and his opponent has 1 of 3 choices. He can either 1) (accidentally) move into the punch, 2) stand still 3) roll with the punch. Let’s try to understand how much force is applied to his opponent’s face. When a boxer hits his opponent, an Impulse is created that will change the motion of the opponent’s head. In each situation, we assume the change in momentum to be the same: Ft = Δp

Rocky IV Final Fight

If the opponent moves into the punch, what will happen to the amount of time the boxer’s fist interacts with his opponent’s face? What happens to the force?  Time decreases because it spends less time changing his face’s momentum  The force increases as time decreases

If the opponent stands still, what will happen to the amount of time compared with if he moves into the punch? What happens to the force that hits the boxer’s face?  The boxer’s fist takes more time interacting with his face  Less Force acting on his face

If the opponent rolls with the punch, what will happen to the amount of time compared with if he stands still? What happens to the force that hits the boxer’s face?  The boxer’s fist takes the most time interacting with his face  Least amount of force acts on the opponent’s face

So assuming that the change in momentum of an object remains the same, what’s the relationship between the force acting on the object and the time it acts on the object?  Inversely Proportional  As the time of interaction increases, the force acting on the object decreases

What is the Impulse-momentum theorem?  The change in an object’s momentum is equal to the impulse acting on the object  I = Δp  If we break into variables  Ft = mΔv  *note – Δ means “change in”

How does an air bag help prevent a person from serious injury?  The bag increases the amount of time to slow your head down  Reduces the force acting on your head

Can you name some situations where we use this concept everyday?  You give some examples…  Airbags  Seatbelts  Padding (in football, gym equipment)  Water barrels at corner of highway exits

Do you remember what Newton’s Third Law of Motion is? (it’s important!)  For every action (or force), there is an equal and opposite reaction (or force)

Imagine two pool balls rolling towards each other. How does Newton’s 3 rd Law apply?  Each ball applies an equal and opposite force on each other  F 1 = F 2

What about the amount of time the two balls interact with each other?  The collisions takes place over one set amount of time; there is no change.  Time is constant

What can we say about the impulse acting on each ball?  Remember…Impulse = Force X Time  The impulse from each ball is equal and opposite

What can we say about the change in momentum of each ball?  Remember…Impulse-Momentum Theorem?  The changes in momentum are equal and opposite

What is the law of Conservation of Momentum?  The total momentum before a collision is equal to the total momentum after the collision  Momentum is ALWAYS CONSERVED!!!!

Astronaut Richard Garriott discussing Momentum

Two skaters initially at rest push against each other so that they move in opposite directions. What is the total momentum before they push off of each other?  Total momentum is zero, since there is no velocity.

What is the total momentum after they push off? Explain this even if they’re moving.  Total momentum is zero, according to conservation of momentum.  The velocities are equal and opposite.

After a gun is shot, explain what happens. Why does this occur?  The gun recoils…shoves backward  Because the bullet is shot outward at high speed, the gun is shoved backwards

What is the law of Conservation of Momentum?  The total momentum before a collision is equal to the total momentum after the collision  Momentum is ALWAYS CONSERVED!!!!

What’s wrong with this clip?

After watching the video clip, explain using Conservation of Momentum why this can’t happen.  The total momentum of the bullet should equal the total momentum of the person.  Approximate bullet speed – 400 m/s Bullet mass – about.01 kg  Thus momentum of bullet is 4 kgm/s  If man has a mass of 90 kg, what is his calculated speed?

An astronaut working in space finds himself drifting away from the shuttle. He forgets his zip cord and has no propulsion device. The only object he has is a large wrench. Using the concept of Conservation of Momentum, what could he do in order to safely get back to the shuttle?  Throw the wrench away from the shuttle  Conservation of momentum says he will be propelled in the opposite direction of the wrench

What is the law of Conservation of Momentum?  The total momentum before a collision is equal to the total momentum after the collision  Momentum is ALWAYS CONSERVED!!!!

Explain why can’t this happen?

What are the 3 types of Collisions  Elastic Collision  Inelastic Collision  Perfectly Inelastic Collision

Elastic Collision  Objects collide and separate w/ NO deformation  Both momentum & kinetic energy are conserved  Approximate Examples:  Playing pool  Marbles  Exact Examples:  Atoms colliding

Inelastic Collision  Objects collide and split apart w/ SOME deformation  Momentum is conserved  Some Kinetic Energy is lost  Examples:  Bumper cars  Boxing punch  Kicking Soccer ball

Perfectly Inelastic Collision  Objects collide and stick together  Momentum is conserved  Some Kinetic Energy is lost  Examples:  Football tackle  Bug on a windshield  Hockey puck and glove

Bill Nye on Momentum