GS104Week7,Thursday

Nov9th

Law of Conservation of Energy:

Energy cannot be created or destroyed;

it may be transformed from one form to

another, but the total energy never

changes

Einitial = Efinal

A ball is thrown up at 20 m/s. Use

conservation of energy to find how high it

goes.

A ball is thrown up at 20 m/s. Use

conservation of energy to find how high it

goes.

Einitial = Kinitial = (1/2)mv2

Efinal = Ug,final = mgh

h = (v2)/(2g) = 20.4 m

What is inertia?

What is inertia?

The tendency of an object to resist changes in motion

What is momentum?

What is momentum?

“Inertia in motion.” “Mass in motion.”

Quantity of motion that an object has.

What is momentum?

“Inertia in motion.” “Mass in motion.”

Quantity of motion that an object has.

Momentum = mass x velocity, p = mv

Is momentum a scalar or vector?

What are the units of momentum?

What is momentum?

“Inertia in motion.” “Mass in motion.”

Quantity of motion that an object has.

Momentum = mass x velocity, p = mv

Is momentum a scalar or vector? (Vector)

What are the units of momentum?

What is momentum?

“Inertia in motion.” “Mass in motion.”

Quantity of motion that an object has.

Momentum = mass x velocity, p = mv

Is momentum a scalar or vector? (Vector)

What are the units of momentum? (kg*m/s)

What has more momentum, a 70 kg

Olympic skier travelling at 30 m/s or

a 1000 kg car driving at 2 m/s?

What has more momentum, a 70 kg

Olympic skier travelling at 30 m/s or

a 1000 kg car driving at 2 m/s?

Olympic skierOlympic skier: p = (70kg)(30m/s) = 2100 kg*m/s

Car: p = (1000kg)(2m/s) = 2000 kg*m/s

Momentum of an object can change

in two ways. What are they?

Momentum of an object can change

in two ways. What are they?

→ Change an object’s mass

→ Change an object’s velocity

How do you change the momentum

of a car?

How do you change the momentum

of a car?

Since we can’t change the car’s mass,

we change the velocity. We need to

apply a change in velocity (acceleration

→ force). Either apply brakes or gas.

How do you change the momentum

of a rocket?

Describe what happens during a rocket

launch.

Rocket Launch

Consider a run away car (no brakes) and

wanting to stop it. You have the choice to

crash the car into a bail of hay or a brick

wall to stop it. Which would you choose?

Why?

What is the change in momentum?

Consider a run away car (no brakes) and

wanting to stop it. You have the choice to

crash the car into a bail of hay or a brick

wall to stop it. Which would you choose?

Why?

What is the change in momentum? (Same)

Consider a run away car (no brakes) and

wanting to stop it. You have the choice to

crash the car into a bail of hay or a brick

wall to stop it. Which would you choose?

Why?

If momentum isn’t different, what is?

Both the hay and wall apply a force, but

the hay would apply a small force for a

longer time. The brick wall would apply a

large force for a shorter time.

The force applied for a period of time

is called impulse.

The force applied for a period of time is

called impulse.

Impulse = force x time, J = Ft

Impulse is also known as the change of

momentum.

Δp = Ft

Impulse-Momentum Principle

The impulse acting on an object produces a change in

momentum of the

object that is equal

in both magnitude

and direction to the

impulse.

GS 104, Homework 1 Due Sept 28th

1 Physics Subtopic

K =

1

2

mv

2

(1)

K

f

� K

i

= W (2)

�K = Fd (3)

�p = Ft (4)

U

g

= mgh (5)

⌃

~

F

net

= m◆◆◆70

~a = 0 (6)

⌃

~

F

net

= 0 (7)

1

A runaway semi-truck (30,000 kg)

traveling at 25 m/s (56 mph) crashes into

a wall and is completely stopped 1.5

seconds after the collision began. What

force did the wall apply to the truck?

A runaway semi-truck (30,000 kg)

traveling at 25 m/s (56 mph) crashes into

a wall and is completely stopped 1.5

seconds after the collision began. What

force did the wall apply to the truck?

Δp = (30,000kg)(25m/s) = 750,000 kg*m/s

F = (750,000 kg*m/s)/(1.5s) = 500,000 N

Two people in rolling chairs are next to

each other. Person 1 pushes on person 2.

If person 2 weighs twice that of person 1,

what do you expect to happen?

Conservation of Momentum:

In an isolated system (no external forces

or friction), the momentum before a

collision and the momentum after a

collision are the same.

pinitial = pfinal

Law of Conservation of Energy:

Energy cannot be created or destroyed;

it may be transformed from one form to

another, but the total energy never

changes

Einitial = Efinal

Your instructor (65 kg) is standing at rest

on frictionless ice while holding the 1.5 kg

physics textbook. If she throws it at 20 m/s

to the east, what is her final velocity?

Your instructor (65 kg) is standing at rest

on frictionless ice while holding the 1.5 kg

physics textbook. If she throws it at 20 m/s

to the east, what is her final velocity?

pinitial = (mHeather + mbook)(vHeather + book)

pfinal = (mHeather)(vHeather) + (mbook)(vbook)

vHeather = 0.46 m/s west

A 100 kg hockey player skating at 15 m/s

collides with another player at rest with a

mass of 90 kg. If they stick together after

the hit, what is their final velocity?

A 100 kg hockey player skating at 15 m/s

north collides with another player at rest

with a mass of 90 kg. If they stick together

after the hit, what is their final velocity?

pinitial = (m1)(v1)+ (m2)(v2)

pfinal = (m1+m2)(v1+2)

v1+2 = 7.89 m/s north

Types of Collisions:

Elastic:

Inelastic:

Perfectly Inelastic:

Types of Collisions:

Elastic: Bounce off one another

Inelastic: Stick to one another

Perfectly Inelastic: No energy lost in collision

Shifting Gears…

Circular MotionWhen you drive around a round-about,

what direction is the force of friction

between your car and the road? Draw a

picture.

Circular MotionWhen you swing a ball attached to a

string in a circle, what direction is the

force of tension of the ball? Draw a

picture.

As astronauts orbit the Earth, what direction is the force that allows them to orbit?

What is this force?

Draw a picture to depict the forces.

As astronauts orbit the Earth, what direction is the force that allows them to orbit?

What is this force? (Fg)

Draw a picture to depict the forces.

Uniform circular motion is circular motion with a constant speed.

For any object in uniform circular motion, what direction is the net force?

Uniform circular motion is circular motion with a constant speed.

For any object in uniform circular motion, what direction is the net force? (Toward the center)

Centripetal ForceAny object in uniform circular motion experiences an inward force (towards the center of the circle of motion). This is known as the centripetal force or “center seeking force.”Beware that the centripetal force is named a force, but that it is truly describing the behavior of motion – going in a circle. The force that is “center seeking” is usually tension, force due to gravity or friction.

Conservation of Angular Momentum:

If the net torque (rotational force) acting

on a system is zero, the total angular

momentum of the system is conserved.

Linitial = Lfinal