Conservation of Energy

Forms of energy

• You add energy to a system by doing work on it.

• If you do work on an object, it may:– Accelerate…adding kinetic energy– Be lifted…adding gravitational potential energy– Bend…adding elastic potential energy

Work = force x distance

The work you do is the energy it gains!

How much work is done?

A car exerts a force of 2500 N against the road as it accelerates over 63 m.

2500 N

63 m

How much work is done?

A 1200 kg car accelerates at 4.1 m/s2 over a distance of 75 m.

1200kg

75m

a=4.1m/s2

How much work is done?

A forklift raises a 250 kg crate 3.2 m above the ground.

250kg

3.2m

How much work is done?

A 75 kg sprinter reaches 11 m/s at 3.1 s in a race.75

kg vf=11 m/s at 3.1 s

How much work is done?

1) A car exerts a force of 2500 N against the road as it accelerates over 60 m.

2) A 1200 kg car accelerates at 4.1 m/s2 over a distance of 75 m.

3) A 75 kg sprinter reaches 11 m/s at 31 m in a race.

4) A forklift raises a 250 kg crate 3.2 m above the ground.

Shall we return to the braking distance problem?

When a car is braking, the tires do work against the road in the direction opposite to its motion.

The work done decreases kinetic energy of the car.

When KE=0 J, the car is at rest

Shall we return to the braking distance problem?

• The road can offer a car exactly 1/3 of its weight in friction with the tires at full braking. Suppose the car has a mass of 1100 kg (weight = 10800 N)

a) How much friction does the road provide?

b) If the car is moving at 25 m/s, how much kinetic energy does it have?

c) How far does it brake before KE0.0 J?

Shall we return to the braking distance problem?

• The road can offer a car exactly 1/3 of its weight in friction with the tires at full braking. Suppose the car has a mass of 1100 kg (weight = 10800 N)

a) How much friction does the road provide?

b) If the car is moving at 50 m/s, how much kinetic energy does it have?

c) How far does it brake before KE0.0 J?

• Triple axel• http://www.youtube.com/watch?

v=XzFGzsn6Skg• Slam dunk• http://www.youtube.com/watch?

v=fVmZnvdzAC0

55kg kid, 2.0 m fall.Make a table of GPE, KE, and velocity

55kg kid, 2.0 m fall.Make a table of GPE, KE, and velocity

• Ht. GPE KE v1) 2.0m2) 1.5m3) 1.0m4) .5m5) 0m

How much work is done?

• An 84 kg science teacher runs up Pikes Peak. The race includes a vertical climb of 2410 m.

How much work is done?

An 84 kg science teacher runs up Pikes Peak. The race includes a vertical climb of 2410 m.

A student runs…

A student runs…

…or walks if he is a SLACKER!

• …up three flights of stairs. How much work is done?

A student runs…

…or walks if he is a SLACKER!

• …up three flights of stairs. How much work is done?

• I dunno. What is YOUR mass, and how high is the lunchroom?

• Find three ways to measure the height of the lunchroom (above the first floor)

Power Lab

1) Measure the change in height from the first to fourth floors.

2) Run (or walk) up the stairs, measure and record the time required

3) Calculate: Work done and power for this trip

Momentum=mass x velocity

p=m x v

• The units of momentum are kg m/s

In a collision:

Momentum is conserved!

Momentumbefore=Momentumafter

p initial=p final

CollisionsBefore:

After:

Elastic CollisionsBefore:

After:

Elastic CollisionsBefore:

After:

Elastic CollisionsBefore:

After:

Elastic CollisionsBefore:

After:

Elastic CollisionsBefore:

After:

Elastic CollisionsBefore:

After:

Elastic CollisionsBefore:

After:

What is the momentum of….?

1) A pitched baseball?2) A sprinter?3) A car on Louisiana Ave.?4) A car on I-25?5) A semi on I-25?6) An aircraft carrier at the dock?

What is the momentum of….?

1) A pitched baseball? (.15kg, 45m/s)2) A sprinter? (60 kg, 10 m/s)3) A car on Louisiana Ave.? (1000 kg, 15 m/s)4) A car on I-25? (1000 kg, 35 m/s)5) A semi on I-25? (15000 kg, 35 m/s)6) An aircraft carrier at the dock? (200000000

kg, 0 m/s)

What is the kinetic energy of….?

1) A pitched baseball? (.15kg, 45m/s)2) A sprinter? (60 kg, 10 m/s)3) A car on Louisiana Ave.? (1000 kg, 15 m/s)4) A car on I-25? (1000 kg, 35 m/s)5) A semi on I-25? (15000 kg, 35 m/s)6) An aircraft carrier at the dock? (200000000

kg, 0 m/s)

A change in momentum:

…if momentum is mass x velocity—

…and the mass of an object can’t change, then it’s a change in velocity

A change in momentum:

…if momentum is mass x velocity—

…and the mass of an object can’t change, then it’s a change in velocity =acceleration x time

=Force/mass x time

Did you notice?

While work is force x distance, a change in momentum is force x time!

m x Dv=m x (a x t)=m x (F/m x t)=F x t

Did you notice?

While work is force x distance, a change in momentum is force x time!

m x Dv=m x (a x t)=m x (F/m x t)=F x t

This is called an impulse:Dp=mDv=Ft

Try it.• If a 10. kg object at rest is pushed with a force

of 20. N (total) for 10. s…

Try it.• If a 10. kg object at rest is pushed with a force

of 20. N (total) for 10. s…

• The impulse is F x t

Try it.• If a 10. kg object at rest is pushed with a force

of 20. N (total) for 10. s…

• The impulse is F x t = 20. N x 10. s= 200 Ns = 200 kg m/s

Try it.• If a 10. kg object at rest is pushed with a force

of 20. N (total) for 10. s…

• The impulse is F x t = 20. N x 10. s= 200 Ns = 200 kg m/s

Did you notice?The acceleration is 2.0 m/s2

The final velocity is 20 m/sThe final momentum is 200 kg m/s

Consider a 15 kg object, accelerated by a 30 N force.

• If the force is applied for 100 m…

• ?a• ?t• ?vf

• ?p• ?KE• ?W

• If the force is applied for 10 s…

• ?a• ?d• ?vf

• ?p• ?KE• ?W

Crash test videos

• http://www.youtube.com/watch?v=sh1-ti8cCiw • Introduction

• http://www.youtube.com/watch?v=FnH_gvhI9OI• Basic crash

• http://www.youtube.com/watch?v=d7iYZPp2zYY• Seat belt and airbag

• http://www.youtube.com/watch?v=fPF4fBGNK0U• Old vs new cars