A weightlifter lifts a barbell 0.65 m with a constant force of 435 N. If it takes 280 W to move the...

A weightlifter lifts a barbell 0.65 m with a constant force of 435 N. If it takes 280 W to move the barbell, how many seconds did it take?

Warm-Up: December 9, 2011

Use the answer key handout to check and correct your homework.

DO NOT WRITE ON THE ANSWER KEY

Homework due: p. 235, 250

Questions?

EnergyChapter 11

Energy is the ability of an object to change itself, other objects, or the world around it.

Itself: Energy can be changed from one form to another

Other objects: Energy can be transferred from one object to another, often through collisions

Surroundings: Energy can be transferred (such as heat) to surroundings

What is Energy?

Kinetic Potential

◦ Gravitational◦ Elastic◦ Electrical◦ Chemical◦ Mass

Types of Energy

The SI unit of energy is the Joule (J) This is the same unit as work

Unit of Energy

2

2

s

mkgmNJ

Kinetic energy is the energy an object has due to its motion

Kinetic Energy

2

2

1mvKE

Calculate the kinetic energy of a 0.145 kg baseball traveling at 40.0 m/s

Example 1

Calculate the kinetic energy of a 1100 kg car backing up at 1.5 m/s.

You-Try #1

The work done to an object is equal to its change in kinetic energy

Work-Energy Theorem

22

2

1

2

1if mvmvFd

KEW

A hockey player hits a 0.16 kg puck with a force of 120 N for a distance of 0.92 m. If before the slapshot the puck was moving forward at 1.4 m/s, what was the puck’s velocity after the shot?

Example 2

A soccer player is running towards a 0.43 kg soccer ball that is moving towards the goal at 0.76 m/s. When she reaches the ball, she kicks it with a force of 110 N for a distance of 0.43 m. What is the ball’s velocity after the kick?

You-Try #2

A collision is called elastic if kinetic energy is conserved

A collision is called inelastic if kinetic energy is not conserved

A collision is called perfectly inelastic if the two objects have the same final velocity (stuck together)

Momentum is always conserved in a closed, isolated system.

Elastic vs. Inelastic Collisions

Potential energy is energy that is stored in an object.

Many types of potential energy:◦ Gravitational◦ Elastic/spring◦ Electrical◦ Chemical◦ Mass

Potential Energy

Gravitational potential energy is a result of the force of gravity between an object and another object (Earth).

m = object’s mass g = acceleration due to gravity (9.8 m/s2) h = height above a reference level (often

the ground)

Gravitational Potential Energy

mghPE

Calculate the potential energy of a 18 kg child at the top of a 2.5 m tall slide.

Example 3

A 75 kg boulder is resting precariously at the edge of a cliff that is 32 m above the ground below. What is the potential energy of the rock?

You-Try #3

An airplane with mass of 3.7x105 kg is flying with a velocity of 89.0 m/s at an altitude of 3.1 km. What is the potential energy of the airplane?

Example 4

A 7.8x103 kg car is at the top of a hill. It drives down the hill where the elevation is lower by 23 m. If the top of the hill is the reference level, what is the car’s potential energy at the bottom of the hill?

You-Try #4

Energy is conserved (just like momentum) Since energy can be converted between its

different forms, it is the sum of energy types that is conserved.

If no work (other than gravity) is done to a system:

Conservation of Energy

ffii KEPEKEPE

Conservation of Energy Equation

ffii KEPEKEPE

22

2

1

2

1ffii mvmghmvmgh

22

2

1

2

1ffii vghvgh

A 0.145 kg baseball is thrown vertically from a height of 1.3 m with a velocity of 5.2 m/s. Assume air resistance is negligible.

a) What is the maximum height of the baseball?b) What is the baseball’s velocity 2.4 m above

the ground?

Example 5

Stanley (one of Mr. Szwast’s nephews) slides down a frictionless slide. The top of the slide is 2.4 m above the ground. The bottom of the slide is 0.3 m above the ground. If he pushes off to give him an initial velocity of 0.5 m/s, what is Stanley’s velocity at the bottom of the slide?

You-Try #5

A cannon shoots a 5.4 kg cannonball straight up at 95 m/s. This was a very bad decision, as the cannon was inside a building at the time. How fast is the cannonball traveling when it hits the ceiling 2.1 m above the cannon?


A 1200 kg car is parked at the top of an icy hill. Its brakes fail, and it begins to slide down the hill. At the bottom of the hill, the car is traveling 7.1 m/s. What is the height of the hill?

Example 6

Top Thrill Dragster is one of the tallest, fastest roller coasters in the world. It shoots passengers out at 120 mph (53.6 m/s) up a track to a height of 420 ft (128 m). What is the velocity of the roller coaster at the top of the track? Ignore friction and air resistance.

You-Try #6

Variable Symbol SI Unit of Measurement SI SymbolTime t seconds s

d m/s

meters per second squared Force

kg p (lower

case)

W Watts

Kinetic Energy PE

Units Quiz Next Class

A raindrop falls from a rain cloud 1100 m above sea level. How fast is it moving just before it hits the ground if the elevation is 89m?

What is wrong with this problem?

Example 7

A 1250 kg rocket is launched with an initial velocity of 13 m/s. What is the maximum height that the rocket can reach?

What is wrong with this question?

You-Try #7

PE & KE Worksheet

Conservation of Energy worksheet

A 2.1 kg rock is thrown horizontally with a velocity of 7.2 m/s at a height of 1.8 m.

Calculate the ball’s kinetic energy Calculate the ball’s gravitational potential

energy


No notes, no calculator, no purple sheet When you are finished, turn it in at the front

table and pick up both worksheets◦ Momentum and Energy Review◦ Momentum and Energy Crossword

No talking until ALL quizzes are submitted. If you appear to be talking (even if you have already turned in your quiz), you will receive a zero and have to retake the quiz.

Units Quiz

Potential Energy & Kinetic Energy – Wednesday Conservation of Energy – Wednesday

◦ Typo on #3: vi = 5.10 m/s (not 2.10) Momentum and Energy Review – Friday Momentum and Energy Crossword – Friday

Worksheets – due dates

An astronaut hits a golf ball on the moon. The golf tee is 0.0032 m above the ground. The golf ball leaves the tee at 17.9 m/s. The ball hits the side of the lunar lander 4.43 m above the ground with a speed of 17.5 m/s. What is the acceleration due to gravity (g) at the surface of the moon?


Work and Power worksheet

WP

JW

WP

NF

md

WP

NF

JW

WP

JW

JW

24

29)6

34

79)5

0.3

12)4

82

990)3

23

540)2

38)1

Homework

WPb

JWa

WP

JW

NF

JW

kgm

JW

3

3

7

4

4

1000.3)59

1000.9)59

126)57

1087.8)56

1075.2)55

988)54

9.59)53

101)52

Kinetic Energy worksheet

JKE

kgm

v

JKE

JKE

sm

1410374.2)5

17.0)4

11)3

3000)2

71)1

Potential Energy worksheet

JPE

mh

kgm

JPE

JPE

8

5

104.3)5

0.2)4

62.0)3

21)2

100.7)1

Conservation of Energy worksheet

Drag. Slower.)8

3.42)7

65.9)6

103)5

60.1)4

371.0)3

54.8)2

65.4)1

sm

f

sm

f

sm

f

sm

f

sm

f

sm

f

sm

f

v

v

v

v

v

v

v

Monday 10th – Elastic and inelastic collisions Tuesday 11th – Review Wednesday 12th – Review Thursday 13th – Benchmark #2 (graphs) Tuesday 18th – Review worksheet Wednesday 19th (minimum day) – Review Thursday 20th (sub) – Test on momentum

and energy Week of 24th – Review for and take final

exam

Rest of the semester

A collision is called elastic if kinetic energy is conserved

A collision is called inelastic if kinetic energy is not conserved

A collision is called perfectly inelastic if the two objects have the same final velocity (stuck together)

Momentum is always conserved in a closed, isolated system.

Elastic vs. Inelastic Collisions

A 2.00 kg black ball is rolling east at 3.00 m/s. A 5.00 kg gold ball is rolling west at 2.00 m/s. If the collision is elastic, what is the final velocity of each ball?

Example 8 – Elastic Collision

This packet is due Thursday. Mr. Szwast will answer questions before we

take a quiz on the material.

Skill Builder #2 - Graphs

A weightlifter lifts a barbell 0.65 m with a constant force of 435 N. If it takes 280 W to move the...

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