Chapter 3Energy

The Goal of this activity is to

• Introduce the student to the terms work, kinetic energy and gravitational potential energy,

• Illustrate the concepts with simple example problems, and

• Test the students understanding with pertinent problems.

Your responsibilities…

• Read Chapter 3, Section 3.1 and 3.2, pages 62 to 69. It would be good to do it right now, before proceeding – its only 7 pages – with pictures – read it now.

• View this presentation (adapted from Professor Martino’s presentations available on my faculty web page).

• On a single sheet of paper describe, and define each of the following energy terms illustrating the concept with one numerical example: Work, Kinetic Energy, (Gravitational) Potential Energy.

• Submit written solutions to the 7 “Practice Problems” in this presentation following the standard HW problem format.

Work

• An applied force acting through a distance parallel to the force

• Units of work (and energy) = joule (J)

• Zero // distance, no work• Displacement

perpendicular to applied force, no work

• An applied force acting through a distance parallel to the force

• Units of work (and energy) = joule (J)

• Zero // distance, no work• Displacement

perpendicular to applied force, no work

Fig 3.3 Work Against Gravity

Practice Problem #1

• A weightlifter bench presses 80kg (approx. 175lbs.) 0.75m straight up.

• a. How much work does she do, assuming constant velocity, in one lift (just the 0.75m straight up)?

• b. How many repetitions would she have to do to burn off 1 hamburger (400 Calories). Note 1 Calorie = 4.186 J.

Practice Problem #2

• A person pushes a stalled 2000 kg car from rest to a final speed of 2.0 m/s. During this time the car moves 20 m. Neglecting friction between the car and the road, find:

• a. the final acceleration of the car • b. the horizontal force exerted on the car

(Hint: Use Newton’s 2nd Law to find the force using the acceleration.)

• C. the work done on the car

fv

Practice Problem #3

A person pushes a 10 kg box at a constant velocity over a distance of 4 m. The frictional force between the box and the floor is 30% the weight of the box. How much work does the person do in pushing the box?

Power • The rate at which work

is done• Units: watts (W) ;

horsepower (hp)• Example: Walking

versus running upstairs• The “power bill” - you

actually pay for “work”(kWh), which is related to energy

• The rate at which work is done

• Units: watts (W) ; horsepower (hp)

• Example: Walking versus running upstairs

• The “power bill” - you actually pay for “work”(kWh), which is related to energy

Units of Power

• Horsepower (1 hp = 550 ft·lb/s)

• Watts (N·m/s or J/s)

• 1 hp = 746 W

• Horsepower (1 hp = 550 ft·lb/s)

• Watts (N·m/s or J/s)

• 1 hp = 746 W

Fig 3.5

Convert 9.8 kW to hp

Practice Problem #4

• An elevator m=800 kg has a maximum load of 8 people or 600 kg. The elevator goes up 10 stories = 30 m at a constant speed of 4 m/s. What is the average power output of the elevator motor if the elevator is fully loaded with its maximum weight? (neglect friction)

(Hint: First determine the time the elevator takes to go up 10 stories, then determine the work the elevator motor exerted. Finally find the power of the motor.)

Motion, Position and Energy

• Work and energy are related

• Energy = ability to do work

• When work is done on something, a change occurs in its energy level

• Work and energy are related

• Energy = ability to do work

• When work is done on something, a change occurs in its energy level

Next: • Relationship

between work and energy associated with position

• Relationship between work and energy of motion

Next: • Relationship

between work and energy associated with position

• Relationship between work and energy of motion

Potential Energy (PE)• Energy associated with

“position”– “Potential” to then do work

• Gravitational Potential Energy (GPE)– Measuring h: need a

reference position (or reference height)

• Work can “change” PE • Potential Energy can

“change” into Kinetic Energy

• Energy associated with “position”– “Potential” to then do work

• Gravitational Potential Energy (GPE)– Measuring h: need a

reference position (or reference height)

• Work can “change” PE • Potential Energy can

“change” into Kinetic Energy

PE = mgh

Practice Problem #6

• A cart is loaded with a brick and pulled at constant speed along an inclined plane to the height of a seat-top. If the mass of the loaded cart is 3.0 kg and the height of the seat top is 0.45 meters, then what is the potential energy of the loaded cart at the height of the seat-top?

Kinetic Energy (KE)

• Energy associated with “motion”

• Results from work or change in potential energy

• Speed squared!

• Double the speed,KE increases by 4

• Energy associated with “motion”

• Results from work or change in potential energy

• Speed squared!

• Double the speed,KE increases by 4

KE = 12mv2

Practice Problem #5

• A 500 kilogram car is driving at 15 meters/second. What's its kinetic energy?

Practice Problem #7

A forklift lifting a crate of mass 100 kg at a constant velocity to a height of 8 m over a time of 4 s. The forklift then holds the crate in place for 20 s.

a. How much power does the forklift exert in lifting the crate?

b. How much power does the forklift exert in holding the crate in place?

Practice Problems adapted from…

• http://zebu.uoregon.edu/~probs/mech/work.html

• http://www.sparknotes.com/testprep/books/sat2/physics/chapter7section6.rhtml

• http://www.physicsclassroom.com/class/energy/u5l1b.cfm