Chapter 5 – WORK and ENERGY

TODAY’s OBJECTIVES

• Recognize the difference between the “scientific” and “ordinary”

work• Define work by relating it to force

and displacement

5.1 – 5.3 Work and Energy Vocabulary

WorkWork done on an object

JouleKinetic energy

Potential energyGravitational Potential Energy

Elastic Potential EnergySpring constant

Mechanical Energy

5.1 WORK

What is work?

In everyday language

In science

To do somethin

g:To learnTo kick a

ballTo thinkTo hold a

chair

The product of the

magnitude ofthe

displacementand the

component of a force

parallel to that

displacement

Work is done – when a force causes a displacement

Work done on a car

Constant horizontal force

= FDisplacement =

Δx = d

NO WORK is done on the object unless the objects moves (i.e. has a

displacement)!!!

W = F ∙ d

Work is done – when a force acts on an object AND the object must move in the direction

of that force

Work done on a crate

Constant horizontal force = F ∙ cos

Displacement = Δx = d

NO WORK is done on the object if the force is non – parallel to the direction of the

displacement

W = F ∙ d

IS WORK DONE ON AN OBJECT?

Example 1 (on a worksheet)

A teacher holds a chair at arm’s

length for several

minutes.

A person carries a bucket of water

along a horizontal path while walking

at constant velocity

IS WORK DONE ON AN OBJECT?

Example 1 (on a worksheet)

No, chair does not move = no

displacement in the direction of force applied

No, the upward force that holds the buckets is

perpendicular to the displacement

of the bucket

1.1 x 102 J

1.72 ×104 J, or 17.2 kJ

W = F × d = 300.0 x 0 = 0 J

Reminder...If WORK is done against GRAVITY

W = F ∙ dW = mg ∙ h

POWERRate of Energy

Transfer

the rate at which work is done or energy is

transformed

POWER

Power =

the rate at which work is done or energy is transformed

Power = force x speed

WHY? UNITS = J/s =

Watt = W

73 W

5.4 ×105 J/h

Useable work: 2.7 ×1011J

Heat: 8.1×1011 J

HORSE POWER (hp)• Another unit

used• 1 hp = 746

watts

Convert to horsepower.

Reminder...If WORK is done against GRAVITY

W = F ∙ dW = mg ∙ h

HOMEWORKPage: 139 - 141Problems: all

18

WORKSHEET EXAMPLEMany mountain roads are built so that they

zigzag up the mountain rather than go straight up toward the peak. Discuss the advantages of

such a design from the viewpoint of energy conservation and power

WORKSHEET EXAMPLEA light bulb is described as “having 60 watts.”

What’s wrong with this statement?

WORKSHEET EXAMPLEA 193.0 kg curtain needs to be raised 7.5 m, at

constant speed, in as close to5.0 s as possible. The power ratings for three

motors are listed as 1.0 kW,3.5 kW, and 5.5 kW.

Which motor is best for the job?

Worksheet EXAMPLEA 193.0 kg curtain needs to be raised 7.5 m, at

constant speed, in as close to5.0 s as possible. The power ratings for three

motors are listed as 1.0 kW,3.5 kW, and 5.5 kW.

Which motor is best for the job?GIVEN

?UNKNOWN

?m = 193.0 kgΔt = 5.0 s

d = 7.5 m

P = ? watts

WORKSHEET EXAMPLETwo horses pull a cart. Each exerts a force of 250.0 N at a speed of 2.0 m/s for 10.0 min.a. Calculate the power delivered by the

horses.b. How much work is done by the two

horses?

Investigation 5 – 1A1. Read the lab procedure

2. Answer all the Concluding Questions

Investigation 5 – 1A and 5 – 1B Two horses pull a cart. Each exerts a force of 250.0 N at a speed of 2.0 m/s for 10.0 min.a. Calculate the power delivered by the

horses.b. How much work is done by the two

horses?

ANSWERS TO

WORKSHEET EXAMPLES

IS WORK DONE ON AN OBJECT?

Example 1 (on a worksheet)

No, chair does not move = no

displacement in the direction of force applied

No, the upward force that holds the buckets is

perpendicular to the displacement

of the bucket

WORKSHEET EXAMPLEMany mountain roads are built so that they

zigzag up the mountain rather than go straight up toward the peak. Discuss the advantages of

such a design from the viewpoint of energy conservation and powerAssuming mechanical energy is

conserved, the same amount of energy is needed to reach the top in

both cases. Because the same amount of work must be done, the path with a longer distance takes more time and

hence requires less power.

WORKSHEET EXAMPLEA light bulb is described as having 60 watts.

What’s wrong with this statement?

Light bulbs don’t have theenergy stored within them;

energy is transferred to them

in the form of electricity at arate of 60 J/s.

WORKSHEET EXAMPLETwo horses pull a cart. Each exerts a force of 250.0 N at a speed of 2.0 m/s for 10.0 min.a. Calculate the power delivered by the

horses.b. How much work is done by the two

horses?