Work and Power

Section 1 - Energy

Section 2 - Work

Section 3 - Power

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Energy
Kinetic energy
Potential energy
Work
Displacement
Power

Vocabulary

Energy

● Energy (E) is defined as the capacity to do

work

● Many forms

● Cannot be created or destroyed, only

converted

● chemical, sound, nuclear, mechanical, electrical, light, thermal,

elastic,

● Measured in Joules (J)

● a Joule is the amount of energy required to move

an object 1 meter with 1 newton of force

Kinetic Energy

Energy due to motion reflects

● the mass (kg)

● the velocity of the object (m/s)

● always use measurements in kg and m/s

KE = 1/2 mv2

Calculate Kinetic Energy

How much KE in a 145 g
baseball thrown at 35.8
m/s?

Calculate Kinetic Energy

Table of Variables
Mass = 145 g ⇒ 0.145 kg
Velocity = 35.8 m/s

Calculate Kinetic Energy

Table of Variables
Mass = 145 g ⇒ 0.145 kg
Velocity = 35.8 m/s
Select the equation and solve:
KE = ½ m v2

KE = ½ (0.145 kg)(35.8 m/s)2

KE = ½ (0.145 kg)(1281.54 m/s/s)
KE = ½ (185.8 kg m/s/s)
KE = 92.9 kg m/s/s, or 92.9 Nm, or 92.9J

Calculate Kinetic Energy

How much KE possessed by

a 68.1 kg female volleyball
player moving downward at
3.2 m/s after a block?

Calculate Kinetic Energy

Table of Variables
● 68.18 kg of mass
● -3.2 m/s
Select the equation and solve:
KE = ½ m v2

● KE = ½ (68.18 kg)(-3.2 m/s)2

● KE = ½ (68.18 kg)(10.24 m/s/s)
● KE = ½ (698.16 kg m/s/s)
● KE = 349.08 Nm or J

Calculate Kinetic Energy

Compare KE possessed by:
● a 220 pound (100 kg) running back

moving forward at 4.0 m/s

● a 385 pound (175 kg) lineman moving

forward at 3.75 m/s

Calculate Kinetic Energy

Table of Variables
m = 100 Kg
v = 4.0 m/s
Select the equation

and solve:

KE = ½ m v2

KE = ½ (100 kg)(4.0

m/s)2

KE = 800 Nm or J

Table of Variables
m = 175 kg
v = 3.75 m/s
Select the equation

and solve:

KE = ½ m v2

KE = ½ (175)(3.75)2

KE = 1230 Nm or J

Potential Energy

● Energy that is stored in an object.

● potential energy can be due to position, stresses

within the object, the chemical composition of
the object, electrical charges, etc.

● Gravitational Potential Energy

●

GPE=mgh

●m=mass

●g=pull of gravity - 9.8m/s on Earth

●h=height off ground

●

Other forms of potential energy are calculated is slightly
different ways

Work - Energy Relationship

● We defined energy as the capacity to do

work. Work is done on an object due to an
unbalanced force. There must be an energy
transfer to create an unbalanced force.

energy transfer—>unbalanced force—> work is done

Work

● The product of an unbalanced force and the

amount of distance along the line of action
of that force.

W=Fd

W=work - measured in Joules

Joule = Newton x meter

F= force - measured in Newton
d=distance - measured in meters

Work = F x d

To calculate work done on an object, we need:
The Force
● The average magnitude of the force
● The direction of the force
The distance
● The magnitude of the change of position
● The direction of the change of position

Calculate Work

● During the ascent phase of a rep of the

bench press, the lifter exerts an
average vertical force of 1000 N
against a barbell while the barbell
moves 0.8 m upward

● How much work did the lifter do to the

barbell?

Calculate Work

Table of Variables:
Force = +1000 N
Displacement = +0.8 m

Force is positive due to pushing upward
distance is positive due to moving upward

Calculate Work

Table of Variables:
Force = +1000 N
Displacement = +0.8 m
Select the equation and solve:

- & + Work

● Positive work is performed when

the direction of the force and
the direction of motion are the
same

● ascent phase of the bench press

● Throwing a ball

● push off (upward) phase of a jump

- & + Work

● Positive work
● Negative work is performed

when the direction of the force
and the direction of motion are
the opposite

● descent phase of the bench press

● catching

● landing phase of a jump

Power

● The rate of doing work

● Work = Fd

Units: Fd/s = J/s = watt

Power

● The rate of doing work

● P=W/t

● P=power-measured in watts

● W=work - measured in joules

● t=time-measured in seconds

● Power is always measured in Watts

● A Watt is a Joule/Second

Units: Fd/s = J/s = watt

Calculate & compare power

● During the ascent phase of a rep of the

bench press, two lifters each exert an
average vertical force of 1000 N
against a barbell while the barbell
moves 0.8 m upward

● Lifter A: 0.50 seconds
● Lifter B: 0.75 seconds

Calculate & compare power

Lifter A
Table of Variables
F = 1000 N
d = 0.8 m
t = 0.50 s

Lifter B’s time would

be .75 sec instead of
.5 sec