Work

1
Work
Work is the product of the applied in the
direction of motion, and the of the
object. W x .

• The unit of work is called a Joule (J).
• One joule is the amount of work done when a force
  of N acts over . m.
• 1J .
• 1-2.m
• 1kg.m2.s-2
• Work is a quantity. (.. Not necessary)
• From the formula we see that if there is no
  .. then there is no work done.
• If the displacement is to the force
  then there is no work done either.

F
2
Work
Work is the product of the resultant force
applied in the direction of motion, and the
displacement of the object. W F ?xcos?

• The unit of work is N.m called a Joule(J).
• The joule is the amount of work done when a force
  of 1N acts over 1m.
• 1J 1N.1m
• 1kg.m.s-2.m
• 1kg.m2.s-2
• Work is a scalar quantity. ( Direction not
  necessary)
• From the formula we see that if there is no
  movement then there is no work done.
• If the displacement is perpendicular to the force
  then there is no work done either.

F
?
?x
3
Energy Kinetic Energy

• Energy is the capacity to do ..
• Energy is a scalar quantity and has the same unit
  as work viz. ...
• When ENERGY is transferred . is done.
• If an object is accelerated from rest.

Energy of movement W f. F m.
gtW vf2 vi2 2a ?x but vi
0 gt ?x vf2 .... W gt EK
...
FORCE
m
m
a m.s-2
m
m
4
Energy Kinetic Energy

• Energy is the capacity to do work.
• Energy is a scalar quantity and has the same unit
  as work viz. Joule.
• When ENERGY is transferred WORK is done.

Energy of movement W f.?x F m.a
gtW ma?x vf2 vi2 2a?x but vi 0
gt ?x vf2 /2a W m.a.(vf2/2a) gt EK
1/2mvf2
FORCE
m
m
a m.s-2
m
m
5
Potential energy
Potential energy is the energy that an object has as a result of its ..or ...

• Object lifted to a height h.
• Energy gained (or transferred) .
• Ep of an object .. to lift the object to
  that height
• W
• F
• W .
• Ep .
• Where h is the displacement of lifted object.

h
6
Potential energy
Potential energy is the energy that an object has as a result of its position or state.

• Object lifted to a height h.
• Energy gained (or transferred) work
  done
• Ep of an object work done to lift the object
  to that height
• W f.?x
• Fg mg
• W mg?x
• Ep mgh
• Where h is the displacement of lifted object.

h
7
Potential energy Bouncing Efficiency
Observation A ball dropped from a height bounces back to a lower height than it was released from.

• Task
• Investigate the efficiency of the bounce of a
  ball.
• Theory
• The efficiency of the bounce can be expressed as
  the of energy of the bounce over the original
  potential energy of the ball.
• Efficiency x100
• Outcomes
• Write up all the steps of the scientific method
  and include Calculations of energy and
  efficiency from at least five different heights.
  Also show at least one calculation of the
  velocity of the ball on impact with the surface.
  Compare the efficiency of at least two different
  balls.

h initial
Initial Potential energy (top) Potential energy
of the bounce
h bounce
8
Mechanical Energy
An object dropped from REST
EK ____ Ep . Ep ____ Ek
.

• Mechanical energy is the SUM of the
• and
  energy of an object at any given time.
• E .
• The energy, which is converted or transferred, is
  equal to the to do this.
• If an object is moving and is brought to rest
  then the to do this
  is equal to the
  .
• At any point during the fall of an object the
• 
  is equal to the potential energy of the object
  it began to fall.

9
Mechanical Energy
EK 0 Ep mgh Ep 0 Ek (mgh)

• Mechanical energy is the sum of the kinetic and
  potential energy of an object at any given time.
• E Ep Ek
• The energy, which is converted or transferred, is
  equal to the work done to do this.
• If an object is moving and is brought to rest
  then the work done to do this is equal to the
  change in kinetic energy. ?Ek work done
• At any point during the fall of an object the
  mechanical energy is equal to the potential
  energy of the object before it began to fall.
• If any energy is lost through work then
• E Ep Ek Work done

10
Mechanical Energy
A 4kg ball is dropped 8m.
Any object that is above the ground has the
POTENTIAL to fall Down. The potential energy
EP mgh (Height) lost would be converted into
velocity (kinetic energy) EK. Ep()
Ek(.) If the object is lifted up then it
.. potential energy. At any time the .of
these two is called the MECHANICAL ENERGY - and
it stays .
E Ep Ek
4kg
m
Ep
Ek .J
E Ep Ek J
Ep Potential Energy LOST
8m
E Ep Ek . 320J
4m
EK kinetic energy .. Ep !
E Ep Ek ..J
Ep
m
Ek
11
Mechanical Energy
4kg
m
Any object that is above the ground has the
POTENTIAL to fall Down. The potential energy
EP mgh (Height) lost would be converted into
speed (kinetic energy) EK. Ep(Top)
Ek(Bottom) If the object is lifted up then it
gains potential energy. At any time the sum of
these two is called the MECHANICAL ENERGY - and
it stays constant.
E Ep Ek 320 0 320J
Ep mgh
Ek 0J
Ep Potential Energy LOST
8m
E Ep Ek 160 160 320J
4m
EK kinetic energy gained Ep lost!
E Ep Ek 0 320 320J
Ep 0
m
Ek 320J
E Ep Ek
12
Conservation of energy - Pendulum

• Energy cannot be created or destroyed but merely
  transferred from one form to another.
• Whenever there is energy conversion there is
  done.

A
C
h
B
The of the pendulum is not required to
calculate the velocity.
Ep (Top) Ek (Bottom) .2 gt V
v..
13
Conservation of energy - Pendulum

• Energy cannot be created or destroyed but merely
  transferred from one form to another.
• Whenever there is energy conversion there is work
  done.

A
C
h
B
The mass of the pendulum is not required to
calculate the velocity.
Ep (Top) Ek (Bottom) Mgh 1/2mv2 gt V
v2gh
14
A pendulum question

• A person of 60kg is lifted to a height of 30m on
  a slingshot pendulum and then released what is
  its maximum speed?

Total Mechanical Energy (Top)
ETop Ep EK mgh 1/2mv2
(60)(10)(30) 0 18000J
m
h
EBottom Ep Ek mgh
1/2mv2 18 000 0 1/2 (60)v2 v2
18000/30 600 v ?600 24.5 m.s-1
15
A pendulum

• A person of 60kg is lifted to a height of 30m on
  a slingshot pendulum and then released what is
  its maximum speed?

Total Mechanical Energy (Top)
ETop Ep EK mgh 1/2mv2
(60)(10)(30) 0 18000J
m
h
EBottom Ep Ek mgh
1/2mv2 18 000 0 1/2 (60)v2 v2
18000/30 600 v ?600 24.5 m.s-1
16
Power

• Power is the .. at which WORK is done.
• Power
• Units Watts (W)

Since v .., Power can be found by P .
If a force of 20N is exerted over a distance of
5m for a time of 30s the power used would be.
17
Power

• Power is the RATE at which WORK is done.
• Power
• Units Watts (W)

Work time
Joules (J) Time (s)
Since v s/t, Power can be found by P F.v
If a force of 20N is exerted over a distance of
5m for a time of 30s the power used would be. W
F.?x (20).5 100J P W/t 100/30 3.3 W