1) Simple machines make work easier by multiplying the input force or changing the direction of force applied. However, no machine is 100% efficient as energy is lost to friction. 2) Mechanical advantage is a measure of how much easier a task has become due to a machine. It is the ratio of the resistance force to the effort force. Actual mechanical advantage accounts for friction. 3) Common simple machines include levers, pulleys, wheels and axles, and inclined planes. Levers can be first, second, or third class depending on the position of the fulcrum. Work is the product of the applied force and distance of force application. Power is the rate of doing work over time.

1. Energy and its Transformations
Simple Machines,
Work & Power
Chapter 15

2. Examples

3. Simple Machines and Work
• A simple machine is a device used to make
work easier.
• It is used to multiply forces or change the
direction of the force.
• No machine is 100% efficient.
• Energy is always lost to friction.
• The person must do more work, but easier
work, i.e. less force.
• A lesser force is applied over a greater
distance.

4. Definitions & Equation
• Effort Force, FE: Force used by person, N
• Effort Distance, ΔdE: Distance person’s force
is applied over, m.
• Resistance Force, FR: Force that comes
from machine, or weight of object, N
• Resistance Distance, ΔdR: Distance object is
moved, m.
• FE x ΔdE = FR x ΔdR

5. Mechanical Advantage
• Mechanical Advantage is a measure of how
much easier the task has become.
• Ideal Mechanical Advantage – no friction:
• IMA = ΔdE = Effort arm length
ΔdR Resistance arm length
• Actual Mechanical Advantage – with friction.
• AMA = FR
FE

6. Levers
• The fulcrum is a pivot that can act to
change the direction and magnitude of the
force.
• Depending on force direction and position
of fulcrum:
– 1st class
– 2nd class
– 3rd class

7. 1 Class Lever
st
• 1st Class Lever: The fulcrum, or pivot, is
located between the 2 forces.
• E.g. lever, seesaw, teeter-totter, scissors.

8. 2 Class Lever
nd
• 2nd Class Lever: The fulcrum is located on
1 side of both forces.
• E.g. wheel barrow, nutcracker

9. 3 Class Lever
rd
• 3rd Class Lever: The effort force is
between the resistance force and the
fulcrum.
• E.g. tweezers, hockey stick

10. Variations of the Lever
• The pulley is a variation of the lever.
• IMA = the number of supporting ropes.
• IMA = ΔdE
ΔdR
• AMA = FR
FE

11. Wheel and Axle
• The wheel and axle acts like a 2nd class
lever. E.g. doorknob, taps, steering wheel
• IMA = radiusE AMA = FR
radiusR FE

12. The Inclined Plane
• The inclined plane trades distance for
force by reducing the force needed to
work against gravity.
• E.g. stairs, ramps, screw

13. Inclined Plane Formula
• IMA = length of slope = ΔdE
height of slope ΔdR
• AMA = FR = mg
FE FE

14. Activity
• A 1st class lever has a AMA of 4. How
much force is needed to lift 5 kg?
• AMA = FR = mg
FE FE
• 4 = 5kg x 9.81 m/s2
FE
• FE = 12.25 N

15. Exam Question
A workman uses a pulley to lift a 50 kg sack of potatoes by pulling downwards on a rope
with a force of 550 N.
x
What is the acceleration of the sack?
A) 1.0 m/s2
B) 2.1 m/s2
C) 4.5 m/s2
D) 11 m/s2

16. Measuring Work
• Work is defined as the energy that comes
from applying a force over a certain
distance.
• W = F Δd = mad (horizontal)
• = magd (against gravity)
• Work is in Joules, J
• Force is in Newtons, N
• Distance is in metres, m

17. Activity
• E.g How much work is done by a boy
pushing a car with a force of 800 N over a
distance of 200m?
• W=Fd
• = 800 N x 200 m
• = 160 000 J = 160 kJ
• Do page 330, Q. 1-4

18. Exam Question
A 200 g brick falls from a wall 4.0 metres above the ground. It hits the ground with a
velocity of 8.5 m/s.
4.0 m
How much work did gravity do on the brick?
A) 8.0 J
B) 7.2 J
C) 3.4 J
D) 1.7 J

19. Exam Question
A sled has a mass of 10 kg.
A child pulls the sled a distance of 20 metres with a force of 10.0 N at an angle of 35° with
respect to the horizontal. During this motion, a force of friction of 4.0 N acts in the opposite
direction of the motion.
How much work is done on the sled by the child over the distance of 20 metres?
A) 1.6 × 102 J
B) 1.1 × 102 J
C) 8.4 × 101 J
D) 3.5 × 101 J

20. Efficiency
• The IMA is always greater than the AMA.
• The MA must be greater than 1.
• % Efficiency = Work output x 100 = AMA
Work input IMA
• The maximum efficiency is 100%.
• It is a measure of what energy is lost to
friction, vibration, and other factors.

21. Power
• Power is defined as the rate at which work
is being done.
• P=W
• Δt
• Work is in Joules,
• Time is in seconds
• Power is in Watts

22. Activity
• What is the power of a bulldozer that does
55000J of work in 1.1s?
• P = W = 55000J = 50000 Watts
Δt 1.1s
• If 100000 J of energy was expended by
the bulldozer, what is its efficiency?
• Do page 334, Q 1-5

23. Exam Question
A horse is hitched up to a buggy with a mass of 500 kg including the people inside.
Disregard the effects of friction.
Starting from rest, the horse exerts a horizontal force of 300 newtons on the buggy over a
distance of 30 metres.
What is the average power that the horse develops over the first 30 metres?
A) 9.0 × 102 W
B) 9.0 × 103 W
C) 4.5 × 105 W
D) 4.5 × 106 W

24. Summary
• Work done on object equals the applied
force times the displacement of the object
in the direction of the force.
• W = FΔd
• The units are Joules, J, or Nm.
• Power is the rate at which work is done
per unit of time.
• P = W/Δt
• The units are Watts, W, or J/s

25. Summary
• The Efficiency of a machine is given by
the formula, Efficiency = Work output x
100
Work input
• A machine is supposed to reduce the
force required by a person to do a physical
task.
• Ideal Mechanical Advantage does not take
into account friction.
• IMA = effort distance

26. Summary
• Actual Mechanical Advantage, AMA, takes
friction into account.
• AMA = Resistance Force
Effort Force

27. Exam Question
A car with a mass of 1000 kg and moving at a speed of 30 m/s comes to rest over a
distance of 100 metres.
What is the force of friction (acting on the wheels of the car) which causes the car to
stop?
A) 3 000 N
B) 3 500 N
C) 4 000 N
D) 4 500 N

28. Activity
• Do Page 338, Q. 1-6