2. What it means to do work
Work – force acting over a distance to
move an object
2 key points for work to be done
An object has to move
The object must move in the direction of
the applied force
3. Work
Work = Force x Distance
W = F x D
Units
Force = Newtons (kg m/s2) - N
Distance = meters – m
Work = joules – J (N m)
Joule – unit of work and energy; 1
newton-meter
4. Work
Example:
If a weight lifter lifts a 800 N barbell a
distance of 2.5 m what is the work done?
W = F x D
800N x 2.5m
W = 2,000 J
6. Power
Power – the rate at which work is done,
or the amount of work per unit of time
Power = work/time p = w/t
Power = force x distance
time
7. Power
Unit for power is called a watt (W)
Watt is a joule/sec
Ex: a 40 watt light bulb does work at a rate
of 40 J/s
Large quantities of power are measured in
kilowatts (kW)
Horsepower (hp) – used to rate electric
motors and gas engines
1 hp = 745.56 watts (746 watts)
9. Simple Machines
Inclined plane – a type of simple machine
that has a flat, slanted surface with no
moving parts
The less slanted the surface, the longer the
distance over which the effort force is applied
IMA = effort distance = length of slope = l
resistance dist. height of slope h
10. Simple Machines
Wedge – an inclined plane that moves
The greater the distance the wedge moves,
the greater the force is on an object
Calculating IMA of a wedge is the same
as an inclined plane (except use width
instead of height)
11. Simple Machines
Screw – an inclined plane wrapped
around a cylinder to form a spiral
Tight threads = longer distance over which
effort is exerted = easier to turn
Spaced threads = shorter distance over
which force is exerted = harder to turn
12. Simple Machines
Lever – a rigid bar that is free to pivot
or turn about a fixed point
Fulcrum – the fixed point
Effort arm – the part of the lever where
the effort force is applied
Resistance arm – the part of the lever
that exerts the resistance force
13. Levers
1st class lever R E
Ex: scissors
2nd class lever R E
Ex: wheelbarrow
3rd class lever E R
Ex: arm swinging
a hammer
14. Levers
IMA = length of effort arm___ = _Le_
length of resistance arm Lr
Ex: A worker uses an iron bar to lift a
manhole cover weighing 65 N. The effort
arm of the lever is 60 cm long. What is
the IMA of the bar?
IMA = 60cm = 6
10cm
15. Simple Machines
Pulley – rope, chain, or belt
wrapped around a grooved
wheel
A fixed pulley is attached to
something that doesn’t move
(ceiling, wall, etc.)
The IMA of a fixed pulley is 1
A movable pulley moves
The IMA of a movable pulley is 2
16. Pulleys
Block and tackle
IMA can be multiplied by counting every
rope supplying an upward force on the
resistance
17. Simple Machines
Wheel and axle – a simple machine
consisting of two wheels of different
sizes that rotate together
IMA = radius of wheel = Rw
radius of axle Ra
