1. Chpt. 5—Work and Machines
Work: is the transfer of energy that occurs when a force makes an object move
For work to be done, a force must make something move
ex: pushing on something that doesn’t move is not doing work

2. Two conditions have to be met to do work on an object
1. object has to move
2. motion of the object must be in same direction as force applied
When work is done, a transfer of energy always occurs
So: Energy is also the ability to do work (not just the ability to cause change)

3. Calculating Work
Work = force X distance OR
W = F X d
*Units for work are Joules
Practice Calculating Work—pg. 128

4. Power
Power: the amount of work done in a certain amount of time ( it is the RATE at which work is done)
Calculating Power
Power = Work / time OR
P = W/t
Units used with power are watts—W (1 J/s)
Practice calculating Power—pg. 130

5. Power and Energy
Power is produced or used any time energy is transferred from one object to another
SO:
Power = energy /time
OR P = E/t
Anytime you do work on an object, you cause its energy to increase
Section 1 Assessment questions

6. Section 2—Using Machines
Machines: a device that makes doing work easier
Machines increase the force applied to an
object OR increase the distance over which a force can be applied
Machines can also change the direction of an applied force
Ex: car jacks, ramps, wedges

7. Effort and Resistance Forces
Two forces are involved when a machine is used to do work
Effort force: the force applied to the machine
Resistance force: the force applied by the machine to overcome resistance
Ex: when you pull a nail out w/a hammer, effort force is applied to the handle, resistance force is applied to the nail by the claw

8. Two kinds of work are involved when using a machine
The work done by you on a machine is called the INPUT work
The work done by the machine is called the OUTPUT work
Remember: Energy is always conserved
A machine cannot create energy, so work OUTPUT is never greater than work INPUT

9. Ideal machines create no friction
So for an ideal machine, Work INPUT = Work OUTPUT,
But, this is not so in normally operating machines

10. Mechanical Advantage: the number of times a machine multiplies the effort force
To calculate mechanical advantage, you divide resistance force by effort force
Some machines simply change direction of effort force (ex: window blinds)
Therefore, the effort force and resistance force are equal, so mechanical advantage is 1.

11. Efficiency: a measure of how much of the work put into a machine is changed into useful output work by the machine
High efficiency means less heat produced from friction
Efficiency = W output/W input X 100%
Friction causes output work to always be less than the input work
SO: Efficiency of a real machine is always less than 100%