Work and Energy - koehlerscience

Chapter 9
 Define work and power
 Calculate work done
 Discover what mechanical advantage is and
how it makes work easier
 Calculate mechanical advantage
What is Work?
 Work is done only when force causes a change in
the motion of an object.
 To calculate work you multiply the force by the
distance that the force is done over.
How to Calculate Work
 The force is equal to the pushing or pulling along
the same line as the direction of motion.
 We use joules (J) to label work.
 Imagine a father playing with his daughter by
lifting her repeatedly in the air. How much
work does he do with each lift, assuming he
lifts her 2.0 m and exerts an average force of
190 N?
1st part of assignment
 Do pg 285 numbers 1-5 (it’s under practice)
 Make sure that you have Newtons and meters
to calculate work.
 If you don’t have Newtons you will have to
calculate them using F=m*a.
 If they give you units in kg convert to N using
N=kg*9.8 m/s/s
 Power is the rate at which work is done.
 So, if you do the same amount of work in less
time, you exerted more power.
 To calculate power use the following equation:
How to Measure Power
 Power is measured in watts (W).
 1 Watt is the amount of power needed to do 1 J
of work in 1 s.
 It takes 100 kJ of work to lift an elevator 18 m.
If this is done in 20 s, what is the average
power of the elevator during the process?
2nd part of homework
 Pg. 287 1-5
 Be careful on numbers 4 & 5 – you need to
calculate work done before you can calculate
Machines and Mechanical
 Does using a machine decrease the work that
you do?
 Example: Does it take more work for you to
lift a car by yourself or to use a jack?
 Answer: It takes the same amount of work!
 Machines do not decrease work, they just
make it easier by distributing distance or
Work Input and Output
 Work input – the work you put into the
machine (W = F x d)
 Work output – the work you get out of the
machine (W = F x d)
 Example: pg. 288 – the guy lifting a box into a
 Work input is equal to work output
Mechanical Advantage
 A quantity that measures how much a
machine multiplies force or distance.
𝑚𝑒𝑐ℎ𝑎𝑛𝑖𝑐𝑎𝑙 𝑎𝑑𝑣𝑎𝑛𝑡𝑎𝑔𝑒 =
𝑜𝑢𝑡𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒
𝑖𝑛𝑝𝑢𝑡 𝑓𝑜𝑟𝑐𝑒
𝑖𝑛𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
𝑜𝑢𝑡𝑝𝑢𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
 When mechanical advantage > 1 the machine
makes the force stronger
 When mechanical advantage < 1 the machine
makes the distance or speed higher
Calculating Mechanical
 Example: Calculate the mechanical
advantage (MA) of a ramp that is 5.0 m long
and 1.5 m high.
 Input (what you do) distance = 5.0 m
 Output (what the machine does) distance =
1.5 m
 MA = 5.0 m/1.5 m = 3.3
Assignment due 2/1
 Pg 290 1-5
 Look at practice hints on left side of page.
 Also, complete study guide for section 9.1