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Notes: _____
Chapter 10 Notes, Part 1
Date: _______
Energy and Work
Work, in scientific terms, is what is done anytime an object is ____________. There must
be motion whenever work is done. The formula for work is:
Since the unit of force is a newton and distance is measured in meters the units for work
are _______________________ (
). This unit is also called a ___________ (
).
Since a newton is a _________________ a joule is a ___________ or a ____________.
Example 1:
How much work is done when a person lifts a 65 kg mass, 2.4 meters in the air?
Example 2:
How high can a crane lift a 10,000 N object if it does 24,000 J of work?
Keep in mind that work is also a vector so that if a force is applied at an angle to the
motion you need to resolve the vector into two right angle components:
Energy and work are interchangeable. Like work, energy is measure in _____________.
Unlike work, motion is not required. There are different forms of energy. One form, called
kinetic energy is caused whenever an object is moving. The formula for kinetic
energy (
) is:
Note that increasing the velocity has a much greater effect on the energy than increasing
the mass. A car traveling at 40 mph has ________ more energy than the same car
traveling at 20 mph; A car traveling at 60 mph has ________ more energy than the same
car traveling at 20 mph!
The Work-Energy Theorem states that the work done equals the change in kinetic energy
or:
__________ is defined as the rate of doing work so power = ________________________ or
The unit of power is a ____________; this is simplified as a __________ (
). However, since
a joule is a _______________ a J/s is a _________________.
Another unit for power, often used to rate motors and engines, is the __________________:
Example 3:
A machine can lift a 250 kg object to a height of 6.5 m in 13 seconds.
(a) How much work does the machine do?
(b) How many watts of power are used?
(c) What is the horsepower rating of the motor?
Notes: _____
Chapter 10 Notes, Part 1
Date: _______
Machines
Machines are devices that do at least one of the following:
(1) change the _______________ of a force;
(2) change the _______________ of a force;
(3) change the _______________ of a force.
There are six different types of simple machines, grouped in two categories:
* There are three classes of of levers:
1st class:
2nd class:
3rd class:
Each machine has its own Ideal Mechanical Advantage (
Mechanical Advantage (
). The ____________
) is always equal to ____________________. The efficiency of
any machine equals ___________________.
An alternative method to calculate efficiency is to divide the work output (
the work input (
) by
):
____________ machines are two or more simple machines that work together such that
the resistance force of the first machine becomes the effort force of the second, and so
on. The IMA is simply the product of each simple machine.
Example 1:
A cart has a weight of 1150 N and it is pulled up a 24 m long plank using only 220 N.
If the top of the plank is 3.0 m off the ground, find : (a) the IMA, (b) the AMA, and
the efficiency.
220N
3.0 m
Example 2:
An 8 pulley system is used to lift a 725 N weight. If it took 124 N to lift the weight,
Determine the IMA, the AMA, and the efficiency of the pulley system.
Example 3:
An 6.5 cm wide by 44 cm long wood splitting wedge is used to split a log. What is the
IMA? If the wedge is 91% efficient, what is the AMA? How much force is needed to
split a piece of wood with this wedge if it takes 1500 N to split the wood?
Example 4:
What is the IMA of a wheel and axle which has a wheel diameter of 56 cm and an axle
diameter of 7.5 cm? If the device is 80% efficient, how much work output is done by
1100 J of work input by this machine?