Teaches students about work & its measurements to physics, ranging from grades 6-8 grades. Also, teaches the difference between simple and compound machines, for example, inclined planes. Finally, teaches students how work can be understood as energy & efficiency.

1. Work & Machines
By Emmanuel Dikolelay

2. Work
The scientific definition of work is different from how we use the word every day: in
science, work is when an applied force makes an object move in the same direction
as the applied force. So towing a car is work because the applied force & resulting
motion are both in the same direction. Lifting a book off of the ground is also work:
You apply a force upward & the book moves upward. The amount of work depends
on both the amount of force applied & the distance across which the force is
applied:
work = force x distance

3. Measurements
Work is measured in joules (J), force is measured in newtons (N), & distance is
measured in meters (m).

4. Calculations
To calculate work, we can only input force that is in the same direction of motion,
meaning, if you are holding a basket of laundry, you won´t be doing work as you
walk down a hall, but as soon as you walk up the stairs you are doing work. Why ?
The force applied to the laundry basket to overcome gravity & pick it up is vertical;
when you work down the hall, the motion of the basket is horizontal (not vertical),
so you cannot include the horizontal distance in your calculation of work. When you
walk up the stairs, the force is vertical (because you have to overcome gravity) & the
motion is also vertical, so you are doing ¨work¨!

5. Calculations (con.)
Sometimes the force is only partially in the direction of motion. For example, if you
can´t fully carry the trash, you lift up a little as you drag the bag (to overcome sliding
friction), so you apply force in two directions: vertical & horizontal. However,
because the bag is only moving across the floor, only the amount of force that is
applied in the horizontal direction is considered ¨work¨.

6. Power
Power is the rate at which work is done, in other words, how quickly or slowly work
is done. More powerful machines perform work faster.
power =
work
time
Power is measured with watts (W), & time is measured in seconds (s).

7. Simple Machines
To make work easier, humans invented machines. When you think of a machine, you
might think of a tractor or a car, but a machine can be super simple. A machine is
anything that makes work easier, even a ramp is a machine. A simple machine
doesn´t reduce the total amount of work that is accomplished, but it decreases the
amount of force required to do the same amount of work by increasing the distance.

8. Simple Machines vs Compound Machines
A simple machine is a machine that does work with a single motion, as opposed to a
compound machine, which combines a number of simple machines to produce a
more complicated machine, like a can opener.
work: applying a force over a distance.
The force must be in the same direction
as movement
J = N x M

9. Inclined Plane
An inclined plane or ramp is an example of a simple machine. It reduces the amount
of effort required by increasing the work´s distance. Think about pushing a really
heavy box into the back of the truck. With a ramp, you can roll the box up the ramp,
which requires much less force than lifting the box into the truck. The box ends up
at the same height, so the same amount of work is accomplished. However, because
you pushed the box across a greater distance, less effort was required to raise the
object the same distance.
Egyptians built their pyramids
using really long ramps.

10. Wedge
A wedge is a movable inclined plane, & it reduces the amount of work required to
split or lift objects.
Some examples of wedges are knives, axes, doorstops, & plows. To reduce the
amount of effort required to cut firewood, people use wedge-shaped axes to split
wood. It has to travel farther, but an axe requires less force than simply pulling a log
in half with your bare hands.

11. Screw
A screw is a wedge (an inclined plane) wrapped around a shaft or post. As you turn
the screw, the wedge pushes the object up the shaft (or the screw into the object).
The amount of force required to secure a screw into the wall is less than it would be
to hammer a nail of the same size. That´s because the spiral of the screw has to go to
a much farther distance because it is turned so many times.

12. Lever
A lever reduces the amount of effort required to lift something. A lever is like a
seesaw: a rigid bar or plank with a pivot point, called a fulcrum. When you apply a
force, called effort, to one side of the fulcrum, the load on the other side also moves.
Think about sitting on a seesaw with a friend, when you push down on the seat, your
friend goes up into the air. Even if your friend is twice your mass, you can lift him by
having him sit closer to the fulcrum. You push over more distance, so you need less
force.

13. Lever Classification on Force
Levers are classified according to where the fulcrum & load are placed & where you
apply the force, or effort:

14. First Class
The fulcrum is somewhere in the middle, & the load & effort are on each side of the
fulcrum (like a seesaw).

15. Second Class
The fulcrum is to one side, the load is to the middle, & the effort is on the other side
(like a wheelbarrow).

16. Third Class
The fulcrum is to one side, the effort is to the middle, & the load is on the other side
(like your arm when you lift a weight).

17. Wheel & Axle
A wheel & axle make it easier to turn something by attaching a larger wheel to an
axle, or rod, which is essentially a smaller wheel. There are two ways wheels & axles
are used:

18. They Increase the Output Force
Turning a larger wheel requires less force than turning a smaller wheel (the larger
wheel is moved a farther distance, so it requires less force for the same amount of
work). Think about twisting open a faucet: It is much easier to turn a faucet handle a
few inches than it would be to turn the pencil-thin neck. The axle exerts the output
force.

19. They Decrease the Distance Required to Move
the Wheel
Turning the smaller wheel requires more force than turning the larger wheel, but
the smaller wheel must be turned a much smaller distance to accomplish the same
work output. This mechanism is used on a bike: You apply the force over a shorter
distance with the pedal, & the back wheel exerts less force over a longer distance.
The wheel exerts the output force.

20. Pulleys
A pulley is a wheel with a rope wrapped around it. The rope fits into a groove on the
wheel, & it is used to either magnify the force you apply to the rope (if you arrange a
system of two or more pulleys) or change the direction of the force so it is easier to
pull.

21. Work as Energy & Efficiency
The energy of an object increases as you do work on the object. For example, when
you push an object, it begins to move, & that motion is a form of energy. Work is
equivalent to energy, so all of the work done on an object is conserved in the form of
energy.
Energy comes in many forms, such as heat & motion. If you are doing work on an
object & some of the energy you expend is lost in the form of heat (such as the heat
the friction produces), you have lost some of your work. How much work, or energy,
you lost to heat determines efficiency. A machine that doesn´t lose much energy to
heat produces more work & is therefore more efficient.