1. Chapter 4: Work and Machines

2. What is work?
• You do work when you exert a force on an
object that causes the object to move some
distance.
• In other words: movement with applied force
= work; no movement = no work

3. More about work…
• In carrying an object, you exert an upward
force. To do work, you must exert force in the
same direction as the object’s motion.

4. And yet more clarification…
• When pulling something such as a sled, you
pull at an angle. Your force has both a vertical
and horizontal part. Only the horizontal part
of the force yields work.

5. Calculating work…
• Work depends on force and distance
• Work = Force (1 Newton) x Distance (1 meter)
• The SI unit of work is the joule

6. What is a machine anyway?
• A device with which you can do work in a way
that is easier or more effective.
• Does not decrease the amount of work
needed to do the job.

7. Machines do 3 things…
Multiplies Multiplies Changes
Force Distance Direction
• Makes work • Makes work • Makes work
easier when easier by easier by
force is multiplying the changing the
multiplied distance over direction in
which you exert which force is
force exerted

8. Machine Vocab
• Input force: what you exert on a machine
• Output force: what the machine exerts
• In some machines, the output force is greater
than the input force…or, the other way around

9. Mechanical Advantage
• The number of times a force exerted on a
machine is multiplied by the machine
• Formula:
Mechanical Advantage= Output Force/Input Force
• If Mechanical Advantage is > 1, output force >
input force

10. Mechanical Efficiency
• In all machines, some work is wasted to
overcome friction
• Comparison of output work to input work is
efficiency
• Formula:
Efficiency=output work/input work x 100%

11. Actual or Ideal?
• Mechanical advantage that a machine
provides in a real situation is called the actual
mechanical advantage
• The mechanical advantage of a machine
without friction is called the machine’s ideal
mechanical advantage

13. 2. Wedge
• A device that is thick at one end and tapers to
a thin edge at the other end
• In a wedge, the inclined plane itself moves
• Multiplies force to do the job

14. 3. Screws
• A screw is an inclined plane wrapped around a
cylinder
• A spiral inclined plane forms the threads of a
screw
• In a screwdriver, input force is applied to the
handle, threads exert output force

15. 4. Levers
• A lever is a rigid bar that is free to pivot, or
rotate around a fixed point
• The fixed point is called a fulcrum
• Levers increase the effect of input force
• Change direction of input force

16. 3 Types of Levers
• First‐class: seesaw, scissors, pliers – fulcrum in
between input and output force
• Second‐class: door, wheel barrow, bottle
opener – output force greater than input force
• Third‐class: baseball bat, shovel, rake – output
force less than input force
• Ideal Mechanical Advantage=
Distance from fulcrum to input force/
distance from fulcrum to output force

17. 5. Wheel and Axle
• A simple machine made of 2 cylindrical
objects that are fastened together and that
rotate around a common axis
• Ideal mechanical advantage=
Radius of wheel/radius of axle

18. 6. Pulley
• A grooved wheel with a rope (or chain, or
even a steel cable) wrapped around it.
• Fixed pulleys change direction of input force
but not the amount of force applied
• A moveable pulley has an ideal mechanical
advantage of 2

19. Compound Machines
• A machine that utilizes two or more simple
machines
• To calculate the ideal mechanical advantage of
a compound machine, you must know the
mechanical advantage of each simple machine
used in the compound machine