Work = Force x Distance  W = F x D

Efficiency = Output work X 100%

                      Input work

Mechanical advantage = Output force

                                           Input force

Ideal Mechanical Advantage = Radius of wheel

Of wheel and axel                   Radius of axel

Chapter 4 – Work and Machines

Section 1 – What is work?

Key Ideas

· Work done on an object when a force causes that object to move some distance.

· The amount of work done on an object is equal to the force on the object in the direction of the force on the object in the direction of the motion multiplied by the distance the object moves.

o Work is when you exert a force on an object that causes that object to move some distance.

o In order to do work on an object, the object must move some distance as a result of your force.

· In order to do work on an object, the force you exert must be in the same direction as the object’s motion.

· The amount of work done on an object can be determined by multiplying force times distance.

· Work = Force X Distance

Key Terms

· Work – Force exerted on an object that causes it to move.

· Joule – A unit of work that is equal to one newton-meter.

Section 2 – Mechanical advantage and efficiency

Key Ideas

· A machine makes work easier by changing the direction or amount of force needed to accomplish a task.

· The efficiency of a machine is the percentage of the input work that is changed to output work.

· An Ideal machine has 100% efficiency.

· Efficiency = Output work X 100%

                      Input work

· The mechanical advantage of a machine is obtained by dividing the output force by the input force.

· Mechanical advantage = Output force

                                           Input force

· A machine’s mechanical advantage is the number of times a force exerted on a machine is multiplied by the machine.

· The ideal mechanical advantage of a machine is the mechanical advantage that is would have if there were no friction.

· The more efficient a machine is, the closer the actual mechanical advantage is to the ideal mechanical advantage

· A machine does not decrease the amount of work you do, it changes the amount of force exerted or changes the direction in which the force is exerted.

Key Terms

· Machine – A device that changes the amount of force exerted or the direction in which force is exerted.

· Input force – The force exerted on a machine.

· Output force – The force exerted on an object by a machine.

· Mechanical advantage – The number of times the force exerted on a machine is multiplied by the machine.

· Efficiency – The percentage of the input work that is converted to output work.

· Actual mechanical advantage – The mechanical advantage that a machine provides in a real situation.

· Ideal mechanical advantage – The mechanical advantage that a

 machine would have without friction.

Section 3 – Simple machines

· There are six basic machines:

1. inclined plane

2. the wedge

3. the screw

4. the lever

5. the wheel and axel

6. the pulley

· A compound machine is a machine that is made from two or more simple machines.

o Gears form a compound machine with one wheel and axel linked to another wheel and axel.

o When you divide the radius of a wheel by the radius of an axel, you are calculating the ideal mechanical advantage of a wheel and axel.

o A screw is an inclined plane wrapped around a central cylinder to form a spiral.

o An axe, a zipper, and a front tooth are examples of a simple machine called a wedge.

o When you bend your arm at the elbow, the fulcrum is at your elbow.

Key Terms

· Inclines plane – A flat surface with one end higher than the other.

· Wedge – An inclined plane that is able to move.

· Screw – An inclined plane wrapped around a central cylinder to form a spiral.

· Lever – A rigid object that pivots about a fixed point.

· Fulcrum –The fixed point around which a lever pivots.

· Wheel and axel – Two circular or cylindrical objects that are fastened together and rotate about a common axis.

· Pulley – A grooved wheel around which is wrapped a rope, chain or cable.

· Compound machine – A device that combines two or more simple machines.

· Gears – Two or more wheels linked by interlocking teeth.

Section 4 – Machine is the human body

Key Ideas

· Most of the machines in your body are levers that consist of bones with muscles attached to them.

· When you bite into something, your front teeth use the principle of the wedge

Key terms

· Tendon – A band of connective tissue that attaches a muscle to a bone.