Work and MachinesChapter 14

Work and Power

• Work – occurs when a force causes an object to move in the direction of the force.

• How do you know if work is being done?1. The object moves as a force is applied. 2. The direction of the object’s motion is the same

as the direction of the force being applied.

Example:

You exert an upward force on the suitcase. But the motion of the suitcase is forward. Therefore you are not doing “work” on the suitcase.

Question:

• If you pulled a wheeled suitcase instead of carrying it, would you be doing work on the suitcase? Why or why not?

Answer:

• Pulling a wheeled suitcase is doing work because the force applied and the motion of the suitcase are in the same direction.

Calculating WORK

• Work = Force x Distance

• Remember:– Force in measured in Newtons– The SI unit for distance are meters– The units for work are newton-meters or joules (J)

Work depends on force and distance

• The force needed to lift an object is equal to the gravitational force on the object – in other words, the objects weight.

Cont’d

• Increasing the amount of force increases the amount of work done.

Cont’d

• Increasing the distance also increases the amount of work done.

POWER• Def: the rate at which work is done.

– Is increased as more work is done in a given amount of time.

• Power= Worktime

• Remember:– Unit for work =Joule– Unit for time= second– J/s = Watts (W)

What is a machine?

• Def: a device that helps make work easier by changing the size or direction of a force.

Input and Output

• Work Input – the work that you do to a machine.

• Work Output – the work done by the machine.

INPUT FORCE

OUTPUT FORCE

Do machines save work?

• NO! Machines make work easier but do not get rid of the work to be done.

Mechanical Advantage

• Def: tells you how many times the machine multiplies force. – Compares the input and output forces

Mechanical Advantage (MA)= output force input force

Example:

• MA = 500 N 50 NMA = 10

INPUT FORCE= 50 N

OUTPUT FORCE= 500 N

Cont’d

• Because the MA of the hand truck is 10, that means that the output force is 10 times bigger than the input force.

• The larger the MA , the easier a machine makes your work.– BUT as your MA increases, the distance that the

output force moves the object decreases.

Mechanical Efficiency

X 100

REVIEW

1. Work is done on a ball when a pitcher throws it. Is the pitcher still doing work on the ball as it flies through the air? Explain.

• No, the pitcher is no longer doing work on the ball as it flies through the air because he is no longer exerting a force on it.– However, work is being done on the ball by the Earth,

which exerts a force on the ball and pulls it back toward the ground.

Review:

2. Explain the difference between work and power?

• Work occurs when a force causes an object to move in the direction of the force.

• Power is the rate at which work is done.

*the more work you do the more power you get.

Review:

3. You lift a chair that weighs 50 N to a height of 0.5 m and carry it 10 m across the room.

*How much work do you do on the chair and when does the work occur?

• Work is done on the chair only when it is picked up, not when it is carried across the room.

W=50 N x 0.5 mW= 25 J

Review

4. Explain how using a ramp makes work easier

• Using a ramp makes work easier because it allows you to apply a smaller input force than you would have to apply when lifting a load straight up.

• However, the smaller force has to be exerted over a longer distance.

Review:

5. Why can’t a machine be 100% efficient?

• A machine can’t be 100% efficient because some of the work input is used to overcome friction.

• Therefore, work input is always greater than work output.

Review:

6. Suppose you exert 15 N on a machine, and the machine exerts 300 N on another object.

What is the machine’s mechanical advantage?

MA = 300 N 15 N

MA = 20

The Types of Simple Machines

• There are 6 types of simple machines:

1.Levers2.Inclined Planes3.Wedges4.Screws5.Wheel and Axle6.Pulleys

LEVERS

• Def: a simple machine consisting of a bar that pivots at a fixed point, called a fulcrum.

• Levers are used to apply force to a load.

First Class Levers

• The fulcrum is between the input force and the load.

• Ex: hammer pulling a nail out, seesaw

Second Class Levers

• The load is between the fulcrum and the input force.

• Ex: wheelbarrow, bottle cap opener

Third Class Levers

• The input force is between the fulcrum and the load.

• Ex: using dumb bell weights, Using a hammer

Mechanical Advantage of Levers

MA = length of input arm / length of output arm

Inclined Planes

• Def: a simple machine that is a straight, slanted surface.

• Ex: a ramp, a door stop

Mechanical Advantage of Inclined Planes

MA = length of slope / height of slope

Wedges

• Def: a double inclined plane that moves.

• Ex: knives, an axe blade

Screws

• Def: an inclined plane that is wrapped in a spiral.

• Ex. screw

Wheel & Axle

• Def: a simple machine consisting of two circular objects of different sizes.

• Ex: Ferris wheels, the steering wheel on a car

Mechanical Advantage of Wheel and Axle

MA = radius of wheel / radius of axle

Pulleys

• Def: a simple machine consisting of a grooved wheel that holds a cable or rope.

• Two types:– Movable– Fixed

*Many combined pulleys are called a block and tackle

Fixed Pulley

LOADEFFORT FORCE

Movable Pulley

EFFORTFORCE

LOAD

LOAD

EFFORTFORCE

Compound Machines

• Def: machines that are made of two or more simple machines.

• Ex: can opener (wedge, wheel & axle, 2nd-class lever), scissors (wedge, and two 1st –class levers

Review:

1. Identify the simple machines that make up tweezers and nail clippers.

2. What would you call this type of machine?

Answers:

1. Each side of the tweezers is a 3rd-class lever. The sharpened edges of the nail clippers are wedges, and the arm that activates the clipper is a 2nd-class lever.

2. Compound Machine

Review:

3. What is the unit for work?

The joule (J)

Review:

4. Which of the following is NOT a simple machine?

a. A water faucet handleb. A jar lidc. A can openerd. A seesaw

* The can opener – it is a compound machine

Review:

5. A machine can increase:a. Distance and the expense of forceb. Force at the expense of distancec. Neither distance nor forced. Both (a) and (b)

*d. Both (a) and (b)

Review:

6. What is the unit for power?

The watt (W)

Review:

7. What is power?

How fast work is being done.