Work, Power, and Simple Machines

Physical Science

Work

• Transfer of Energy• Occurs when a force makes an object

move a distance

Work

• Work is done when an object moves in the direction of the force

• Work is not being doing when there is no movement, or when the movement is not in the direction of the force.

Work

1. Pushing on a book forward and the books moves forward.

2. Pulling on a wall and the wall doesn’t move

3. Push on a car forward and it rolls backwards

Work

Work is a product of force & distance•Work (J)•Force (N)•Distance (m)

•1 J = 1 N·m

FW

d÷

Work

1. A man weighing 750 N climbs a flight of stair s 5-m high. How much work did he do?

2. Kayla does 40 J of work while pushing a shopping cart 30 meters. How much force was applied?

3. How much work does a 25N force do to lift a potted plant from the floor to a shelf 1.5m high?

4. Jenny’s bookbag weighs 35.3N; 88.25J of work is done to the bookbag when she lifts it off the floor. How far did she lift the bag?

Work 5. How much work does a crane that lowers 1000N of material

a distance of 150m do?6. How much work is done when a 2N weight is raised a vertical

distance of 1m?7. Calculate the amount of work done when moving a 567N

crate a distance of 20 meters.8. If it took a bulldozer 567.6 joules of work to push a mound of

dirt 30.5 meters, how much force did the bulldozer have to apply?

9. A frontend loader needed to apply 137 N of force to lift a rock. A total of 223 joules of work was done. How far was the rock lifted?

Power

• Power Is the rate of doing work• To increase power, you can

increase the amount of work done in a given time– Work (J)– time (s)– Power (watt, W)– 1 W = 1J/s• Power is the product of work

divided by time

PW

t÷

Power

1. A motorcycle does 2000-J of work in 25 seconds. How much power is produced?

2. Mary produced 20-W of power in 0.5 seconds doing push-ups. How much work was done?

3. Beth moves a 15 N walker (zombie) 20 meters in 10 seconds. How much power was produced?

4. You exert a vertical force of 72N to lift a box to a height of 1.0m in a time of 2s. How much power is used to lift the box?

5. Your family is moving into a new apartment. While lifting a box 1.5m straight up to put it in on a truck, you exert an upward force of 200N for 1.0s. How much power is used if the upward force is 15.0N and you do the work in 2.0s?

Power6. You lift a book from the floor to a bookshelf 1.0m above the

ground. How much power is used if the upward force is 15.0N and you do the work in 2.0s?

7. You lift a large bag of flour from the floor to a high counter, doing 100 Joules of work in 2s. How much power do you use to lift the bag of flour?

8. A machine produces 4000 Joules of work in 5 seconds. How much power does the machine produce?

9. A 480N student climbs the stairs from the second floor to the fourth floor, 16m above, in 22s. How much power did she generate?

10. A 14kg mass is lifted a height of 6.0m in 3 s. How much power is generated?

Simple Machines

• Simple machines are devices that makes doing work easier

Simple Machines

• Increase Force – (ex: car jacks)

• Increase the distance a force is applied – (ex: using a ramp)

• Changing the direction of an applied force – (ex: ax blade)

Simple Machines

• Machines that does work with only one movement of the machine

Lever

Pulley

Wheel and Axle

Wedge Screw

Inclined Plane

Simple Machines

1. Lever: A bar that is free to pivot or turn around a fixed point (ex: teeter totter)

2. Wheel & Axle: consists of two disks or cylinders, each one with a difference radius (door knob)

Simple Machines

3. Inclined Plane: A sloping surface (ex: ramp)

4. Wedge: An inclined plane with one or two sloping sides (ex: knife)

Simple Machines

5. Screw: An inclined plane wrapped in a spiral around a cylindrical post (ex: jar lids)

6. Pulley: Grooved wheel with a rope, chain, or cable running along the groove (ex: wishing well)

Compounds Machines

• When two or more simple machines operate together

• Example: Can Opener– Wheel & Axel– Level– Wedge

Mechanical Advantage & Efficiency

J. McLeod

Input Force & Output Force

Input Force• The force you

exert on a machine

• Symbol = Fin

Output Force• Force exerted

by the machine• Symbol = Fout

Input Work & Output Work

• Input Work–Work done by

you on a machine–Symbol = Win

• Output Work–Work done by

the machine–Symbol = Wout

Input & Output Work: The Relationship

• Input work equals Output work in an ideal machines

• Win = Wout

• Why?–Law of

conservation of Energy• Energy is not

created nor destroyed

Mechanical Advantage

• Is the number of times that a machine increases an input force

• Two Versions– Actual Mechanical

Advantage– Ideal Mechanical

Advantage

Actual Mechanical Advantage (AMA)

• Determined by measuring the actual forces on a machines

• Ratio of the output force to the input force

Actual Mechanical Advantage (AMA)

• Determined by measuring the actual forces on a machine

• Ratio of output force to the input force• FR= Resistance Force (output force)

• FE = Effort Force (input force)

• Force (N)

1. What is the actual mechanical advantage of a machine who’s input force is 30-N but produces an output force of 90-N?

2. You test a machine and find that it exerts a force of 10 N for each 2 N of force you exert operating the machine. What is the actual mechanical advantage of the machine?

Actual Mechanical Advantage (AMA)

Ideal Mechanical Advantage (IMA)

• Is the mechanical advantage in the absence of friction

• Because friction is always present, the actual mechanical advantage of a machine is always less than the Ideal

• dE = displacement of effort force (input distance)

• dR = displacement of resistant force (output distance)

• Displacement (m)

Ideal Mechanical Advantage (IMA)1. A woman drives a car up onto wheel ramps to perform some repairs. If

she drives a distance of 1.8 meters along the ramp to raise the car 0.3 meter, what is the IMA?

2. A construction worker moves a crowbar through a distance of 4 meters to lift a load 0.5 meter off the ground. What is the IMA of the crowbar?

3. A student working in a grocery store after school pushes several grocery carts together along the ramp. The ramp is 3m long and rises 0.5m. What is the IMA of the ramp?

4. A construction worker moves a crowbar through a distance of 0.50m to lift a load 0.05m off of the ground. What is the IMA of the crowbar?

5. The input distance of a simple machine is 2.5m. If the output distance of the machine is 1.0m, what is the IMA?

What is efficiency?• The percentage of

work input that becomes work output

• Because there is always some friction, the efficiency of any machine is always less than 100 percent

Efficiency Formula

• Wout = work output (J)

• Win = work input (J)

Efficiency Formula Practice1. You have just designed a machine that

used 1000 J of work from a motor for every 800 J of useful work the machine supplies. What is the efficiency of your machine?

2. Determine the efficiency of a machine that requires a work input of 224 foot pounds for a work output of 200 foot pounds.