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 Pushing on a book forward and the books moves forward. Pulling on a wall and the wall doesn’t move Push on a car forward and it rolls backwards

W = Fd W d Work ÷ F Work is a product of force & distance Work (J) Force (N) Distance (m) 1 J = 1 N·m W = Fd F W d ÷

Work A man weighing 750 N climbs a flight of stair s 5-m high. How much work did he do? Kayla does 40 J of work while pushing a shopping cart 30 meters. How much force was applied? How much work does a 25N force do to lift a potted plant from the floor to a shelf 1.5m high? 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 How much work does a crane that lowers 1000N of material a distance of 150m do? How much work is done when a 2N weight is raised a vertical distance of 1m? Calculate the amount of work done when moving a 567N crate a distance of 20 meters. 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? 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?

P = W/t P = Fd/t W t Power ÷ P 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 P W t ÷ P = Fd/t

Power A motorcycle does 2000-J of work in 25 seconds. How much power is produced? Mary produced 20-W of power in 0.5 seconds doing push-ups. How much work was done? Beth moves a 15 N walker (zombie) 20 meters in 10 seconds. How much power was produced? 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? 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?

Power 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? 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? A machine produces 4000 Joules of work in 5 seconds. How much power does the machine produce? A 480N student climbs the stairs from the second floor to the fourth floor, 16m above, in 22s. How much power did she generate? 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 Inclined Plane Wheel and Axle Lever Pulley Wedge Screw

Simple Machines Lever: A bar that is free to pivot or turn around a fixed point (ex: teeter totter) Wheel & Axle: consists of two disks or cylinders, each one with a difference radius (door knob)

Simple Machines Inclined Plane: A sloping surface (ex: ramp) Wedge: An inclined plane with one or two sloping sides (ex: knife)

Simple Machines Screw: An inclined plane wrapped in a spiral around a cylindrical post (ex: jar lids) 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 The force you exert on a machine Symbol = Fin Output Force Force exerted by the machine Symbol = Fout

Input Work & Output 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) AMA = FR/FE

Actual Mechanical Advantage (AMA) What is the actual mechanical advantage of a machine who’s input force is 30-N but produces an output force of 90-N? 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?

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) IMA = dE/dR

Ideal Mechanical Advantage (IMA) 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? 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? 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? 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? 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 Practice 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? Determine the efficiency of a machine that requires a work input of 224 foot pounds for a work output of 200 foot pounds.