St. 15, 16, 17: Work, Power, and Simple Machines

What is Work? Work is a transfer of energy when a force makes an object move. Movement must be in the direction of the force.

1. Work = __________ x _____________ Finding Work 1.  Work = __________ x _____________ 2.  Work is measure in newton-meters, also called the ___________________

Calculating Work W = Fd Quick Practice: 1.  A couch is pushed with a force of 40 N and moves a distance of 2 m.  How much work was done on the couch? 2.  A lawn mower is pushed with a force of 100 N, if 12,000 J of work is being down to mow the lawn, what is the total distance covered by the mower? 3. How much work is done in holding a thousand pound barbell over your head?

More practice A teacher applies a force to a wall and becomes exhausted? No, no displacement A book falls off a table and free falls to the ground Yes the force of gravity causes the book to fall A waiter carries a tray full of meals above his head by one arm across the room The force did not cause the displacement A rocket accelerates through space Yes the expelled gases push the rocket through space

Simple Machines Machines make work easier to do!! Change the size of the force needed Change the distance Change direction Some further examples

You can reduce the amount of force needed by increasing the distance You can reduce the amount of force needed by increasing the distance.   W = force x distance W is the same in both pictures (225J), though it is easier to move the box using a ramp

Last set of problems A desk exerts an upward force to support a computer resting on it. Does this force do work? You carry two heavy bags of groceries upstairs to your kitchen. Will you do more work on the bags if you carry them one at a time? How much work is required to lift a 25kg plant from the floor to the shelf 1.0m high?

What is Power? Power measures the time it takes to do work. Unit: Watt Suppose you and your friend move boxes that weigh 50 kg each. You both move the boxes 10m but your friends moves it in 5 seconds and you move it in 3 seconds. You have both done the same amount of work, but you had more POWER. Power = Rate of doing work Power (watts) = work (joules) / time (sec) P = W / t

P = W / t 1. In lifting a brick, 50 J of work are done. How much power is needed if the brick is lifted for 5 seconds. 2. If a runner's power is 100 W, how much work is done by the runner in 20 seconds.

3.  If a runner's power is 100 W, how much work is done by the runner in 2 minutes? 4.  If a force of 100N is used to push a desk 10m in 5 seconds. How much power is needed?  

Two physics students, Will N Two physics students, Will N. Andable and Ben Pumpiniron, are in the weightlifting room. Will lifts the 100-pound barbell over his head 10 times in one minute; Ben lifts the 100-pound barbell over his head 10 times in 10 seconds. Which student does the most work? ______________ Which student delivers the most power? ______________ Explain your answers.

During a physics lab, Jack and Jill ran up a hill During a physics lab, Jack and Jill ran up a hill. Jack is twice as massive as Jill; yet Jill ascends the same distance in half the time. Who did the most work? ______________ Who delivered the most power? ______________ Explain your answers.

Bellringer 1. How much work is done on a 20kg box to lift the box 2 meters? 2. A crane lifts a steel girder in 45 seconds. It required the crane to do 875,000 J of work. How much power did the crane use? 3. A person lifts a 200 N object a distance of 3 m in 5 seconds. How much work did the person do? How much power did the person use? 4. A person is leaning against a wall for 20 minutes. How much work was done?

What Is a Machine? A machine is any device that makes doing work easier. Examples of machines: Lever Wheel and Axel Screw Incline Plane Pulley Wedge

Input and Output Forces Input force Fin (Effort) = force applied to the machine Output force Fout (Resistance) = force applied by the machine Fin Fout

Identify the input and output forces

Ideal Machines Mechanical Advantage = output force (newtons) input force (newtons) You use a pair of pliers to crack a pecan. It takes 1200 N of force to crack the pecan but you only exert 400 N of effort force. What is the mechanical advantage?

Efficiency Efficiency = how much work put into the machine is changed into useful energy, most machines can be made more efficient by reducing friction   Eff: work output/ work input x 100% You have just designed a machine that uses 1000J of work from a motor for every 800J of useful work the machine supplies. Eff: