Work and Simple Machines

Section 8.1 Objectives Determine when work is being done on an object. Calculate the amount of work done on an object. Calculate the amount of power needed to perform a certain task. Explain the difference between work and power. Terms to Learn: Work, Joule, Power and Watt

Work Recall :an object begins moving only when an unbalanced force acts on it. Work = product of force and distance. Work occurs when an objects moves in the same direction of the force. 2 things must happen : Object must move. Motion must bin same direction as the force.

Work or Not? Use the scientific definition, to determine what is work and what is not? a teacher lecturing to her class: Work - Not Work ? WHY? Not Work --- motion is not the same direction as the force. a mouse pushing a piece of cheese with its nose across the floor: Work - Not Work ? WHY? Work --- motion is in the direction of the force.

FORCE EXERTED BY THE MOUSE   MOTION of the CHEESE   The mouse is using a force to move the cheese a distance; Both the force and the motion are in the same direction.

Not Work because he has not made the wall move Work or Not? Ex: If a student pushes a wall with all of his strength? A student carrying a book? Not Work because he has not made the wall move (if the wall does move, then work has been done) NOT WORK because the force and motion are NOT in the same direction.

Work Work = Force x distance W = F x d SI unit for Force = Newtons (N) SI unit for Distance = Meters (m) SI unit for Work = Joules (j) Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

Same work, different force Force = 450 N Distance = 1 m Force = 150 N Distance = 3 m W = F d W = F d

Power- How fast work is done! Power is the rate at which work is done. Doing work at a higher rate requires more Power. To increase power, you can: increase the amount of work done in a given time, OR you can do a given amount of work in less time. Power = Work OR Force x Distance Time Time SI unit for power = Watt (w) SI unit for work = Joules (J) SI unit for time = Seconds (s)

Math Practice If it takes you 10 seconds to do 150 joules of work on a box to move it up a ramp, what is your power output? A light bulb is on for 12 s, and during that time it uses 1,200 J of electrical energy. What is the wattage (power) of the light bulb?

Check for Understanding 1.Two physics students, Ben and Bonnie, are in the weightlifting room. Bonnie lifts the 50 kg barbell over her head (approximately .60 m) 10 times in one minute; Ben lifts the 50 kg barbell the same distance over his head 10 times in 10 seconds. Explain your answers. Which student does the most work? Which student delivers the most power? They do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads. Ben is the most powerful since he does the same work in less time.

Force=Mass x Acceleration 2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s2 a distance of 10 meters in 5 seconds? This problem requires you to use the formulas for force, work, and power all in the correct order. Force=Mass x Acceleration Work=Force x Distance Power = Work/Time 3. If an identical stack of books is lifted the same distance and one person does the job twice as fast, which of the following is done twice as much? a. Work output b. Work input d. Efficiency Force=10 kg x 2 m/s2 Force=20 N Work = 20 N x 10 m Work = 200 Joules Power = 200 J / 5 s Power = 40 watts c. Power

Section 8.1 Objectives Please answer below Determine when work is being done on an object. How do you calculate the amount of work done on an object. How do you calculate the amount of power needed to perform a certain task. Explain the difference between work and power.

Section 8.3 Objectives Identify and give examples of the six types of simple machines. Analyze the mechanical advantage provided by each simple machine. Identify the simple machines that make up a compound machine. Terms to learn: Lever, pulley, wheel and axle, inclined plane, wedge, screw and compound machine.

Simple Machines Ancient people invented simple machines that would help them overcome resistive forces and allow them to do the desired work against those forces.

Simple Machines A machine is a device that helps make work easier to perform by accomplishing one or more of the following functions: transferring a force from one place to another, changing the direction of a force, increasing the amount of a force increasing the distance or speed of a force.

Simple Machines The six simple machines are: 1. Lever 2. Wheel and Axle 3. Pulley 4. Inclined Plane 5. Wedge 6. Screw

The Lever (1ST, 2ND, OR 3RD CLASS) A lever is a rigid bar that rotates around a fixed point called the fulcrum. Fulcrum – a fixed point The bar may be either straight or curved. In use, a lever has both an effort (or applied) force and a load (resistant force).

TIP: A First class lever has the Fulcrum in the center! I = input force or effort O = output force/ weight F = Fulcrum – fulcrum is between the input force and output force Examples: See-saw, scissors, tongs. TIP: A First class lever has the Fulcrum in the center!

TIP: A Second class lever has the Output in the center! – output force located between the input force and fulcrum. Example: Wheelbarrow, bottle opener, nutcracker, crowbar I = input force or effort O = output force/ weight F = Fulcrum TIP: A Second class lever has the Output in the center!

TIP: A Third class lever has the input in the center! I = input force or effort O = output force/ weight F = Fulcrum – input force is between the fulcrum and output force. Provide a mechanical advantage that is always less than 1. Examples: Hockey stick, golf club, screwdriver opening paint can, broom, tweezers, hammer, stapler TIP: A Third class lever has the input in the center!

Wheel and Axle a simple machine consisting of a large wheel rigidly secured to a smaller wheel or shaft, called an axle. When either the wheel or axle turns, the other part also turns. One full revolution of either part causes one full revolution of the other part.

Inclined Plane a slanted surface along which a force moves an object/ weight from a lower to higher elevation. Ramps make lifting a heavy object easier because less force is needed to move the object over a longer distance.

Wedges and Screws A wedge is a V-shaped object whose sides are two inclined planes sloped towards each other. A screw is an inclined plane wrapped around a cylinder.

Pulley a simple machine that consists of a rope that fits into a groove in a wheel. The mechanical advantage of a moveable pulley is equal to the number of ropes that support the moveable pulley.

Made with more than one machine Compound Machines - a combination of two or more simple machines that operate together.

Section 8.3 Objectives Please answer these on your objectives sheet Identify and give examples of the six types of simple machines. Analyze the mechanical advantage provided by each simple machine. Identify the simple machines that make up a compound machine.

USE THE FOLLOWING SLIDES FOR WHEN SECTION 8-2 (in/out/eff) is being addressed

Section 8.2 Objectives Explain how a machine makes work easier. Describe and give examples of the force-distance trade off that occurs when a machine is used. What is the formula to calculate mechanical advantage. What is the formula to calculate mechanical efficiency. Explain why machines are not 100% efficient. Terms to learn Machine, work input, work output, mechanical advantage, mechanical efficiency

Mechanical Advantage It is useful to think about a machine in terms of the input force (the force you apply) and the output force (force from the machine to perform a task). When a machine takes a small input force and increases the amount of the output force, a mechanical advantage has been produced.

Mechanical Advantage The mechanical advantage is the number of times the machine increases or multiplies an input force. Mechanical Advantage = Output Force Input Force The ideal mechanical advantage is the advantage in the absence of friction. Never happens because all objects experience friction.

Mechanical Efficiency Mechanical Efficiency – the percentage of work input that becomes work output. Can never be 100% due to friction ME = Work Output x 100 Work Input

Section 8.2 Objectives Please answer the following on your objective sheet Explain how a machine makes work easier. Describe and give examples of the force-distance trade off that occurs when a machine is used. What is the formula to calculate mechanical advantage. What is the formula to calculate mechanical efficiency. Explain why machines are not 100% efficient.