Work and Simple Machines
What is work? The scientific definition of work is: using a force to move an object a distance Measured in Joules
W=FD Work = Force x Distance Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?
What s work? Which is and which isn t. A scientist delivers a speech to an audience of his peers. A body builder lifts 350 pounds above his head. A mother carries her baby from room to room.
What is Power? *Power is the Amount of Work done in a given amount of time. *Power = Work / Time * Measured in Watts (W) or per second (J/S)
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. Which student does the most work? Which student delivers the most power? Explain your answers.
Ben and Bonnie do the same amount of work; they apply the same force to lift the same barbell the same distance above their heads. Yet, Ben is the most powerful since he does the same work in less time. Power and time are inversely proportional.
2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s 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
2. How much power will it take to move a 10 kg mass at an acceleration of 2 m/s/s 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 Force=10 x 2 Force=20 N Work=Force x Distance Work = 20 x 10 Work = 200 Joules Power = Work/Time Power = 200/5 Power = 40 watts
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 force increasing the distance or speed of a force.
The six simple machines are: 1) Lever 2) Wheel and Axle 3) Pulley 4) Inclined Plane 5) Wedge 6) Screw
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 which is applied to the task). When a machine takes a smaller input force and increases the amount of output force, a mechanical advantage has been produced. If a machine increases an input force of 10 pounds to an output force of 100 pounds, the machine has a mechanical advantage (MA) of 10. MA = output/input
Graphic Organizer Simple Machine Inclined Plane Wedge Definition Drawing Mechanical Advantage Example Screw Lever Wheel & Axle 1 st Class 1 st Class 2 nd Class 2 nd Class 3 rd Class 3 rd Class Pulley
SIMPLE - Simple MACHINES Machines Inclined Plane: An inclined plane is a flat, sloped surface.
Inclined Plane The mechanical advantage of an inclined plane is equal to the length of the slope divided by the height of the inclined plane. While the inclined plane produces a mechanical advantage, it does so by increasing the distance through which the force must move.
Although it takes less force for car A to get to the top of the ramp, all the cars do the same amount of work. A B C
- Simple Machines Wedge A wedge is a device that is thick at one end and tapers to a thin edge at the other end. In other words it uses two combined inclined planes to do work.
- Simple Machines Screws A screw can be thought of as an inclined plane wrapped around a cylinder.
MA of an screw can be calculated by dividing the number of turns per inch.
- Simple Machines Levers A lever is a ridged bar that is free to pivot, or rotate, on a fixed point.
- Simple Machines Levers Levers are classified according to the location of the fulcrum relative to the input and output forces.
- Simple Machines Simple Machines in the Body Most of the machines in your body are levers that consist of bones and muscles.
To find the MA of a lever, divide the output force by the input force, or divide the length of the resistance arm by the length of the effort arm. MA=OF/IF Or MA= RA/EA
- Simple Machines Wheel and Axle A wheel and axle is a simple machine made of two circular or cylindrical objects fastened together that rotate about a common axis. In other words, a set of levers rotating around an axle.
- Simple Machines Wheel and Axle You can find the ideal mechanical advantage of a wheel and axle by dividing the radius of the wheel by the radius of the axle.
- Simple Machines Pulley A pulley is a simple machine made of a grooved wheel with a rope or cable wrapped around it.
- Simple Machines Compound Machines A compound machine is a machine that utilizes two or more simple machines.
- How Machines Do Work Input and Output Work The amount of input work done by the gardener equals the amount of output work done by the shovel.
What is Efficiency? Some output force is lost due to friction. The comparison of work input to work output is called efficiency. No machine has 100 percent efficiency due to friction. Efficiency = Output work/input work X 100%
- How Machines Do Work Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Read and Understand What information have you been given? Input Work (W input ) = 250,000 J Output Work (W output ) = 200,000 J
- How Machines Do Work Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Plan and Solve What quantity are you trying to calculate? The efficiency of the lawn mower = What formula contains the given quantities and the unknown quantity? Efficiency = Output work/input work X 100% Perform the calculation. Efficiency = 200,000 J/250,000 J X 100% Efficiency = 0.8 X 100% = 80% The efficiency of the lawn mower is 80 percent.
- How Machines Do Work Calculating Efficiency You do 250,000 J of work to cut a lawn with a hand mower. If the work done by the mower is 200,000 J, what is the efficiency of the lawn mower? Look Back and Check Does your answer make sense? An efficiency of 80 percent means that 80 out of every 100 J of work went into cutting the lawn. This answer makes sense because most of the input work is converted to output work.
- How Machines Do Work Calculating Efficiency Practice Problem You do 20 J of work while using a hammer. The hammer does 18 J of work on a nail. What is the efficiency of the hammer?
- How Machines Do Work Calculating Efficiency Practice Problem Suppose you left your lawn mower outdoors all winter. Now it s rusty. Of your 250,000 J of work, only 100,000 J go to cutting the lawn. What is the efficiency of the lawn mower now?