Work and Machines Chapter 14

Work and Power Work – occurs when a force causes an object to move in the direction of the force. How do you know if work is being done? The object moves as a force is applied. The direction of the object’s motion is the same as the direction of the force being applied.

Example: You exert an upward force on the suitcase. But the motion of the suitcase is forward. Therefore you are not doing “work” on the suitcase.

Question: If you pulled a wheeled suitcase instead of carrying it, would you be doing work on the suitcase? Why or why not?

Answer: Pulling a wheeled suitcase is doing work because the force applied and the motion of the suitcase are in the same direction.

Calculating WORK Work = Force x Distance Remember: Force in measured in Newtons The SI unit for distance are meters The units for work are newton-meters or joules (J)

Work depends on force and distance The force needed to lift an object is equal to the gravitational force on the object – in other words, the objects weight.

Cont’d Increasing the amount of force increases the amount of work done.

Cont’d Increasing the distance also increases the amount of work done.

POWER Def: the rate at which work is done. Power= Work time Remember: Is increased as more work is done in a given amount of time. Power= Work time Remember: Unit for work =Joule Unit for time= second J/s = Watts (W)

What is a machine? Def: a device that helps make work easier by changing the size or direction of a force.

Input and Output Work Input – the work that you do to a machine. Work Output – the work done by the machine. OUTPUT FORCE INPUT FORCE

Do machines save work? NO! Machines make work easier but do not get rid of the work to be done.

Mechanical Advantage Def: tells you how many times the machine multiplies force. Compares the input and output forces Mechanical Advantage (MA)= output force input force

Example: MA = 500 N 50 N MA = 10 OUTPUT FORCE = 500 N INPUT FORCE

Cont’d Because the MA of the hand truck is 10, that means that the output force is 10 times bigger than the input force. The larger the MA , the easier a machine makes your work. BUT as your MA increases, the distance that the output force moves the object decreases.

Mechanical Efficiency Def: a comparison of a machine’s work output with the work input. Mechanical Efficiency= work output work input *the 100 means that the answer is a percentage. X 100

REVIEW Work is done on a ball when a pitcher throws it. Is the pitcher still doing work on the ball as it flies through the air? Explain. No, the pitcher is no longer doing work on the ball as it flies through the air because he is no longer exerting a force on it. However, work is being done on the ball by the Earth, which exerts a force on the ball and pulls it back toward the ground.

Review: 2. Explain the difference between work and power? Work occurs when a force causes an object to move in the direction of the force. Power is the rate at which work is done. *the more work you do the more power you get.

Review: 3. You lift a chair that weighs 50 N to a height of 0.5 m and carry it 10 m across the room. *How much work do you do on the chair and when does the work occur? Work is done on the chair only when it is picked up, not when it is carried across the room. W=50 N x 0.5 m W= 25 J

Review 4. Explain how using a ramp makes work easier Using a ramp makes work easier because it allows you to apply a smaller input force than you would have to apply when lifting a load straight up. However, the smaller force has to be exerted over a longer distance.

Review: 5. Why can’t a machine be 100% efficient? A machine can’t be 100% efficient because some of the work input is used to overcome friction. Therefore, work input is always greater than work output.

Review: 6. Suppose you exert 15 N on a machine, and the machine exerts 300 N on another object. What is the machine’s mechanical advantage? MA = 300 N 15 N MA = 20

The Types of Simple Machines There are 6 types of simple machines: Levers Inclined Planes Wedges Screws Wheel and Axle Pulleys

LEVERS Def: a simple machine consisting of a bar that pivots at a fixed point, called a fulcrum. Levers are used to apply force to a load.

First Class Levers The fulcrum is between the input force and the load. Ex: hammer pulling a nail out, seesaw

Second Class Levers The load is between the fulcrum and the input force. Ex: wheelbarrow, bottle cap opener

Third Class Levers The input force is between the fulcrum and the load. Ex: using dumb bell weights, Using a hammer

Mechanical Advantage of Levers MA = length of input arm / length of output arm

Inclined Planes Def: a simple machine that is a straight, slanted surface. Ex: a ramp, a door stop

Mechanical Advantage of Inclined Planes MA = length of slope / height of slope

Wedges Def: a double inclined plane that moves. Ex: knives, an axe blade

Screws Def: an inclined plane that is wrapped in a spiral. Ex. screw

Wheel & Axle Def: a simple machine consisting of two circular objects of different sizes. Ex: Ferris wheels, the steering wheel on a car

Mechanical Advantage of Wheel and Axle MA = radius of wheel / radius of axle

Pulleys Def: a simple machine consisting of a grooved wheel that holds a cable or rope. Two types: Movable Fixed *Many combined pulleys are called a block and tackle

Fixed Pulley EFFORT FORCE LOAD

Movable Pulley EFFORT FORCE LOAD EFFORT FORCE LOAD

Compound Machines Def: machines that are made of two or more simple machines. Ex: can opener (wedge, wheel & axle, 2nd-class lever), scissors (wedge, and two 1st –class levers

Review: Identify the simple machines that make up tweezers and nail clippers. 2. What would you call this type of machine?

Answers: Each side of the tweezers is a 3rd-class lever. The sharpened edges of the nail clippers are wedges, and the arm that activates the clipper is a 2nd-class lever. 2. Compound Machine

Review: 3. What is the unit for work? The joule (J)

Review: 4. Which of the following is NOT a simple machine? A water faucet handle b. A jar lid c. A can opener d. A seesaw * The can opener – it is a compound machine

Review: 5. A machine can increase: Distance and the expense of force b. Force at the expense of distance c. Neither distance nor force d. Both (a) and (b) *d. Both (a) and (b)

Review: 6. What is the unit for power? The watt (W)

Review: 7. What is power? How fast work is being done.