Work and Machines Chapter 8

The Scientific Meaning of Work Answer: No mental effort might seem like a lot of work, but you are not using a force to move an object. Definition: work occurs when a force causes an object to move in the direction of the force. Use the definition above, is doing homework, work?? http://www.youtube.com/watch?v=pbsSiFgW-Xw&feature=related

Working Hard, or Hardly Working? Applying a force doesn’t always result in work being done. For example, pushing a car that won’t budge. If you can’t get it to move you haven’t done any work!

Work continued Force and motion in the same direction: For work to be done, the object must move in the same direction as the force. For example if you are carrying a heavy suitcase through the airport it is not considered work because the direction of force is upward, and the direction of motion is forward!

Work or Not Work?

Calculating Work Work = force x distance W= F x d If a man applies a force of 500 N to push a truck 100 m down the street. How much work does he do?

W = F x d Answer: W = 500 N x 100 m W = 50,000 J The unit used to express work is the newton-meter, but is more commonly called the joule (J). http://www.youtube.com/watch?v=IvcOYOO0Fvw&feature=related

Nerdy Science Joke Break… Did you hear about the criminals who never had to do any work? Answer: They were joule thieves.

Power Power is the rate at which work is done. It is how fast work happens! P = W / t Power = Work time The unit used to express power is joules per second (J/s), which is more simply called the watt (W). For example if you do 50 J of work in 5 seconds, your power is 10 J/s or 10 W.

Try it… Calculate Power: What is the power of a small motor that can do 500 J of work in 25 seconds? http://www.youtube.com/watch?v=5EsMmdaYClQ

Nerdy Science Joke… Q: “What is the unit of power?” A: “Watt.” Q: “I said, what is the unit of power?” A: “WATT!” Q: “I SAID…”

Quiz 8.1 1. In which situation do you do more work? W = F x d a. You lift a 75 N bowling ball 2 m off the floor. b. You lift two 50 N bowling balls 1 m off the floor. 2. The rate at which work is done is: a. Joule b. Time c. Power d. Work 3. Power is: a. Work/time b. time/Work c. Force x distance c. Work/Force

Quiz 8.1 continued… 4. The unit used for Power is: a. newton-meter b. Joule c. meter per second d. Watt 5. You push a 100 N box 3 m across the floor. How much work has been done? a. 33.3 J b. 300 J c. 33.3 W c. 300 W

Machines A machine is a device that helps make work easier by changing the size or direction of a force. The work that you do on a machine is called work input. Called input force The work done by the machine is called work output. Called output force

The force-distance trade off W = 450 N x 1 m = 450 J W = 150 N x 3 m = 450 J

Mechanical Advantage Mechanical Advantage tells you how many times the machine multiplies force. MA = output force input force

Finding Advantage You apply 200 N to a machine, and the machine applies 2,000 N to an object. What is the mechanical advantage? Remember MA = output force/input force Answer: MA = 2000 N = 10 200 N http://www.youtube.com/watch?v=oWiZ_5qvs7I&feature=related

Mechanical Efficiency Is a comparison of a machine’s work output with the work input. Mechanical Efficiency = work output X 100 work input

Simple Machines

Levers A lever is a simple machine consisting of a bar that pivots at a fixed point, called a fulcrum. There are 3 types of levers. http://www.youtube.com/watch?v=PW7ztbwJKBk&feature=related http://www.youtube.com/watch?v=wV1pYkTtsxg&feature=related

First Class Levers The fulcrum is between the input force and the load

Second Class Levers The load is between the fulcrum and the input force.

Third Class Levers The input force is between the fulcrum and the load.

Inclined Planes A simple machine that is a straight, slanted surface. An inclined plane allows you to apply a smaller force over a greater distance. http://www.youtube.com/watch?v=pG_kT565-XQ&feature=related

Wedges A wedge is a double inclined plane that moves. Wedges are often used to cut materials. They allow you to exert your force over an increased distance.

Screws An inclined plane that is wrapped in a spiral. When you turn a screw, you exert a small input force over a large turning distance, but the screw itself doesn’t move very far.

Mechanical Advantage of Screws The threads on the left screw are closer together and wrap more times around, so that screw has a greater mechanical advantage that the one on the right.

Wheel and Axle A simple machine consisting of two circular objects of different sizes. Examples include a door knob, a crank, a fishing reel… http://www.youtube.com/watch?v=C-UXry7OiXM&feature=related

Pulleys A simple machine consisting of a grooved wheel that holds a rope or a cable. A load is attached to one end of the rope, and input force is applied to the other end.

Fixed Pulleys only change the direction of the force. Movable Pulleys are attached to the object that is being moved.

Compound Machines Machines that are made of two or more simple machines.