Simple Machines

Before we begin, let’s review what work is… What is work? In which of the following situations is work being done? 1) Mr. Wood gives notes to his class for 40 minutes. 2) Mr. Barita lifts 350 pounds above his head. 3) Ms. Kennedy carries her cat from room to room. 4) Mr. Morrish pushes a laptop cart down the hall. 5) Mrs. Saunders lifts a piece of cake to put it in her mouth. 6) Mrs. Fisher picks her baby up out of his car seat. 7) Mrs. Walker dances around with a book above her head. 8) Mrs. Zanoun throws her computer out of the classroom window.

Now let’s take a look at power. What is power? 2 physical science students, Mike and Kymora, are in the weightlifting room. Kymora lifts the 50 N barbell over her head (approximately.60 m) 10 times in one minute; Mike lifts the 50 N barbell the same distance over his head 10 times in 10 seconds. Which student does the most work? (W = F × d) Which student delivers the most power? (P = W/t) Be ready to explain your answers.

Machines Sometimes you need a little help to do work. That’s where machines come in. A machine is a device that helps make work easier by changing the size or direction of a force. Suppose you need to get the lid off of a can of paint. How would you do this using a screwdriver? This example illustrates that 2 kinds of work are always involved when a machine is used – the work done on the machine, which is called work input, and the work the machine does on another object, which is called work output.

Machines Do NOT Save Work Do machines help you do less work? NO!!! Work output can NEVER be greater than work input! Why? Some of the work done by the machine is used to overcome the friction created by the use of the machine. If a machine decreases the amount of force necessary, the distance over which the force is exerted increases. (Example: a ramp) If a machine increases the amount of force necessary, the distance over which the force is exerted decreases. (Example: a hammer)

Mechanical Advantage A machine’s mechanical advantage tells you how many times the machine multiplies the force. In other words, it compares the input force with the output force. You can find mechanical advantage using the following equation: Mechanical Advantage (MA) = output force input force *****The larger the mechanical advantage, the easier a machine makes your work!*****

Finding the Mechanical Advantage 1) You apply 200 N to a machine, and the machine applies 2,000 N to an object. What is the MA? 2) You apply 10 N to a machine, and the machine applies 10 N to another object. What is the MA? 3) Which of the following makes work easier to do? a) a machine with a MA of 15 b) a machine to which you apply 15 N and that exerts 255 N

Mechanical Efficiency We already talked about how some of the work done by a machine is used to overcome the friction created by using the machine. The less work a machine has to do to overcome friction, the more efficient it is. Mechanical efficiency is a comparison of a machine’s work output with the work input. Mechanical efficiency = work output work input **Efficiency is expressed as a %** It tells you what % of the work input gets converted into work output × 100

Types of Machines There are 6 simple machines: – Lever – Inclined plane – Wedge – Screw – Wheel and axle – Pulley

Levers A lever consists of a bar that pivots at a fixed point, called a fulcrum. Examples: a hammer, a wheelbarrow, and a bottle opener To increase the mechanical advantage of a lever, increase the length of the effort arm.

Inclined Planes An inclined plane is a straight, slanted surface. Example: a ramp The longer the inclined plane is compared with its height, the greater the mechanical advantage. MA = ramp length ÷ ramp height

Wedges A wedge is a double inclined plane that moves. Examples: knife, doorstop, axe head, and chisel The longer and thinner the wedge is, the greater the mechanical advantage.

Screws A screw is an inclined plane that is wrapped in a spiral. The longer the spiral on a screw is, and the closer together the threads are, the greater the screw’s mechanical advantage.

Wheel and axles A wheel and axle consists of 2 circular objects of different sizes. Examples: doorknob, wrench, ferris wheel, and steering wheel The mechanical advantage can be determined by dividing the radius of the wheel by the radius of the axle.

Pulleys A pulley consists of a grooved wheel that holds a rope or a cable. There are 2 kinds of pulleys: fixed and movable Fixed pulleys ONLY CHANGE THE DIRECTION OF A FORCE. A fixed pulley is attached to something that does not move. You pull down on the rope & that lifts the load up. Ex: window blinds & elevators Movable pulleys are attached to the object being moved. It does not change the direction of the force – it increases the output force, but you must exert a force over a greater distance than the load is moved. The more ropes there are, the greater the MA