Physical Science Ch 5 (Part II) : Simple Machines

A pulley is a grooved wheel with a rope or chain in the groove, which rotates around a fixed point.

A single fixed pulley is attached to an immovable surface. It changes the direction of the force, but not the size. So the M.A. is 1.

A single movable pulley is not attached to a stationary surface, and is free to move. A single movable pulley has a mechanical advantage of 2.

Does the mechanical advantage of 2 for a single movable pulley mean that you’ll increase the amount of work you get out of the pulley?

No, it doesn’t. If you get twice as much force out then you have to pull out twice as much rope, but the amount of work (f x d) would not be effected. So how could you change the amount of work produced by the machine?

A block and tackle is a combination of fixed and movable pulleys. The M.A. is equal to the number of rope segments supporting the weight.

What is the mechanical advantage of this block & tackle?

A wheel and axle consists of 2 different sized wheels which rotate together around the same axis.

The mechanical advantage of a wheel and axle is determined by dividing the radius of the wheel by the radius of the axle. M.A. = R w / R A

A gear is a wheel with interlocking teeth which prevent slippage.

An inclined plane (ramp) is a slanted surface used to raise or lower objects. Inclined planes are usually stationary.

A wedge is a movable inclined plane.

What does sharpening a knife do to it’s mechanical advantage? Is a sharper object always going to perform better than a dull one?

A screw is an inclined plane wound around a cylinder. The closer the threads, the greater the mechanical advantage.

Which screw would have the greater M.A.? Which would be hardest to drive into a block of wood? Which would require more turns? A B

The front sprocket on a bike has 2 gears which act as axles, while the circle created by the pedal acts as the wheel. The outer gear has a radius of 6 inches and the smaller inner one has a radius of 4 inches. If the pedal goes in a circle with a radius of 12 inches, calculate the mechanical advantage provided by each of the different gears.

In the previous example, which gear would you want to be in if you were pedaling up a steep hill? Why?

The lugnut on a large dump truck has a radius of 5 cm, and requires a force of 1,800 N to loosen. Lenny, however, can only produce 150 N of force. He knows that if he gets a long enough wrench then he can produce that much force. How long would the wrench need to be?

Carl is applying 200 N of force to push a wheelbarrow up a ramp. If the ramp is 6 m long and allows him to roll the wheelbarrow to a height of 1.5 m, what is the mechanical advantage of the ramp? How much force would Carl need to apply if he were not using the ramp?

How much force would Waylon need to apply in order to lift a 375 N weight, using the pulley shown? How much rope would he need to pull out in order to raise the weight 4 m?

Using the 3 pulleys shown below, how much force would Ned need to apply to each to lift a 400 N weight? Why wouldn’t he just always use the one which requires the least force.

In the diagram below, the inclined plane is 5 m long and 2 m high. The wheel and axle have radii of 25 inches and 10 inches respectively. How much force would be required to move the weight up the inclined plane? (assume no friction) How much work is being done by the machine?