Work, Simple Machines, and Mechanical Advantage

Work In scientific terms, work is done when a force causes an object to move in the direction of the force. W = F x D: Work is calculated as force times distance. The unit of work is the newton-meter, or joule. The object must move in the direction of the force for work to be done.

Work or Not?

YES, as long as the box moves in the direction of the force. Work or Not? YES, as long as the box moves in the direction of the force.

Work or Not?

Work or Not? NO – work was done on the backpack when it was first picked up, but is not being done now.

Work or Not?

YES! The bag is moving up as it is being lifted up. Work or Not? YES! The bag is moving up as it is being lifted up.

Work or Not? NO, work is not being done on the bag since the applied force is not in the direction of movement.

Remember: Work is a transfer of energy You can apply a force but not do work You can set an object in motion, and it may keep moving even though you are no longer doing work

Question: Work is being done when: you apply a force to an object. an object is moving after you applied a force to it. you exert a force that moves an object in the direction of the force. you do something that is difficult.

Answer: Work is being done when: you apply a force to an object. an object is moving after you applied a force to it. you exert a force that moves an object in the direction of the force. you do something that is difficult.

Power Power is a measure of how fast work is done. W/t = P: Power is calculated as work divided by time. The unit of power is the joule per second, or watt.

What can a machine do? A simple machine does NOT reduce work A machine makes work easier by changing the size or direction (or both) of a force. A machine can increase force or distance, but not both.

Changes direction of the force

Changes the size of the force

Trades force for distance

Six Simple Machines Inclined Plane Wedge Screw Lever Pulley Wheel & Axle

Terminology - Synonyms Where your hands are Where the load is Effort Input Output DISTANCE Load Output Resistance Input DISTANCE

Mechanical Advantage is… A multiplier that shows how much a machine is amplifying a force

Mechanical Advantage

Mechanical Advantage Resistance Force = Input Distance = Effort Distance = Output Force Effort Force = Output Distance = Resistance Distance = Input Force

Other forms of the MA formula:

Mechanical Efficiency Mechanical Efficiency = Work Output X 100 Work Input

Inclined Plane Reduces force, but increases distance The mechanical advantage of an inclined plane is length divided by height. Wedges and screws are types of inclined planes.

MA = L/H

Wedge A wedge generates outward forces A wedge is a type of inclined plane. Its mechanical advantage is its length divided by its greatest thickness.

Screw A screw is an inclined plane wrapped around a cylinder Uses a small force over a longer distance The closer the threads, the lower the force required

Levers – Three Classes Can change direction & size of force In a first-class lever, the fulcrum is between the force and the load. In a second-class lever, the load is between the force and the fulcrum. In a third-class lever, the force is between the fulcrum and the load.

First Class Second Class Third Class

First Class Lever Change direction & maybe force

Second Class Lever Change force, but not direction

Third Class Lever Don’t change direction, output < input

Pulleys Change direction and may change input force Types of pulleys include fixed pulleys, movable pulleys, and block and tackles.

Wheel and Axle The mechanical advantage of a wheel and axle is the radius of the wheel divided by the radius of the axle.

Compound Machines Compound machines consist of two or more simple machines. Compound machines have low mechanical efficiencies because they have more moving parts and therefore more friction to overcome.

Question 1 Which of the following simple machines is a wedge? A. Wheelbarrow B. Doorknob C. Ax D. Truck ramp

Answer Which of the following simple machines is an example of a wedge? A. Wheelbarrow B. Doorknob C. Ax D. Truck ramp

Question 2 What is the pivot point of a lever called? A. Load B. Fulcrum C. Wedge D. Effort

Answer What is the pivot point of a lever called? A. Load B. Fulcrum C. Wedge D. Effort

Question 3 A student pushes on a lever with a force of 40N. The 150N load does not move. Which of the following is true? A. Input force is 40 N and work done is 0 J. B. Input force is 40 N and work done is 600 J. C. Output force is 40 N and work done is 150J. D. Output force is 150N and work done is 40J.

Answer A student pushes on a lever with a force of 40N. The 150N load does not move. Which of the following is true? A. Input force is 40 N and work done is 0 J. B. Input force is 40 N and work done is 600 J. C. Output force is 40 N and work done is 150J. D. Output force is 150N and work done is 40J.