Chapter 8 Work and Machines Section 1 Work and Power

If you had a choice what would you prefer to do, work or not work? To Work or Not to Work If you had a choice what would you prefer to do, work or not work? Give some examples of work.

The Scientific Meaning of Work Work occurs when a force causes an object to move in the direction of the force. The object must move for work to occur!

Force and Motion in the Same Direction You’ll know work is being done on an object if two things occur: 1. the object moves as the force is applied 2. the direction of the objects motion is the same as the direction of the force applied.

Force and Motion in the Same Direction

How do we calculate work? Work = force x distance Force expressed in Newtons Distance is expressed in meters So the expression for work is the Newton meter or simply the joule Work = # Newtons x # meters = # Joules

Practice A man applies a force of 500N to push a truck 100m down the street. How much work does he do? In which situation do you do more work? a. You lift a 75 N bowling ball 2m off the floor. b. You lift two 50 N bowling balls 1m off the floor

Power-How Fast Work Is Done Power is the rate at which work is done To calculate power (P) divide the amount of work done (W) by the time (t) it takes to do that work or P = W/ t Unit for work is joule Unit for time is second So the unit for power is joules / second or J/s Or more commonly called a watt (W)

Chapter 8 Work and Machines Section 2 What is a Machine?

Machines - Making Work Easier A machine is something that makes work easier by changing the size or direction of a force

Machines - Making Work Easier Work input: the work you do on a machine Work output: the work done by the machine Output force: the force that the machine applies that opposes the forces of you and the machine Machines do not increase the amount of work that is done because work output can never be more than work input

Machines - Making Work Easier Machines make work easier because they change the size or direction of the input force If force decreases then distance must increase and vice versa

Mechanical Advantage (MA) =output force A machine’s mechanical advantage tells how many times a machine multiplies force…it compares the input force to the output force Mechanical Advantage (MA) =output force input force

Mechanical Advantage The larger the mechanical advantage, the easier the machine makes your work!

Mechanical Advantage The output force is the same as the input force if the machine changes direction of a force and the MA is 1. The input force is greater than the output force allows lower force over a longer distance and has a MA less than 1. The output force is greater than the input force gives a MA of greater than 1.

Mechanical Efficiency While output cannot be greater than input the input is always greater than the output because some of the work done by the machine is to overcome gravity. The less work a machine has to do to overcome friction the more efficient it is. Mechanical Efficiency = work output x 100 work input

Mechanical Efficiency Mechanical Efficiency = work output x 100 work input Mechanical efficiency tells what percentage of the work input gets converted into work output. So reducing friction in a machine makes it more efficient.

Chapter 8 Work and Machines Section 3 Types Of Machines

Types of Machines: Levers Lever: a simple machine consisting of a bar that pivots at a fixed point called a fulcrum There are three classes of levers: First class levers Second class levers Third class levers

Types of Machines: Levers First class levers: always change the direction of the input forces

Types of Machines: Levers Second class levers: do not change the direction of the input forces; output force is greater than the input force

Types of Machines: Levers Third class levers: do not change the direction of the input forces; and do not increase the input force

Types of Machines: Inclined Planes An inclined plane is a simple machine that is a straight, slanted surface A ramp is an example of an incline plane MA of an inclined plane:

Types of Machines: Wedges A wedge is a double inclined plane that moves A wedge applies an output force that is greater than the input force MA of wedges: The longer and thinner the wedge, the better.

Types of Machines: Screws A screw is an inclined plane that is wrapped in a spiral MA of screws: same advantage as a long inclined plane

Types of Machines: Wheel and Axle A wheel and axle is a simple machine consisting of two circular objects of different sizes

Types of Machines: Wheel and Axle The MA of this machine can be determined by dividing the radius of the wheel by the radius of the axle

Types of Machines: Pulleys A pulley is 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 an input force is applied to the other end. There are two types of pulleys: 1) fixed 2) movable

Types of Machines: Pulleys

Types of Machines: Pulleys

Compound Machines Compound machines are made of two or more simple machines. The block and tackle we just talked about is a compound machine

Compound Machines

Mechanical Efficiency of Compound Machines Generally, the more moving parts a machine has, the lower its mechanical efficiency What would happen if the mechanical efficiency of a car was too high?