5.1 Work

What is work?

To some people is something they do to earn money or work is exerting a force with your muscles

• however science uses the term work differently

Work Makes Something Move

Remember a force is a push or pull

Work - the transfer of energy that occurs when a force makes an object move

Doing Work

2 conditions that are required for work to be done on an object:

• The applied force must make the object move

• The movement must be in the same direction as the applied force

Force and Direction of Motion

Even if you are holding a stack of books and walking forward, your arms are exerting a force upward however there is no work taking place on the books

Work and Energy

When work is done a transfer of energy occurs

Remember when the height of an object above the Earth’s surface increases, the potential energy of the object increases

By carrying a book up the stairs, you are doing work

Calculating Work

The amount of work done depends on the amount of force exerted and the distance over which the force is applied

Work Equation: W = Fd

Force is measured in newtons and distance is measured in meters

Work and energy are both measured in joules

When is work done?

The distance you use to calculate the work you did is how far the object moves while the force is being applied

Power

Power - the amount of work done in one second; it is the rate at which work is done

Calculating Power

Power - the rate at which work is done

Power equation: P = W/t

The SI unit for power is the watt (W).

• One watt equals one joule of work done in one second

Power and Energy

Doing work is a way of transferring energy from one object to another; power is the rate at which work is done and energy is transferred

Power equation for energy transfer: P = Et

5.1 Summary

Work and Energy

• Work is done on an object when a force is exerted on the object and it moves in the direction of the force

• If a force, F, is exerted on object while the object moves a distance, d, in the direction of the force, the work done is W = Fd

• When work is done on an object, energy is transferred to the object

Power

• Power is the rate at which work is done or energy is transferred

• When work is done, power can be calculated from the equation P = W/t

• When energy is transferred, power can be calculated from the equation P = E/t

5.2 Using Machines

What is a machine?

Machine - a device that makes doing work easier

Some simple machines include: knives, scissors, and doorknobs

Making Work Easier

Machines can make work easier by increasing the force that can be applied to an object

Machines can make work easier by increasing the distance over which a force can be applied

Machines can also make work easier by changing the direction of an applied force

Increasing Force

A car jack increases an applied force upward; the upward applied force is greater than the downward force you exert on the handle. However, the distance you push the handle downward is greater than the distance the car is pushed upward

Force and Distance

The work done in lifting an object depends on the change in height of the object; the same amount of work is done whether the mover pushes the objects up the ramp or lifts them straight up. If work stays the same and the distance is increased, then less force will be needed to do the work

Changing Direction

Some machines change the direction of the force you apply, like a car jack: you exert a force downward and the force exerted by the jack on the car is upward

The Work Done by Machines

When you use a machine such as a crowbar, you are trying to move something that resists being moved

Input and Output Forces

Two forces are involved when using a machine to do work:

• Input

• Output

Input force - the force that is applied to the machine

Output force - the force applied by the machine

Two kinds of work need to be considered when you use a machine

• The work done by you on the machine

• The work done by the machine

Conserving Energy

Remember energy is always conserved

When you do work on the machine, you transfer energy to the machine, and the machine transfers the energy to the object

• not all energy is transferred to the object due to heat and friction

Ideal Machines

Remember work is calculated by multiplying force by distance

If there were a perfect machine that caused no friction, none of the input or output work would be converted to heat

Ideal (perfect) machine equation: Win=Wout

Ideal Mechanical Advantage

The mechanical advantage of a machine without friction is called the ideal mechanical advantage (IMA)

The IMA can be calculated by dividing the input distance by the output distance

IMA would be the mechanical advantage of the machine if there were no friction

Efficiency

Since for real machines there is always energy converted into heat by frictional forces, the output work of a machine is always less than the input work into the machine

Efficiency - a measure of how much of the work put into a machine is changed into useful output work by the machine

Calculating Efficiency

The output work is divided by the input work

Efficiency Equation: efficiency = Wout/Win×100%

The efficiency of an ideal machine is always 100%

The efficiency of a real machine is always less than 100%

Increasing Efficiency

Machines can be made more efficient by reducing friction which can be done by adding a lubricant

A lubricant fills in the gaps between the surfaces, enabling the surfaces to slide past each other more easily

5.2 Summary

Work and Machines

• Machines make doing work easier by changing the applied force, changing the distance over which the force is applied, or changing the direction of the applied force

• Because energy cannot be created or destroyed, the output work cannot be greater than the input work

• In a real machine, some of the input work is converted into heat by friction

Mechanical Advantage and Efficiency

• The mechanical advantage of a machine is the output force divided by the input force:

  MA = Fout/Fin

• The efficiency of a machine is the output work divided by the input work times 100%:

  efficiency = Wout/Win ×100%

5.3 Simple Machines

Types of Smily Machines

Simple machine - a machine that does work with only one movement of the machine

There are 6 types of simple machines

• Lever

• Pulley

• Wheel and Axle

• Inclined Plane

• Screw

• Wedge

Levers

Lever - a bar that is free to pivot or turn around a fixed point; the fixed point that the lever pivots around is called the fulcrum

• Ex. Wheelbarrow, rake, swinging of a baseball bat

The output force produced by a lever depends on the length of the input and output arms

There are 3 different classes of levers that depends on:

• Locations of the fulcrum

• The input force

• The output force

First-Class Lever

The fulcrum is located between the input and output forces; the output force is always in the opposite direction to the input force in a first-class lever

Second-Class Lever

The output force is located between the input force and the fulcrum; the output force is always greater than the input force for a second-class lever

Third-Class Lever

The input force is applied between the output force and the fulcrum

• the output force is always less than the input force in a third-class lever

Ideal Mechanical Advantage of a Lever

Ideal mechanical advantage: IMA

IMA can be calculated for any machine by dividing the input distance by the output distance

• for a lever, the input distance is the length of the input arm and the output distance is the length of the output arm

IMA of a lever equation: IMA = LinLout IMA

Pulleys

Pulley - a grooved wheel with a rope, chain, or cable running along the groove

The two sides of a pully are called the input arm and output arm

A pulley can change the direction of the input force or increase input force, depending on whether the pulley is fixed or movable

3 kinds of pulleys:

• Fixed Pulleys

• Movable Pulleys

• Block and Tackle

Fixed Pulleys

A fixed pulley is attached to something that doesn’t move

Because a fixed pulley changes only the direction of force, the IMA is 1

The distance you pull the rope downward, equals the distance the weight moves upward

Movable Pulleys

A movable pulley is where one end of the rope is fixed and the wheel is free to move

The distance you pull the rope upward is twice the distance the weight moves upward

The Block and Tackle

The block and tackle is a system of pulleys consisting of fixed and movable pulleys

The IMA of a pulley system is equal to the number of rope segments that support the weight

The IMA of a block and tackle can be increased by increasing the number of pulleys in the pulley system

Wheel and Axle

Wheel and axle - a simple machine consisting of a shaft or axle attached to the center of a larger wheel, so that the wheel and axle rotate together

• Ex. Doorknobs, screwdrivers, faucet handles

Usually the input force is applied to the wheel, and the output force is exerted by the axle

Mechanical Advantage of the Wheel and Axle

A wheel and axle is another modified lever; the center of the axle is the fulcrum and the input force is applied at the rim of the wheel, the output force is exerted at the rim of the axle

The IMA of Wheel and Axle Equation: IMA = rw/ra

The IMA of a wheel and axle can be increased by increasing the radius of the wheel

Gears

A gear is a wheel and axle with the wheel having teeth around its rim

Inclined Planes

Inclined plane - a sloping surface that reduces the amount of force required to do work

Mechanical Advantage of an Inclined Plane

IMA of an Inclined Plane Equation: IMA = l/h

The Screw

Screw - an inclined plane wrapped in a spiral around a cylindrical post

The IMA of a screw is related to the spacing of the threads; if the IMA is larger, the threads are closer together but then more turns of the screw are needed to drive it into some material

The Wedge

Wedge - another type of simple machine where the inclined plane moves through an object or material

A wedge is an inclined plane with one or two sloping sides; it changes the direction of the input force

Compound Machines

Compound machines - two or more simple machines that operate together

• Ex. Car, can opener

5.3 Summary

The Lever Family

• A lever is a bar that is free to pivot about a fixed point called the fulcrum

• There are three classes of levers based on the relative locations of the input force, output force, and the fulcrum

• A pulley is a grooved wheel with a rope, chain, or cable placed in the groove and is a modified form of a lever

• The IMA of a lever is the input arm divided by the output arm

• A wheel and axle consists of a shaft or axle attached to the center of a larger wheel

The Inclined Plane Family

• An inclined plane is a ramp or sloping surface that reduces the force needed to do work

• The IMA of an inclined plane is the length of the plane divided by the height of the place

• A screw consists of an inclined plane wrapped around a shaft

• A wedge is an inclined plane that moves and can have one or two sloping surfaces

Ch. 5 Study Guide (Reviewing Main Ideas)

5.1 Work

1. Work is the transfer of energy when a force makes an object move

2. Work is done only when force produces motion in the direction of the force

3. Power is the amount of work, or amount of energy transferred, in a certain amount of time

5.2 Using Machines

1. A machine makes work easier by changing the size of the force applied, by increasing the distance an object is moved, or by changing the direction of the applied force

2. The number of times a machine multiplies the force applied to it is the mechanical advantage of the machine. The actual mechanical advantage is always less than the ideal mechanical advantage

3. The efficiency of a machine equals the output work divided by the input work

4. Friction always causes the output work to be less than the input work, so no real machine can be 100 percent efficient

5.3 Simple Machines

1. A simple machine is a machine that can do work with a single movement

2. A simple machine can increase an applied force, change its direction, or both

3. A lever is a bar that is free to pivot about a fixed point called a fulcrum. A pulley is a grooved wheel with a rope running along the groove. A wheel and axle consists of two different-sized wheels that rotate together. An inclined plane is a sloping surface used to raise objects. The screw and wedge are special types of inclined planes

4. A combination of two or more simple machines is called a compound machine