Work and Energy

Reading: Textbook Chapter 13 p.429-461

Work, Power and Machines

Work is the transfer of energy to a body by the application of a force that causes the body to move in the direction of the force.Work is only done when the force cause a change in position or motion of the object in the direction of the applied force.

work = force x distance

W = F x d

Work is measured in units of newtons time meters, N*m. These units are called joules (J).

Power is a quantity that measures the rate at which work is done or energy is transformed.

power = work/ time

P = W / t

Power is measured in units called watts (W). A watt is the amount of power required to do 1 J of work in 1 s, abut as much power as you need to lift an apple over your head in 1 s.

Complete the learning activity Work and Power.

Go to assignments and do questions 1-4.

Machines and Mechanical Advantage

Machines help us by redistributing the work and changing the direction of an input force. Machines can increase or decrease the force by changing the distance over which the force is applied. Different amounts of force can do the same amount of work. For example, lifting a heavy box into a truck or sliding it up a ramp into the truck.

Mechanical advantage tells how much a machine multiplies force or increase distance.

mechanical advantage = output force\input force = input distance\ output distance

Complete the learning activity Mechanical Advantage.

Go to assignments and do questions 5-6.

There are 6 types of simple machines: lever, pulley, wheel and axle, inclined plane, wedge, and screws. They are the basis for all other forms of machines.

The Lever Family

All levers have a rigid arm that turns around a point, called the fulcrum. The force is transferred from one part of the arm to another. A claw hammer is an example of a first-class lever. First-class levers have the fulcrum in the middle of the arm. A wheelbarrow is an example of a second-class lever. The fulcrum is at one end of the arm and the input force is at the other end of the arm and in the same direction as the output force. Third-class levers have the fulcrum at one end of the arm and the input force is in the middle of the arm. An example of a third-class lever is a human arm.

Go to Levers- Enchanted Learning for good explanation and examples of each class of lever.

Pulleys are modified levers. The rod in the center of the pulley is the fulcrum. Multiple pulleys are put together in a single unit called a block and tackle.

Fixed pulley

moveable pulley

multiple pulley

A Wheel and Axle is a lever or pulley connected to a shaft. When the wheel is turned the axle also turns. A crank or screwdriver are also examples of wheel and axles.

The Inclined Plane Family

Inclined planes multiply and redirect force. A ramp is an inclined plane.

A Wedge is a modified inclined plane. A wedge functions like two incline planes used to split or cut.

A screw is an inclined plane wrapped around a cylinder. So as a screw is turned you are sliding an object up the inclined plane.

Click here for an interactive lab on Simple Machines that will show how they make work easier.

Compound Machines

A compound machines is made of one or more than one simple machine.

Go to assignments and do question 7-9.

What is Energy?

Potential Energy is the energy that an object has stored because of the position, shape or condition of the object. An apple in a tree has gravitational potential energy because of its position above the ground. A larger apple would have more potential energy than a smaller apple. Gravitational potential energy depends on mass and height.

grav. PE = mass x free-fall acceleration x height

PE=mgh

Kinetic Energy is the energy of a moving object due to the object's motion. Kinetic energy depends on mass and speed. For example a falling apple can do more work than a falling cherry at the same speed. A peach pit with the same mass as a 22 bullet does less work that a bullet shot from a gun because of the speed. Kinetic energy therefore depends on speed more than mass.

kinetic energy = 1/2 x mass x speed squared

KE = 1/2mv(squared)

Atoms and molecules have kinetic energy.

Go to assignments and do questions 10-11.

Mechanical Energy is the amount of work an object can do because of the object's kinetic and potential energy.

Chemical energy is potential energy stored in the chemical bonds. All living things get their energy from the sun in the form of chemical energy stored in the food you eat.

The sun gets energy from nuclear reactions. Nuclear power plants use a different process called nuclear fission. Nuclear energy is a kind of potential energy stored by the forces holding subatomic particles together in the nuclei of atoms.

Electrical Energy is formed from the flow of charged particles. Moving electrons increase temperature and create magnetic fields.

Light Energy can carry energy through empty space. The energy from the sun travels to Earth as light energy and is turned into chemical energy through photosynthesis in plants.

Law of Conservation of Energy states that energy is neither lost or created, but is just transferred or changes form.

Go to assignments and do questions 12-15.