Lesson 1 unit 4

EnergyEnergy can be defined as the capacity to

work or to accomplish a task. Example: burning fuel allows an engine to do

the work of moving a car.

Forms of EnergyChemical Potential Energy

In chemical reactions, new molecules are formed and chemical potential energy is released or absorbed.

Sound EnergyProduced by vibrations; the energy travels by

waves through a material to the receiver.

Radiant EnergyComponents of the electromagnetic spectrum

have characteristics of waves, such as wavelengths, frequencies, and energies; they travel in a vacuum at the speed of light (3.0 x 108 m/s).

Nuclear EnergyThe nucleus of every atom contains

energy. Nuclear fission – The breaking down of atomsNuclear fusion – the joining of atoms

Electrical EnergyElectrons in a electric circuit can transfer

energy to the components of the circuit.

Thermal EnergyThe more rapidly atoms and molecules move,

the greater their total thermal energy.

Gravitational Potential EnergyA raised object has stored energy due to its

position above some reference level

Kinetic EnergyEnergy of motion, every moving object has this

energy

Elastic Potential EnergyIs stored in objects that are stretched or

compressed.

Energy TransformationsThe 9 forms of energy listed above are able to

change from one to another; this change is called energy transformation. A energy transformation equation can be used to summarize the changes in a transformation. Example: a microwave

Electrical energy Radiant Energy Thermal Energy

Energy-Transformation TechnologyA device used to transform energy for a

specific purpose.

Work – The energy transferred to an object by a force applied over a measured distance.

As the force or the distance increases, the work also increases.

W is the work done on the objectF is the magnitude of the applied force in the

direction of the displacement∆d is the magnitude of the displacement

Because the applied force and displacement are in the same direction there are no signs needed making work a scalar quantity, it has magnitude but no direction.

Work is measured in Newton-meters (N•m)One Newton-meter is referred to as a joule

Example 1A store employee exerts a horizontal applied

force of magnitude 44 N on a set of carts. How much work is done by the employee when pulling the carts 15m? Express the answer in joules and kilojoules.

F = 44 N∆d = 15 mW = ?

W = F ∆dW = ( 44 N) x ( 15 m) W = 6.6 x 102 J

Therefore, the work done by the employee in pulling the cars is 6.6 x 102 J, or 0.66 kJ.

Negative Work If the force is opposite in direction to the

displacement, the work done is negative. W = -F∆d

Consider the previous example but this time a second employee exerts a horizontal force in the opposite direction to the 44 N force.

Since the force is in the opposite direction to the displacement, the work done by the second employee on the carts is negative.

Now the total work done by the two employees on the carts is the sum of the positive and negative values.

Work done by the 44 N forceWork done by the 14 N forceTotal work done by the forces on the carts

Negative work can also occurs with kinetic friction because the force of kinetic friction always happens in the direction opposite to the direction of motion of the object.

W = -FK∆d

-FK is the magnitude of the force of kinetic friction

Example 2A toboggan carrying two children (total mass

85kg) reaches its maximum speed at the bottom of a hill. It then glides to a stop 21 m along a horizontal surface. The coefficient of friction between the toboggan and the snowy surface is 0.11.

A.Calculate the magnitude of the force of kinetic friction acting on the toboggan

B.Calculate the work done by the force of kinetic friction on the toboggan

m = 85 kgg = 9.8 N/kgµk = 0.11

FK = ?

FK = µkFN

FK = µkmg

FK = (0.11)(85 kg) (9.8 N/kg)

FK = 92 N

The magnitude of the kinetic force is 92 N

Calculate the work done by the force of kinetic friction on the toboggan

∆d = 21 mW = ?

W = -FK∆dW = -(92 N) (21 m)W = - 1.9 x 103 J

Therefore, the work done by the kinetic friction on the toboggan is -1.9kJ

The work done by friction has been transformed into thermal energy. This is observed as a increase in temperature.

Work done in raising objectsTo lift an object to a higher position, an

upward force must be applied against the downward force of gravity acting on the object.

If the force applied and the displacement are both vertically upward and no acceleration occurs, the work done by the upward force is positive and calculated with the formula:

W = F∆d.

Work done in raising objectsThe force in this case is equal in magnitude

to the weight of the object or the force of gravity on the object.

F = mg

Example 3A bag of groceries of mass 8.1 kg is raised

vertically at a slow, consistent velocity from the floor to a countertop, for a distance of 92 cm. Calculate

A.The force needed to raise the bag of groceries at a constant velocity

B.The work done on the bag of groceries by the upward force.

m = 8.1 kgg = 9.8 N/kgF = ? F = mgF = (8.1 kg) (9.8 N/kg)F = 79 NTherefore, the force needed is 79 N.

The work done on the bag of groceries by the upward force.

∆d = 92 cm = 0.92 W =?W = F∆d W = (79 N) (0.92 m) W = 73 JTherefore, the work done on the bag of

groceries by the upward force is 73 J.

Zero WorkSometimes an object can experience a force, a

displacement, or both, yet no work is done on the object.

Example: Holding a box in your hands. The box is not moving so no work is done so the displacement is zero. A puck on an air hockey table is moving but it does

not have force acting parallel to the movement as friction is negligible.

Work and springs When a spring or rubber band is stretched,

the more difficult it becomes to stretch. If a typical spring was stretched and then graphed the force applied to it against the stretch experienced by the sting, the line would be straight.

The area under the triangle can be calculated by the equation

this yields the work done by the force used to stretch the spring by an amount ∆x. The slope of the line, is found by using the equation

or

this represents the force constant of the spring, k. The force constant represents the stiffness of the spring.

Questions1. Using an energy transformation equation, show

how energy is transformed for each of the following:

1. A hotdog grilled on an outdoor BBQ2. A truck acceleration on a highway

2. A Truck does 3.2 kJ of work pulling horizontally on a car to move 1.8 m horizontally in the direction of the force. Calculate the magnitude of the force.

3. A store clerk moves a 4.4 kg box of soap at a constant velocity along a shelf by pushing it with a horizontal force of magnitude 8.1 N. The clerk does 5.9 J of work on the box. How far did the box move?

A 150 g text book is lifted from the floor to the shelf 2.0 m above. CalculateThe force needed to lift the book without acceleration. The work done by the force on the book to lift it up to a

shelf. An Electric forklift is capable of doing 4.0 x 105 J

of work on a 4.5 x 103 kg load, What height can the truck lift the load?

A student pushes against a large tree with a force of magnitude 250 N, but the tree does not move. How much work has the student done on the tree?

Draw a graph of a spring that has the following dataCalculate the work done in stretching of

the spring represented in the graph you just drew. a)0.12 m b)0.24 m

Force (N)

Stretch (m)

5 0.0610 0.1215 0.1820 0.24