Work and Energy (2)

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WorK Work in physics is defined as the ability of the forces acting on an object

Work The work done on an object is the product of the force applied and the displacement

When a force is parallel to the direction of the motion of the body WorK

The work done by a force may be positive or negative

The work done by a force The work done by a force is positive if the force and is negative if the force displacement are in the and displacement are in same direction. opposite directions.

Work

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Look at the examples below. Is work being done or not?

Look at the examples below. Is work being done or not?

wThheetondetthfhieenidftoiisorpncleaocfisewmpoaerrnkatlilsel W = Fd

W = Fd W = work (Joule) F = Force (Newton) d = Displacement (meter)

Example 1.If a force of 5 N acts on an object is displaced through 2 m in the direction of force, then work done is....

Example 2. Kaew pushes the table with the force of 50 N to the back of the room, which is a metres away. How much does kaew work?

Example 3. A porter lifts a luggage of 15 kg from the ground and puts it on his head 1.5 m above the ground. Calculate the work done by him on the luggage

Force carry hold

Example 4. Mek carries the box that weighs 2 kg and puts it on the shelf that is 1.5 metres hight from the floor. How much dose Mek have to work?

Example 5. A book weighing 0.1 kilograms is lifted 2 meters. How much work was done?

Example 6. A force of 15 newtons is used to push a box along the floor a distance of 3 meters. How much work was done?

Example 7. It took 50 joules to push a chair 5 meters across the floor. With what force was the chair pushed?

Example 8.A force of 100 newtons was necessary to lift a rock. A total of 150 joules of work was done. How far was the rock lifted?

Example 9. A young man exerted a force of 9,000 N on a stalled car but was unable to move it. How much work was done?

Energy ability to do work

Mechanical Energy The energy an object has from its motion or its potential for motion

Mechanical Energy Kinetic Energy Potential Energy “The moving energy” “Energy stored in an object's position”

Kinetic Energy - Kinetic energy is the energy of motion - If something is moving it has Kinetic enegy

Ek = Kinetic Energy (Joule) m = mass (kilograme) v = velocity(m/s)

Example 1. Determine the kinetic energy of a 5 kg roller coaster car that is moving with a speed of 20 m/s.

Example 2. If the roller coaster car in the above problem were moving with twice the speed, then what would be its new kinetic energy?

PoGtreanvtiiatal tEinoenragly (GPE) Elastic Potential Energy (EPE) Potential Energy

POTEGNRTAIAVILTAENTIEORNGAYL(GPE) Gravitational potential energy is the energy stored in an object as the result of its vertical position or heigh. The energy is stored as the result of the gravitational attraction of the Earth for the object

POTEGNRTAIVAILTAET NIEORNGAYL(GPE) GPE = mgh

GPE =Wxh=mxgxh GPE = gravitational potential energy (Joule) w= weight (Newton) m = mass (kilograme) g = 10 (m/s2) h = height (meter)

Example 1.A 20 kg suitcase is raised 3.0 meters above the ground. What is its potential energy?

Example 2. Consider the roller coaster on the right. How much potential energy does the coaster have at each point A, B, C and D. If it has a mass of 150 kg?

Example 3. Consider the rock being dropped on the right. if the rock has a weight of 5 N. What is the potential energy at each point?

Example 4. A 6 kg mass is lifted from a position that is 2 m above a table. to a position that is 7 m above the table. What is its change in potential energy?

Example 5. What is the mass of an object that is hanging 10 m above the surface of the earth and has a potential energy of 2000 J?

Example 6. An object has the Gravitational potential energy of 4000 Joule and has a mass of 200 kilograms. How high is it above ground?

Power

POWER Power is the rate of change of work or the amount of work done in a certain amount of time

Power In the SI system of measurement, power is measured in Watts (symbol: W)

How do we measure power?



Example 1. A machine does 30 J of work in 10 seconds. How much power is that?

Example 2. a light bulb uses 600 J of energy in 6 seconds. what is the power of the light bulb?

Example 3. A bucket of water with a total weight of 50 Newtons is lifted at a constant velocity up to 10 meters deep well. if it takes 20 seconds to raise the bucket this distance, the Power required to lift the bucket is....

Example 4. A crane lifts an 800 Newton beam 75 meters to the top of a building in 30 seconds. a. How much work was done? b. How much power does the crane use?

Example 1.(a) How much work is done in lifting an object that weighs 250 Newton to a height of 4 meters? 2.(b) At what rate is work done if the 250 Newton object is lifted to a height 4 meters in 4s?