ENERGY Energy

Introduction Energy is always present, but never visible! We see the evidence of energy Pushing a wheelchair Jumping Eating Movement, sound, heat, light all provide evidence of energy

Trebuchet Video: Consider these questions How is a trebuchet constructed? Which simple machine do you see? What is the purpose of the counterweight? What type of energy does the trebuchet have when stationary? What happens when the firing pin is released? How would you differentiate between potential and kinetic energy? http://www.teachersdomain.org/asset/hew06 _vid_trebuchet/

Work- Energy Correlation In order for work to be done, a force must be applied to an object and cause it to move in the same direction as the force. Work transfers energy from one object to another What is energy?

What is energy? Energy is the ability of an object to produce a change in itself or the environment. It is the ability to do work.

Types of Energy Mechanical Energy(ME) - enables an object to do work A. Kinetic Energy (KE) B. Potential Energy(PE)

Kinetic Energy Kinetic energy is defined as the energy of a moving object Examples: Thrown football Waterfall Rock falling from a cliff

Potential Energy Potential energy is defined as the energy in matter due to its position or the arrangement of its parts Often referred to as stored energy The more potential an object has, the more potential energy is has Has many different forms including Gravitational potential energy Elastic potential energy Chemical potential energy Electrical potential energy

Toy observations Describe your toy. Take a few minutes to determine how your toy works. Make a short statement about how the toy works. State how you think your toy uses energy. How many forms of energy does your toy display? Name the types of energy you observe. Switch toys with another table and repeat the above observations.

Gravitational Potential Energy When something is lifted or suspended in air, work is done on the object against the pull of gravity This work is converted to a form of potential energy, called gravitational potential energy Once the object falls the potential energy is converted to kinetic energy

Elastic Potential Energy Occurs when an object resists being pulled out of shape Examples: a stretched rubber band, a spring, trampoline, our skin The elastic potential energy in a rubber band can be used to do work- such as a toy plane . . . A rubber band untwists and causes a propeller to spin

Chemical Potential Energy Chemical potential energy is the energy stored in molecules (including in us) Examples: food, gasoline When gasoline is burned through combustion, the arrangement of molecules changes, and energy is released The released energy is used to do work

Electric Potential Energy Electrical potential energy is the result of energy from a battery, a power plant, a hydroelectric dam, solar cells, or a windmill Electric potential energy can be converted to sound, light, motion, etc.

Work and Energy Work and Energy are related When work is waiting to be done, we call the energy potential When work is being done, we call the energy kinetic

Rubber Band In order to make a rubber band fly, you must do work on it While it is stretched, the rubber band has potential energy Once you release it, it has kinetic energy If you pull it back a greater distance (therefore doing more work) the potential is greater The result is that the rubber band will go faster and further (more potential energy leads to greater kinetic energy, thus more work can be done)

Situations to consider Is work done in the following situations? Pushing against a wall? No, there is no displacement, no work, so kinetic energy does not change Pushing at constant speed? No, work done by the applied force is equal to work done by the frictional force, so kinetic energy does not change Free fall Gravity is the only force acting, so there is a net force and work is done . . . Kinetic energy changes

Kinetic Energy Because energy is a property of matter, it can be quantified The equation for kinetic energy is: Where “m” is the mass and “v” is the velocity of the object The unit for energy is joules

Example 1 What is the kinetic energy of a 45 kg object moving at 13 m/s?

Example 2 The kinetic energy of a boat is calculated at 52,000 J. If the boat has a mass of 39,000 kg, with what velocity is it moving?

Potential Energy Remember that potential energy is the energy of position To quantify potential energy use the following: Where “m” is mass, “g” is gravity and “h” is the height in meters

Example 3 A 3.8 kg object is lifted to a height of 3 meters. What is the potential energy of the object?

Example 4 A 30 kg child climbs 15 m up a tree. When he stops to look around, what is the child’s potential energy?

State whether each of the following has kinetic energy, potential energy, or both. 1. If an object is at rest, it certainly does NOT possess this form of energy. 2. Depends upon object mass and object height. 3. The energy an object possesses due to its motion. 4. The amount is expressed using the unit joule (abbreviated J). 5. The energy stored in an object due to its position (or height). 6. The amount depends upon the arbitrarily assigned zero level. 7. Depends upon object mass and object speed. 8. If an object is at rest on the ground (zero height), it certainly does not possess this form of energy.

Work-Energy Theorem Work is the transfer of energy from one object to another, or . . The change in KINETIC ENERGY!

Total Energy At any point, the total energy of an object ME = KE + PE

Law of Conservation of Energy Energy cannot be created or destroyed. The total energy in a system before an interaction equals the energy after an interaction. This is true for a closed, isolated system (no external forces present)

Law of Conservation of Energy

Example 1 A 12 kg rock is at the edge of a 95 m cliff. a. What is the rock’s initial PE and KE?   b. If the rock falls to the ground, what is its final PE and KE just before it hits? c. What is the rock’s velocity just before it hits the ground?

Example 2 A 33 kg cart rests at the top of a hill. If the cart has a PE of 4800J, what is the height of the hill? When the cart has reached point B (h = 5.0 m), what is its KE?

Example Problem A greyhound at a race track can run at a speed of 16.0 m/s. What is the KE of the 20.0 kg greyhound as it crosses the finish line?

Example Problem 2 Determine the kinetic energy of a 625-kg roller coaster car that is moving with a speed of 18.3 m/s.

Example 3 Missy Diwater, the former platform diver for the Ringling Brother's Circus, had a kinetic energy of 12 000 J just prior to hitting the bucket of water. If Missy's mass is 40 kg, then what is her speed?

Example Problem Legend has it that Isaac Newton “discovered” gravity when an apple fell from a tree and hit him in the head. If a 0.20 kg apple fell 7.0 m before hitting Newton, what was its change in PE during the fall?