The ability to make something happen, to create a change Energy The ability to make something happen, to create a change
I. Types of Mechanical Energy A. Potential Energy 1. potential energy is stored energy 2. Does not involve motion or change 3. The object has potential to cause change
B. Gravitational Potential Energy (PE) 1. determined by its position above the earth. 2. PE = mgh m - mass (kilograms) g – gravity (9.8 m/s2) h – height (meters)
Both mass and height are proportional to the amount of potential energy!
C. Kinetic Energy 1. the energy of motion 2. Anything moving has kinetic energy 3. KE = 1/2mv2 m – mass (kilograms) v – velocity (m/s)
Both mass & velocity are proportional to the amount of energy
II. Units of Measurement A. Energy is measured in Joules (J) B. 1J = kg • m/s2
III. Law of Conservation of Energy Energy cannot be created or destroyed, but can change forms. The total of all the energies is constant. Look at the sum of the energies in the examples.
IV. Mechanical Energy 1. The total amount of kinetic and potential energy in a system. 2. ME = KE + PE 3. Also measured in Joules 4. What happens to the mechanical energy as KE & PE are converted into each other?
Example Problems 1. What is the potential energy of a 50N brick on top of a 4m ladder?
Example Problems 2. If a student drops a book having a gravitational potential energy of 5 joules from a height of 1.1 meters what is the mass of the book?
Example Problems 3. An SUV with a mass of 2,450 kilograms is traveling at 21 meters/second southeast. What is the kinetic energy of the SUV?
Example Problems 4. A jet skier with a total mass of 250 kilograms is traveling south with a kinetic energy of 40,500 J. What is the velocity of the jet skier?
Example Problems 5. A cat with a kinetic energy of 160 J is traveling up a tree at a velocity of 8 meters per second. What is the cat’s mass?
Practice Problems 1. Calculate the kinetic energy of a 3kg toy cart that moves at 4m/s. a. Calculate the kinetic energy of the same cart at twice the speed.
Practice Problems 2. Calculate the potential energy of a 90N block of ice sitting on a table 3m above the ground.
Practice Problems 3. Calculate the potential energy of a weightlifter holding a 100kg barbell above his head at a height of 2.2 m.
Practice Problems A 0.5kg sphere at the top of an incline has a potential energy of 6.0 joules. How tall is the incline? a. The sphere rolls halfway down the incline. What will the object’s new potential energy be?
Practice Problems 5. A 0.173kg baseball is thrown with a velocity of 2m/s. What I the kinetic energy of the baseball?
Practice Problems 6. A 3.1 kg cat sitting on a tree branch has a potential energy of 153.7 joules. How far above ground is the tree branch?
Practice Problems 7. A 1000kg roller coaster car sits at the top of a 30m drop. What is the gravitational potential energy of the roller coaster car?
Practice Problems 8. When a 5kg mass is lifted from the ground to a height of 10 meters, the gravitational potential energy of the mass is increased by approximately how much?
Practice Problems 9. A 50kg shell is shot from a cannon at Earth’s surface to a height of 450m. What is the gravitational potential energy of the shell at this height?
Practice Problems a. What is the change in the potential energy when the shell falls to a height of 225m?
Practice Problems What is the gain in kinetic energy when the shell falls to the height of 225m? c. What is the velocity of the shell at this new height?