Energy Problems Review for Potential energy, Kinetic energy, Total Energy work, power.
Conservation of energy Energy cannot be created or destroyed. It can be transformed from one form to another, but the total amount of energy never changes.
Example: When a roller coaster goes down a hill some energy is “lost through friction.” Not really, it simply transforms to heat and sound.
PE = mgh Where : –Pe= Energy in Joules –m = mass (in kilograms) –g = gravitational acceleration of the earth (9.8 m/sec 2 ) –h = height above earth's surface (in meters) A cart is loaded with a brick and pulled at constant speed along an inclined plane to the height of a seat-top. If the mass of the loaded cart is 3.0 kg and the height of the seat top is 0.45 meters, then what is the potential energy of the loaded cart at the height of the seat-top? Potential Energy Answer: PE = aprox 13.5 Joules
Kinetic Energy KE = (1/2)mv 2 Where: –KE = Energy (in Joules) –m = mass (in kilograms) –v = velocity (in meters/sec) Determine the kinetic energy of a 1000-kg roller coaster car that is moving with a speed of 20.0 m/s. Answer: KE = 200,000 Joules
Total Energy Te=Ke+Pe Where: –Te = total energy –Ke = kinetic energy –Pe = potential energy A roller coaster starts falling from a height of 50 m. When the roller coaster hits the the 1 st drop it is still 20 meters above the ground. What would the KE and PE energy be at this point? And what is the velocity? Roller coaster: 200 kg
Work Two things enter into into every case where work is done The application of force And the movement of something W=FD F= force D= Distance Work is measured in Joules Force in Newtons Distance in meters
Examples… Carrying something up stairs, lifting anything, etc. How much work is a person using if he pushes a block with a force of 6N for 10 meters? F= (6)(10) F=60 Joules
Power The rate at which work is done Measured in Watts Power = Work done/ time interval Examples- lifting weights, climbing stairs, etc. Question: if a forklift is replaced with a new forklift that has twice the power, how much greater a load can it lift in the same amount of time?
Quiz Questions How much work is done on a 100 N boulder you carry horizontally across a 10 meter room? No work. “its carried horizontally” How much work is done if the boulder is lifted 1 meter? (100N)(1m) = 100 J
Quiz Questions What's the power expended when a 500 Newton barbell Is lifted 2.2 m in 2 sec? P=work/time interval W= (2.2m)(500N) = 1100 J P= 1100J/ 2 sec= 550 Watts
Questions What is the KE of 3kg toy that moves at 4m/s ? KE=.5mv squared KE= (.5) 3kg (4m/s) squared KE= 24 J
Questions a 5kg toy is dropped from the top of a building. Just before the toy hits the ground its KE is 7000 J’s. How tall is the building? SO…. PE= 7000 M= 5kg H= ? G= 9.8 7000= 5kg (9.8) (?) H= 142.85 m