Kinetic Energy
What two variables directly determine the amount of kinetic energy an object possesses?
What are mass and velocity (or speed)?
Gravitational potential energy depends on mass, gravitational acceleration (g), and this measurement relative to a zero level.
What is height (or vertical position)?
What is energy cannot be created or destroyed, only transferred or transformed?
What is the Law of Conservation of Energy or 1st Law of Thermodynamics?
The SI unit used to measure work, energy, kinetic energy, and potential energy.
What is the Joule (J)?
Power is defined as the rate at which this is done over time.
What is Work?
If you double the speed of a baseball while keeping its mass constant, its kinetic energy increases by this factor.
What is 4 times (or quadruples)?
If an object is sitting at rest on the designated zero-level reference ground, its gravitational potential energy is equal to this value.
What is 0 Joules?
At what point on a frictionless coaster hill or ramp is potential energy at its minimum while kinetic energy is at its maximum?
What is the very bottom of the track/ramp?
A person applies a horizontal force of 50 N to push a crate across a floor a distance of 4.0 m. How much work was done on the crate?
What is 200 J?
W=Fd= (50 N)(4.0 m)= 200 J
What is the standard SI unit of Power?
What is Watts?
Calculate the kinetic energy of a 3.0 kg bowling ball rolling down a lane at 6.0 m/s.
What is 54 J?
(KE= 1/2mv2) = (1/2)(3.0 kg)(6.0 m/s)2= 54 J
Calculate the potential energy of an 8.0 kg shelf mounted 2.5 m above the floor.
What is 200 J?
PE=mgh (8.0 kg) (9.8 m/s2) (2.5 m)= 196 J= 200 J
A 10 kg object is dropped from a height where its initial PE is 500 J. On a frictionless system, what is its kinetic energy just before hitting the ground?
What is 500 J?
A student holding a heavy 15 kg box walks horizontally across a flat classroom at a constant speed for 10 m. How much work is done by the student's upward lifting force on the box?
What is 0 J?
(because the upward force is perpendicular to the horizontal displacement)
A machine performs 1200 J of work in 4 seconds. What is the power output of the machine?
What is 300 Watts?
P=W/t= (1200 J)/ (4 seconds)= 300 W
A 0.5 kg soccer ball has a speed of 10 m/s. What is the Kinetic Energy of the ball?
What is 25 J?
(KE= 1/2mv2) = (1/2)(0.5 kg)(10 m/s)2= 25 J
A mountain climber doubles her height above sea level while maintaining her mass. Her new potential energy compared to her original potential energy is this.
What is double (2 times as much)?
A sledder starts from rest at the top of a hill with 1000 J of mechanical energy. Halfway down the hill, their potential energy is 400 J. Assuming no friction, what is their kinetic energy at this midpoint?
What is 600 J?
TME= KE + PE
KE= TME- PE = 1000 J - 400 J = 600 J
Calculate the work required to lift a 12 kg suitcase vertically at constant speed onto a shelf 1.5 m high
What is 180 J?
F=mg= (12 kg) (9.8 m)= 117.8 N or 120 J
W=Fd= (120 N) (1.5 m)= 180 J
Two lifters raise the exact same 200 N weight to a height of 2.0 m. Lifter A takes 2.0 seconds and Lifter B takes 4.0 seconds. How do their work and power outputs compare?
What is they do equal amounts of work BUT lifter A generates twice as much power (200 W vs 100 W)?
If Car A (2.0 kg) has twice the mass of Car B (1.0 kg), BUT travels at twice the speed (10.0 m/s) vs Car A (5.0 m/s), which car has more kinetic energy AND by how much?
What is Car B by 2 times as much?
CAR A: (KE= 1/2mv2) = (1/2)(2.0 kg)(5.0 m/s)2= 25 J
CAR B: (KE= 1/2mv2) = (1/2)(1.0 kg)(10.0 m/s)2= 50 J
A 2.0 kg bird sitting on a tree branch has 147 J of gravitational potential energy relative to the ground. How high up is the branch?
What is 7.5 m?
h=PE/mg = 147 J/ (2.0 kg) (9.8 m/s2) = 7.5 m
An object is at rest on the top of a track 22 m in the air with a mass of 15 kg. The object moves 2 meters down the track. What is the objects new Kinetic Energy?
What is 200 J?
TME= PE + KE= (15 kg) (9.8 m/s2) (22 m) + 0 J = 3234 J or 3200 J
KE= TME- PE= 3200 J- ((15 kg) (9.8 m/s2) (20 m))= 200 J
A warehouse worker needs to load a heavy crate with a mass of 45.0 kg onto a storage shelf that is elevated 2.20 meters above the ground. The worker lifts the crate vertically at a constant speed. Calculate the total work done by the worker to lift the crate onto the shelf.
What is 970 J?
F=mg= (45.0 kg) (9.8 m/s2)= 441 N or 440 N
W=Fd = (440 N) (2.20 m) = 968 J or 970 J
An elevator motor lifts an 800 kg elevator car up a height of 10 m in 5.0 seconds at a constant speed. What is the power rating of the motor?
What is 20,000 Watts?
F=mg= (800 kg) (9.8 m/s2)= 7840 N or 8000 N
W=Fd= (8000 N) (10 m)= 80000 J
P=W/t= (80000 J)/ (5.0 seconds)= 16000 W or 20000 J