Work
You push a box across the floor with a constant force of 45 N for a distance of 3 meters. How much work do you do on the box?
135 Joules
A 3 kg book is lifted from the floor to a shelf 2 meters high. What is the change in the GPE?
U = mgh
U = (3)(9.8)(2) = 58.8 J
A ball with a mass of 2 kg is moving at 3m/s. What is its kinetic energy?
KE = 0.5 x 2 (3)2 = 9 joules
What is the formula for potential energy stored in a spring?
1/2 kx2
What is the basic formula for power in terms of work and time?
A worker pushes a 20kg crate across a flat floor by applying a force of 100 N at an angle of 30 degrees below the horizontal. The crate moves 5 meters horizontally. What is the work done by the worker on the crate?
W = 100(5)cos(30)
W = 500(0.866) = 433 J
U = mgh
U = (0.5)(9.8)(15) = (0.5)(147) = 73.5 J
A 0.5 kg soccer ball is kicked at 8 m/s. Find the ball's kinetic energy.
0.5 X 0.5 X (8)2 = 16 joules
In the formula 1/2 kx2, what does "k" represent?
The spring constant
Watts (W)
A sled is pulled with a force of 60 N at an angle of 45 degrees to the horizontal over a distance of 10 meters. How much work is done by the pulling force?
W = 60(10)cos(45)
W = 600(0.707) = 424 J
A 1200 kg car is on top of a hill that is 50 meters higher than the surrounding ground. What is the GPE of the car relative to the ground?
U = mgh
U = (1200)(9.8)(50) = (1200)(490) = 588,00 J
A 1500 kg car at 20 m/s. What is its kinetic energy?
KE = 1/2mv^2
KE = 1/2(1500)(20)^2
KE750(400) = 300,000 J
How much potential energy is stored if a spring with a spring constant of 50 N/m is compressed 0.2 meters?
1 joule
A machine does 300 joules of work in 3 seconds. How much power does it output?
100 Watts
A 5 kg object is lifted vertically upward with a force of 60 N over a distance of 3 meters. Find the work done by the lifting force.
W = 60(3)(1) = 180 J
An elevator (mass 800 kg) rises from ground level to the 10th floor, which is 30 meters above the ground. What is the change in GPE?
U= (800)(9.8)(30) = (800)(294) = 235,200 J
A 1200 kg car accelerates from 10 m/s to 25 m/s over a distance of 200 meters. What is the change in kinetic energy of the car?
KEi = 1/2(1200)(10)^2= 60,000 J
KEf = 1/2(1200)(25)^2=375000J
375000-60000=315000 J
If you double the compression of a spring, by what factor does the potential energy change?
Four times (Quadruples)
True or False: You generate more power if you do the same amount of work faster.
True
A person pulls a 10 kg wagon with a horizontal force of 50 N, but the handle makes an angle of 20 degrees above the horizontal. The wagon moves 8 meters. How much work is done by the person?
W = 50(8)cos(20)
W =400(0.940) = 376 J
A 1000 kg rollercoaster car is at the top of 40 m hill. It then descends to a 10 m high point without any friction. How much GPE is lost?
Ui = mgh = (1000)(9.8)(40) = 392000 J
A 1200 kg car increases its speed from 5 m/s to 20 m/s. How much work was done on the car to cause this change in kinetic energy?
270,000 Joules.
A spring stores 8 joules of energy when stretched 0.5 meters. What is the spring constant?
64 N/m
A motor outputs 600 watts of power to lift a 100 N object. How fast is the object lifted (in meters per second)?
6 m/s