Work and Power
What formula is used to calculate Work?
W = F d cos(theta)
What is the potential energy of a 100kg boulder sitting on a 10m tall hill?
9800J
A ball rolls up a hill and comes to a stop at the top. What kind of transformation of energy has taken place? From ______ to _______
From kinetic energy to gravitational potential energy
Which type of collision has two objects hit into each other and then stick together?
Inelastic
Gretta pushes a box with three times the force over twice the distance compared to Carl. How much more work does Gretta do than Carl?
W=Fd W=(3)(2) W=6 times as much force
What are the variables that affect how much Gravitational Potential Energy an object will have?
Mass, Height, g
What is the difference between a momentum of 10 kgm/s and -10 kgm/s
They are going in opposite directions (direction)
The total amount of energy in a closed, isolated system...
remains constant. Conservation of Energy
the total momentum of a bird and a balloon before they collide is 1000 kg m/s. What will the total momentum of the two after the collision be?
According to conservation of momentum: P after = P before so total momentum after = 1000 kgm/s
What is the work done to lift a 100kg box up 4 meters?
W=(100kg)(9.8)(4) W= 3920J
What are the variables that determine how much Kinetic Energy an object will have?
mass and velocity
Two identical cars are going the same speed when they hit a wall. If car A has a much larger impact time than car B, which one experiences a larger impact force?
As impact time increases, impact force decreases, so Car B will have a larger impact force than car A
A Ball being held at rest over the railing of a parking deck has a potential energy of 100J. Some time after the ball is dropped, it has a potential energy of 35J. How much kinetic energy will it have at this point?.
KE1 + PE1 = KE2 +PE2 0 + 100 = KE2 + 35 100 - 35 = KE2 KE2= 65J
What causes a change in momentum?
an Impulse causes a change in momentum
How do simple machines make work easier?
Reduce the force by extending the distance over which the work is done.
If an object moves at a constant speed and zero acceleration, will it have kinetic energy? Why or why not?
Yes, it will be KE=1/2mv^2
If the impact time of a collision gets 4 times longer, by what factor will the impact force change?
impact time gets bigger, impact force gets smaller. so 1/4
In order for energy to enter or leave a system, what has to be done?
Work
A ball has an initial momentum of 10 kgm/s when it hits a child who is at rest. After the collision, the ball has a final momentum of 2kgm/s. What is the final momentum of the child?
Total momentum before = total momentum after P ball before + P child before = P ball after + P child after 10 + 0 = 2 + P child after Pchild after = 8 kgm/s
A waiter exerts an upward force of 5N on a tray to move it 10 m across the room to the right. How much work is done on the tray?
0J
By what factor would the kinetic energy of a ball increase if it's velocity were tripled?
it would go up by a factor of 9 since the velocity is directly proportional and squared
What is the impact time needed to make a 100kg ball go from 20m/s to a stop with an impact force of 10N?
Ft=mv (10)t=(100)(20) 10t=2000 t=200 sec
A ball is thrown straight up, what kind of energy will it have at the top of its path?
Only potential energy. (velocity at the top of a projectile's arc is 0)
A car with a mass of 700kg is going 3m/s when it hits a 4kg box that is at rest. After the collision, the box bounces off with a velocity of 2m/s. What is the final velocity of the car?
total momentum before = total momentum after P car before + P box before = P car after + Pbox after (700)(3) + (4)(0) = (700)V + (4)(2) 2100 = 700V + 8 2092 = 700V V= 2.99 m/s