Momentum & Energy
The kinetic energy of a pendulum is greatest at the point where it swings ______________
closest to the ground
A workman can lift barrels a vertical distance of one meter or he can roll them up a 2 meter ramp to the same elevation. If he uses the ramp, the effort (input force) _________
decreases. when distance is increased, force is decreased to keep "work" the same.
If you drop an object from rest, and it is freely falling, its potential energy at its maximum height will be _________________ it's kinetic energy just as it hits the ground.
equal to
In order to catch a baseball with minimal force on your hand, you should _______________________ over which your change the momentum of the ball.
increase the time
Compared to the force that brings a light car to a stop, the force required to bring a heavy truck, traveling at the same speed, to a stop is ______________
larger.
A simple machine is used to lift a heavy load. The input force (effort) was ______________________ the output force (load). The input distance was _____________________ the output distance (height the load was lifted).
less than ; longer than
The purpose of a simple machine is to enable you to _________________________________________________________________________
lift a heavy load with less force
If the speed of an object doubles then the _______________ of the object doubles and the ____________ is four times greater
momentum ; Kinetic energy
A moving object has __________________________________________
momentum and kinetic energy
What does an object have when moving that it doesn't have at rest?
momentum.
In order to do work more quickly you need ______________________
more power
A job is done slowly and an identical job is done quickly. Both jobs require the same amount of work but the job that is done more quickly requires ____________ power.
more.
An object has momentum. therefore, this object must be
moving
Suppose a gun were made of a strong but very light material. Suppose also that the bullet is more massive than the gun itself. Would you be willing to shoot this gun? explain your answer.
no. the mass of the gun compared to the massive bullet will cause a bad recoil.
Using 1000J of work, a toy elevator is raised from the ground to the second floor in 20 seconds. How much power does the elevator require?
p= w/t. 1000J/20 = 50 watts.
An object is raised above the ground gaining a certain amount of potential energy. If the same object is raised twice as hight it gains ____________________
potential energy.
When you are in the way of a fast moving object and you can't get out of its way, you will suffer a greater force of impact if you decrease the momentum of this object over a ____________________ period of time.
shorter
when you jump from an elevated position you usually bend your knees upon reaching the ground. by doing this the time of impact is about 10 times more than it would be in a stiff legged landing. In this way the average force your body experiences is reduced by:
ten.
Padded dashboards is cars are safer in an accident than non-padded ones because
the cushion increases the time over which momentum is changed.
A 1000N object and a 2000N object are raised the same distance. Raising the heavier object requires more
work.
An object with 100J of potential energy freely falls to the ground. It's kinetic energy just as it hits the ground is
100J
A machine is 75% efficient. this means that _____% of the work put into the machine is applied to useful work output and ______% is applied against friction.
25% ; 100%
A 2kg ball is thrown 3m/s. What is the ball's momentum?
2kg x 3 m/s = 6kg m/s
A TV set is pushed a distance of 2 meters with a force of 20N that is in the same direction as the set moves. How much work is done on this TV set?
2m x 20N = 40J
The work input to a machine is 500J. If the work output is 400J then the efficiency of this machine is
400J/500J = .8 x 100 = 80%
which requires more work: lifting a 50N sack vertically 2 meters of lifting a 50N sack vertically 4 meters? Explain your answer.
50N for 4 meters. same net force but longer distance.
A freight train rolls along a track with considerable amount of momentum. If it rolls at the same speed but twice as much mass, it's momentum is
Doubled.
Gym mats reduce the force on a gymnast because the mats ______________ the time over which her momentum is changed.
Increase
Which has the greatest momentum: a sitting cat, a walking cat, a running cat?
a running cat.
A 8lb object is lifted to 60ft above the ground. When released this object will hit the ground with _________ ft-lb of kinetic energy.
1/2 mass x distance above ground = ft-lb of KE 4 x 60 = 240 ft-lb
A 4kg ball has a momentum of 12kg m/s. what is the ball's speed?
12kg / 4kg = 3 m/s
A car moves four times as fast as an identical car. compared to the slower car, the faster car has _____ times more kinetic energy and will need _____ times more stopping distance.
four times = 4^2 = 16 16 ; 16
A rifle recoils from firing a bullet. The speed of the rifle's recoil is small because the ________________ of the rifle is large.
mass.
If an object has kinetic energy then it must also have
momentum
The reason the speed of a rifles recoil is small, as compared to the speed of the bullet it fired, is that the rifle has _______________________
more mass.
A car moving at 20 mi/hr skids 10ft with locked brakes. how far will the car skid if it is traveling at 60 mi/hr?
ms = Vfinal/Vinitial d = (multiple of speed)^2 x di 60/20 = 3 d = (3^2) x 10 = 90 answer = 90 ft.
a truck and a car has the same momentum. the car must be moving _______________
much faster than the truck.
A cannonball shot from a cannon with a short barrel will leave the barrel with a lower velocity than one shot from a cannon with a long barrel because the cannonball in the short barrel experiences a force over a ___________ time and over a ____________ distance.
shorter ; shorter
If work input equals work output then how can the use of a simple machine be advantageous?
small amount of effort to lift heavy load