Work and Energy
An object at rest may have
energy (potential or gravitational energy)
In physics, work is defined as
force times distance
If Nellie Newton pushes an object with twice the force for twice the distance, she does
four times the work
When a car's speed triples, its kinetic energy
increases by nine times
Potential energy is the energy an object has because of its
location
If the velocity of a moving object doubles, then what else doubles?
momentum
An object that has kinetic energy must be
momentum (moving)
A small economy car (low mass) and a limousine (high mass) are pushed from rest across a parking lot, equal distances with equal forces. The car that receives more kinetic energy is the
neither one—they receive the same amount of kinetic energy.
Kinetic energy of an object is equal to
one half the product of its mass times its speed squared
A job is done slowly, and an identical job is done quickly. Both jobs require the same amount of work but different amounts of
power
A 30 kg girl runs up the staircase to a floor 5 m higher in 8 seconds. What is her power output?
power=work done/ time interval work=force*distance=(mass x acceleration) x distance power=(30kg x 10m/2^2 x 5m)/8s power=1,500/8 power= 187.5W
If possible, determine the power expended when a barbell is raised 5.0m in 2s
power=work done/time interval power=0.5/2s=0.25W
The amount of potential energy possessed by an elevated object is equal to
the work done in lifting it
If you lift two loads up one story, how much work do you do compared to lifting just one load up one story?
twice as much
Power is defined as
work done on an object divided by the time taken to do the work
Sue can easily lift a 45.0-N rock with the help of a lever. When she pushes down with 20.0 N of force, she lifts the rock 0.3 meters. How far does she move her arms to do this?
0.7 m
In raising a 3000-N piano with a pulley system, it is noted that for every 2m of rope pulled down, the piano rises 0.2m. Ideally, what force is needed to lift the piano?
FORCEin*DISTANCEin=FORCEout*DISTANCEout FORCEin*2=3000*0.2 FORCEin*2=600 FORCEin*2/2=600/2 FORCEin=300N
True or False: More power is needed to carry a heavy suitcase slowly up a flight of stairs than to carry the suitcase quickly up the same flight of stairs.
False
The unit of work is the
Joule (J)
The 4.0-kg head of an ax is moving at 4.0m/s when it strikes a log and penetrates 0.01m into the log. What is the average force?
KE=1/2*m*v^2 KE=1/2*4.0kg*4.0m/s^2 = 32J Force=32J/0.01 = 3,200N
What amount of work can a 600 W motor do in 4 minutes?
Power=work done/time interval power=600W*4min (60s) - (multiply 4 and 60 and then change 600W to 600J) Power=600J*240s=144,000J
Which has a greater kinetic energy, a car traveling at 30km/h or half-as-massive car traveling at 60km/h?
The 60km/h car
True or False: Energy transforms from one form to another with no net loss or gain.
True
True or False: The energy an object has by virtue of its locatin is its potential energy.
True
What amount of work is done on a chair that pushed 9m across a floor by a horizontal 30-N net force?
Work=force*distance W=30N*9m= 270J
A woman can lift barrels a vertical distance of 1 m or can roll them up a 2 m ramp to the same elevation. If she uses the ramp, the applied force required is
1/2 as much
It takes 80J to push a large box 8m across a floor. Assuming the push is in the same direction as the move, what is the magnitude of the force on the box?
10N
An arrow in a bow has 70 J of potential energy. Assuming no loss of energy to heat, how much kinetic energy will it have after it has been shot?
70J
How many joules of work are done on a box when a force of 25 N pushes it 3 m?
75J
As a pendulum swings back and forth
at the end points of its swing, its energy is all potential; at the lowest part of its swing, its energy is all kinetic; kinetic energy is transformed into potential energy; potential energy is transformed into kinetic energy (all of the above)