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Energy Calculations
PE or KE
Energy Conversions
Work or No Work
100
At the top of a roller coaster hill, 20m high, a 60 kg student has this amount of potential energy.
What is 11,760 J ? GPE = mgh GPE = (60)(20)(9.8) GPE = 11,760 J
100
Sheep are grazing on grass in a field. The chemical energy stored in the grass would fit under this type of energy.
What is potential energy?
100
As a ball is launched from the ground, at the launch location it has a maximum amount of _________ and little if any _______.
What is kinetic energy , potential energy?
100
A student pulls a toy wagon by its handle along a road?
What is work is done on the wagon by the person pulling it. Positive work specifically
200
A 60 kg student starts at the top of a 20 m high roller coaster hill and then moves halfway down the hill. How much kinetic energy do they have when they are half way down the hill?
What is 5,880 J? GPE is now (60)(10)(9.8) 5880 J Initial energy is 11, 760 J so this means that they now have 5,880 J of KE
200
A ball is sitting on top of shelf where a small child cannot reach it. This type of energy is associated with the ball sitting at rest on the shelf.
What is potential energy (specifically gravitational potential energy)?
200
As an object is dragged along a surface which has a lot of friction some energy is transferred to the surface in the term of thermal energy. Is this a violation of the law of conservation of energy?
What is NO this is not a violation of law of conservation of energy. The law says that energy can be transferred or transformed. This is how we track energy in a system as objects interact with each other.
200
A waiter applies an upward force on a tray of food as they carry the tray horizontally across the dining room. Does the waiter do work on the tray?
What is NO work? recall F and d are at 90 degrees here
300
A 5 kg ball starts at the top of a 2m high ramp. At the bottom of the ramp how much Kinetic energy does the ball have?
What is 98 J?
300
A basketball is falling through the hoop. This type of energy is associated with the falling ball.
What is Kinetic energy (specifically mechanical energy)?
300
An 80 kg cliff diver's kinetic energy increases by 9400J as he falls. How far did the diver fall?
What is 12 m? the change in PE to KE is 9400 this means that the PE "lost" is 9400 we know that PE =mgh we know m, g, and PE so we find h h = PE/ (mg) h= 9400/ (9.8 * 80) h=11.989 or 12 m
300
A 12N force is applied at an angle of 30 degrees to a box as it is moved right along a surface for a distance of 6 m. How much work, if any, does the force do on the box?
What is 31.18 J? W = Fdcos theta W = (12)(6)cos (30) W = 31.17 J
400
A 3kg object is at the top of a 5 m high ramp. At the bottom of the ramp how fast will it be going?
What is KE f= PE i KEf = 147 J KE = 1/2 mv^2 v = sqrt (2KE/m) v = sqrt (2*147/3) v = 9.899 or 9.9 m/s
400
The blades of a fan are rotating to keep the room cool. This type of energy is associated with the rotating fan blades.
What is kinetic energy (specifically mechanical energy)?
400
Two 0.75 kg blocks sit at the top of a 6.8 m frictionless ramp which is at an angle of 19 degrees with the ground. If one block falls straight down to the ground and the other slides down the incline. How does the work done by the gravitational force compare on each of these?
What is the work done by the gravitational force is the same? This one is a little tricky Recall we said work is W=Fdcos theta recall also that we said the gravitational force is equal to mg so our formula becomes W=mgdcos theta Recall also that the cosine of theta is designed to pull out the component of the displacement which is in the direction of the force. The force is down for work we are only concerned with the portion of the displacement which is down. The start at the same height and both end up on the ground so the height change is the same. therefore the work done by the gravitational force is the same.
400
An object moving at constant velocity will have __________ net work done on the object. Why?
What is NO, because the net force is zero so individual forces may do work on the system but the energy one force is adding to the object is begin "removed" by some other force. Recall work energy theorem states W = delta KE no change in v then no change in KE so no work!
500
In the sun, hydrogen atoms combine to form helium atoms. This process releases light and thermal energy. This form of energy is related to changes in the nucleus of an atom.
What is nuclear energy?
500
The change in energy as a ball is thrown up and as the ball is falling down.
What is kinetic energy to potential energy (as the ball goes up) and potential energy to kinetic energy (as the ball goes down)?
500
The work energy theorem states
What is the work done on an object is equal to the change in kinetic energy of the object?
500
A 5 N force is applied to a 6kg object as it slides across a rough surface at constant velocity. (a) does friction do work on the object as it moves along the ground? (b) what is the coefficient of kinetic friction between the object and the ground?
What is Yes it does negative work on the object we would need to know the distance it is displaced in order to calculate the work. What is .09? Ff = FA Ff = 5N Ff = mu FN 5N = mu (6)(9.8) mu = 5/(6*9.8) mu =.09