Help. Work done when going in circle.?

Question

Hi. I was just wondering about work when ex. a rollercoaster is going along a loop .the work done by friction would that be 2Pi * r * Ff? and why is this? Because I thought work in circular motion was 0 ...what about anything going in a curve ..If I were to push a pall in a curved path with a force .


I have one more ques that ties into this:
How do you know what forces doing the work are important? like when I push a car along the floor, the force I exert is the only one considered to change the energy ...
I understand this cause it's an external force. And what ABout other forces?.but isn't friction an internal force and it changes the total energy?? How do I determine this for all cases?

Answer 1

For the specific example the phrase work in a circular motion is 0 refers to the placement of the body. You can consider that the rollercoaster requires work to get to the top of the loop, but then it gets it back as the coaster goes to the bottom. that is an example of loop work =0. but in truth the coaster has friction througout the range of motion, which is where the 2 pi r Ff comes in.


In general, it helps to consider the energy of an object at two points in time to see which forces needed to be considered.
for pushing a car, is there a change in speed? if no then there is no change in kinetic energy. does the car change elevation? if no then there is no change in potential energy.
why are you pushing? to overcome friction. Can that energy input be recovered? no (the wheels and axels get warm) so the friction is a lost ennergy and would be considered in your evaluation of the system.

so in the future think about the final form of the energy, and if it can be used to do work. PE and KE can. a generator creates charge which can be stored and recovered (minus losses). a change in angular speed of a flywheel is a change in KE that can be recovered. charge used to separate hydrogen and oxygen can be covered as the h2 and O have chemical energy.
wind drag results in warming the air. that thermal energy cannot be recovered. a pump raises liquid elevation or pressure. those can be recovered and will be 0 in a loop. sometimes in classes you get a lot of questions about changes in pressure of a gas, the compression work and the energy stored. when a gas is compressed it heats and usually you can't recover that energy, but the gas under pressure can do work.

Also, as a general comment, if there is a force changing internal energy then the lost energy is likely to be negligible. For example that's why the common class questions assume a block sliding on a frictionless surface. the change in PE of the block is large compared to the friction losses. same with 'frictionless pulleys'.
but if the total change in energy is small, then the frictional loses are important (eg pushing a car with no change in speed)

Answer 2

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