Solve Work Energy Power Homework with Friction Present

[GrandMaster87]

Homework Statement

Hi Guys

I have a problem when working with conservation of energy and work energy theorem. I have a problem when friction is present in the system.

I don't know when to add the friction to get the total work done or when to minus the friction to find work done.

Will someone be able to help me out?

1. Let's say someone is going UP a ramb and friction is present.
2. a skier is going down a slope and friction is present along the course.
3. a person is going down a slope , friction present and they ask you to find the skiers velocity at the bottom.
4. a car is moving up a slope and friction is present and you need to work out the velocity at the top

My Answer.

When someone is going up a ramp we add the work done by friction to obtain the work done.

2. when someone is going down a ramp we minus the friction to find work done.

Im not to sure about this. In my exam i lost the 13 marks for this question. Can someone just maybe give me some equations to use when friction is present for the different situations because when friction is present i get really confused.

Thanks a lot for reading..looking forward to your replies

 

[rl.bhat]

Work done = F*d*cosθ.
Frictional always acts in the opposite direction to the displacement.

 

[tomcue]

Hello,

I completely understand your confusion when working with friction in the context of conservation of energy and the work-energy theorem. In order to correctly solve these types of problems, it is important to understand the role of friction and how it affects the work done in a system.

To start with, it is important to remember that friction is a non-conservative force, meaning that it does not conserve energy in a system. This means that the work done by friction will not be included in the total energy of the system. Instead, we must consider the work done by friction separately and add or subtract it from the total work done in the system.

In the first situation, when someone is going up a ramp and friction is present, we must add the work done by friction to the work done by the person to obtain the total work done. This is because friction is acting in the opposite direction of the person's motion, so it is essentially doing negative work and reducing the total work done.

In the second situation, when a skier is going down a slope and friction is present, we must subtract the work done by friction from the work done by the skier to find the total work done. This is because friction is acting in the same direction as the skier's motion, so it is essentially doing positive work and increasing the total work done.

In the third situation, when a person is going down a slope and friction is present and we are asked to find the skier's velocity at the bottom, we can use the work-energy theorem, which states that the total work done on an object is equal to the change in its kinetic energy. In this case, we would use the work done by the person (minus the work done by friction) to calculate the kinetic energy at the bottom, and then use that to find the velocity.

In the fourth situation, when a car is moving up a slope and friction is present and we need to find the velocity at the top, we can use the work-energy theorem again, but this time we would use the work done by the car (plus the work done by friction) to calculate the kinetic energy at the top, and then use that to find the velocity.

In summary, when working with friction and conservation of energy, it is important to remember that friction does not conserve energy and must be considered separately. By using the work-energy theorem and correctly identifying the direction of friction, you can solve these types of problems with confidence.