# Conservation of Energy With Friction

• jakeginobi
In summary, When a block with a mass of 10 kg is pulled 13m across a floor by a 50N force, with friction doing 380J of work, the final velocity of the block can be found using the work-energy theorem. This involves setting up the equation KEfinal - KEinitial = Wfriction + W50 N force, and finding expressions for each term to solve for the final velocity. It is important to consider the direction and sign of the work done by friction and the 50N force, as they can affect the result. In this problem, the final velocity is 7.3 m/s, not 8.72 m/s as originally calculated, because the 50N force and friction have
jakeginobi

## Homework Statement

A 10 kg initially at rest is pulled 13m across a floor by a 50N force. if friction does 380J of work over this distance, what is the block's final velocity? http://imgur.com/a/zM1MX

W=Fd, Ek=1/2mv^2

## The Attempt at a Solution

Since the block was at initially at rest the Energy initially is 0, so I set up the equation like this 0 = Ek after + Efriction. Since friction is 380J i moved it to the other side, then did 1/2mv^2 = 380J to find V which I got 8.72 m/s, but the answer is 7.3m/s. Did I set up the equation wrong?

You forgot to include the 50 N force in your calculation.

kuruman said:
You forgot to include the 50 N force in your calculation.
How would the entire equation look like in terms of Energy like Ek before = Ek after - E friction?

It is the work-energy theorem.

KEfinal - KEinitial = Wfriction + W50 N force

You need to find expressions for each one of the terms and put it together.

kuruman said:
It is the work-energy theorem.

KEfinal - KEinitial = Wfriction + W50 N force

You need to find expressions for each one of the terms and put it together.

Oh okay thank you. Last question is work and energy the same thing, and are they both scalar quantities?

jakeginobi said:
Oh okay thank you. Last question is work and energy the same thing, and are they both scalar quantities?
Yes, they're both scalars. Energy is the more generic term, while work is used for energy transfer.
In thermodynamics, work refers to energy transfer in mechanical form at the macroscopic level, while heat refers to energy transfer at the level of uncoordinated motion of molecules. Exactly where one draws the line is not defined, and may depend on context.
In regard to this thread, note that in kuruman's equation both the work terms on the right refer to work done on the object. They will not both be positive.

haruspex said:
Yes, they're both scalars. Energy is the more generic term, while work is used for energy transfer.
In thermodynamics, work refers to energy transfer in mechanical form at the macroscopic level, while heat refers to energy transfer at the level of uncoordinated motion of molecules. Exactly where one draws the line is not defined, and may depend on context.
In regard to this thread, note that in kuruman's equation both the work terms on the right refer to work done on the object. They will not both be positive.
http://imgur.com/a/VHZyU For a question like this, does it matter which direction the velocity is? would I take the resultant velocity to find kinetic energy, so 1/2(m)(500m/s)^2? Or do I need to break it into x and y components?

jakeginobi said:
would I take the resultant velocity to find kinetic energy, so 1/2(m)(500m/s)^2?
I assume you mean for the initial KE. Yes, try that.

haruspex said:
Yes, they're both scalars. Energy is the more generic term, while work is used for energy transfer.
In thermodynamics, work refers to energy transfer in mechanical form at the macroscopic level, while heat refers to energy transfer at the level of uncoordinated motion of molecules. Exactly where one draws the line is not defined, and may depend on context.
In regard to this thread, note that in kuruman's equation both the work terms on the right refer to work done on the object. They will not both be positive.
Sorry last question, about kuruman's equation, why won't both be positive since they're both scalars I thought direction wouldn't affect it?

jakeginobi said:
Sorry last question, about kuruman's equation, why won't both be positive since they're both scalars I thought direction wouldn't affect it?
Scalars can be positive or negative. The direction of the vector quantities that go into calculating a scalar may (or may not) affect the sign of the scalar. So the general answer is "it depends".

For kinetic energy, ##E=\frac{1}{2}mv^2##, it turns out that one is taking the scalar product of a vector ##\vec v## with itself and multiplying the result by ##\frac{1}{2}m##. Direction does not matter. But in the case at hand, I think you are asking not about kinetic energy, but about work.

We are considering the work done by friction and the work done by the 50N force. Is the work done by friction on the 10 kg object positive or negative? Is the work done by the 50N force on the 10 kg object positive or negative?

jbriggs444 said:
We are considering the work done by friction and the work done by the 50N force. Is the work done by friction on the 10 kg object positive or negative? Is the work done by the 50N force on the 10 kg object positive or negative?
And remember, the work done by a constant force on an object is the product of three quantities, the magnitude of the force doing the work, the magnitude of the displacement of the object and the cosine of the angle between the force and the displacement. So, under what circumstances is the work done by the force on the object positive, zero or negative?

jbriggs444

## 1. What is the law of conservation of energy with friction?

The law of conservation of energy with friction states that energy cannot be created or destroyed, but it can be transformed from one form to another. In the case of friction, some of the energy is transformed into heat, sound, or other forms of energy, resulting in a loss of energy.

## 2. How does friction affect the conservation of energy?

Friction plays a crucial role in the conservation of energy. It converts a portion of kinetic energy into other forms, reducing the overall amount of energy in a system. This decrease in energy is accounted for by the increase in the internal energy of the system due to the heat generated by friction.

## 3. Can friction ever increase the amount of energy in a system?

No, friction can never increase the amount of energy in a system. As stated by the law of conservation of energy, the total amount of energy in a system remains constant. Friction simply transforms the energy into other forms, but it cannot create or add energy to a system.

## 4. What are some examples of conservation of energy with friction?

One example of conservation of energy with friction is when a car is braking. The kinetic energy of the car is converted into heat due to friction between the brake pads and the wheels. Another example is rubbing your hands together, which produces heat due to friction between your hands.

## 5. How can we reduce the loss of energy due to friction?

There are a few ways to reduce the loss of energy due to friction. One way is to use lubricants to reduce the surface contact between two objects, thus reducing friction. Another way is to use smoother surfaces or materials with lower coefficients of friction. Additionally, reducing the speed or force of the moving objects can also help decrease energy loss due to friction.

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