? on change in mechanical energy

In summary, the conversation discusses a physics problem involving a block sliding down a rough incline. The goal is to find the change in mechanical energy due to friction, but the given values and heights are causing confusion. The concept of mechanical energy is related to potential energy and kinetic energy, and the correct approach is to use the equation ME(initial) = ME(final) + Wnc. The distance that affects gravitational PE is the vertical height, and the final velocity at the bottom of the incline is needed to solve the problem.
  • #1
ken.adams
1
0
I am having problems with this physics problem.
Starting from rest a 6.0 kg block slides 2.2m down a rough 30deg incline in 2.0s. find the change in mechanical energy due to friction. answer in J.

I know that change in mechanical energy is obtained by E=Ef-Eo or Wnc=(KEf+PEf)-(KEo-PEo) and I have used that but must be using the wrong #s.
The given values are easy to plug in but I am not sure on heights and velocities. the block travels down the incline 2.2 m which is only a height drop of 1.1m do I use 2.2 or 1.1m?, and I figure the velocity to be 2.2m/2.0s= 1.1 m/s ,but since the block starts at rest is my initial velocity is 0 and final 1.1 right and I have tried using 1.1 and 2.2 as starting heights and 0 as final height, but cannot get the right answer.
any help is appreciated
thanks
ken
 
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  • #2
Thread moved to Homework Help.

Welcome to the PF, Ken. I'm not familiar with the term "mechanical energy". How is mechanical energy related to potential energy (PE) and kinetic energy (KE)? The wording of the proble is a bit confusing, but it sounds like the friction force on the block is not enough to keep it from starting to slide on its own when released, so there will be some acceleration down the incline due to gravity, and hindered by the friction force. You can write a FBD equation for that interaction of forces and derive the total acceleration, which will head you in the direction of getting your answers.
 
  • #3
ken.adams said:
I know that change in mechanical energy is obtained by E=Ef-Eo or Wnc=(KEf+PEf)-(KEo-PEo) and I have used that but must be using the wrong #s.
That looks OK. Think of it this way: ME(initial) = ME(final) + Wnc.
Compare the initial mechanical energy to the final mechanical energy.
The given values are easy to plug in but I am not sure on heights and velocities. the block travels down the incline 2.2 m which is only a height drop of 1.1m do I use 2.2 or 1.1m?,
Which distance affects gravitational PE?

and I figure the velocity to be 2.2m/2.0s= 1.1 m/s
That would be the average velocity--you need the final velocity at the bottom of the incline. How does the average velocity relate to the initial and final velocity?
 

1. What is mechanical energy?

Mechanical energy is the energy that an object possesses due to its motion or position. It is a combination of kinetic energy (energy of motion) and potential energy (energy of position).

2. How does mechanical energy change?

Mechanical energy can change in two ways: through work done on the object or through the force of gravity. When work is done on an object, its mechanical energy increases. When work is done by the object, its mechanical energy decreases. Similarly, when an object moves against the force of gravity, its mechanical energy decreases, and when it moves with the force of gravity, its mechanical energy increases.

3. What is the law of conservation of mechanical energy?

The law of conservation of mechanical energy states that in a closed system (no external forces acting on the system), the total amount of mechanical energy remains constant. This means that mechanical energy can be transformed from one form to another (e.g. potential energy to kinetic energy), but the total amount of mechanical energy remains the same.

4. Can mechanical energy be converted into other forms of energy?

Yes, mechanical energy can be converted into other forms of energy, such as thermal energy, sound energy, or electrical energy. This conversion usually occurs through the process of work, where the mechanical energy of an object is used to do work on another object, resulting in a transfer of energy.

5. How is mechanical energy important in everyday life?

Mechanical energy is important in everyday life because it is the energy that powers many of the machines and devices we use, such as cars, elevators, and appliances. It is also essential for activities like sports and exercise, where our bodies convert chemical energy into mechanical energy to perform physical tasks. Understanding mechanical energy is crucial for advancements in engineering, transportation, and technology.

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