Why Does My Calculation of Kinetic Energy Yield a Negative Result?

[couris]

Hello,
http://fm92-1.com/physics.jpg"
The initial speed is 0 m/s
We let go of the rope
What is the system's kinetic energy when the little block is at 15 cm from the ground?

What's wrong with my work?
$$E_{t}=E_{P}+E_{F}+E_{K}\Rightarrow E_{K}=E_{t}-E_{P}-E_{F}$$
where Et is total energy, Ep is potential energy, Ef is friction energy and Ek is kinetic energy.

$$E_{K}=0.75*9.81*0.45-0.75*9.81*0.15-0.1*2.5*9.81=-2.4$$
The answer is 1.47, but i get -2.4
What to do?
Thank you

 

[Poop-Loops]

Your last term, energy lost due to friction, is calculated wrong. Friction energy is force times distance. What you have is force. You need to multiply it by a distance (how far the block moves) to get energy.

I don't know if you get the right answer, but that's one thing that's wrong that I could find.

 

[Moetasim]

for bringing up the topic of energy conservation. The law of conservation of energy states that energy cannot be created or destroyed, it can only be transformed from one form to another. In this case, the energy of the system is initially in the form of potential energy (due to the height of the block) and is then transformed into kinetic energy (due to the block's motion) as it falls.

Your approach to solving for the kinetic energy is correct, using the equation EK = ET - EP - EF. However, there are a few errors in your calculations. First, the potential energy should be calculated using the mass of the block, not the coefficient of friction. Also, the friction energy should be calculated using the force of friction, not the coefficient of friction. Finally, the units in your calculations are not consistent, which can lead to incorrect results.

To get the correct answer of 1.47, the calculations should be as follows:

EK = ET - EP - EF
EK = (0.75 kg)(9.81 m/s^2)(0.45 m) - (0.75 kg)(9.81 m/s^2)(0.15 m) - (0.1 N)(2.5 m/s^2)(0.45 m)
EK = 3.329 - 1.115 - 0.225
EK = 1.989 J

It is important to double check your calculations and make sure all units are consistent to avoid errors. I hope this helps clarify the issue. Keep up the good work in your scientific studies!