Find the potential and kinetic energy of the box

In summary, the conversation discusses finding the potential and kinetic energy of a 8.63 kg box sliding down a frictionless incline of angle 20 degrees. The method used is to calculate the velocity using acceleration due to gravity and time, then using the equations .5mv^2 and mgh to find the kinetic and potential energy respectively. The answer obtained is 1925.2569 J for potential energy and 157.3863974 J for kinetic energy. The person has entered this answer into an online system, but it is not accepted. They speculate that there might be an error in the program or a problem with the significant figures entered.
  • #1
Houyhnhnm
7
0
A 8.63 kg box slides down a long, frictionless incline of angle 20 degrees. It starts from rest at time t = 0 at the top of the incline a height 24.6 m above ground.
Find the potential and kinetic energy of the box at t = 1.8s. Answer in units of J.

acceleration is gsin20
so I got velocity
by gsin20 x 1.8 seconds
and then I used .5mv^2 to find KE
and then for PE
I found PE at the highest point
which is mgh
and then subtracted KE from that

I got 1925.2569 J for PE and 157.3863974J for KE (ignore sig figs)
I entered this into UTexas and got it wrong.
Please help.
 
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  • #2
Maybe I've had too much to drink this eveing but I can't see where you went wrong.
 
  • #3
Yeah neither can I
 
  • #4
Where is this UTexas thing?

Doug
 
  • #5
Originally posted by Houyhnhnm
I got 1925.2569 J for PE and 157.3863974J for KE (ignore sig figs)
I entered this into UTexas and got it wrong.
Please help.
Your method is fine. I assume you are entering your answer into some online system? If so, they can be picky; it might not tell you you are correct unless you give exactly the answer it expects. Try varying the number of significant figures you enter. (I hope you didn't enter all 8-10 "significant" figures!)
 
  • #6
This UTexas thing is at hw.utexas.edu. It's an online program that my physics teacher uses to torture us. Doc Al, the program specifically says "Enter what your calculator says" and it allows a 1% error anyways so I'm pretty sure that's not the problem.
 
  • #7
It's always possible that the program might contain an error. Have you asked your teacher about this problem?
 
  • #8
Heh...we've been snowed in for 2 days...problem has some time to fester...
 

FAQ: Find the potential and kinetic energy of the box

What is potential energy?

Potential energy is the energy an object possesses due to its position or state. It is stored energy that has the potential to do work.

How do you calculate potential energy?

Potential energy can be calculated using the formula PE = mgh, where m is the mass of the object, g is the acceleration due to gravity, and h is the height of the object above the ground.

What is kinetic energy?

Kinetic energy is the energy an object possesses due to its motion. It is the energy required to put an object of a given mass in motion.

How do you calculate kinetic energy?

Kinetic energy can be calculated using the formula KE = 1/2 * mv^2, where m is the mass of the object and v is the velocity of the object.

How are potential and kinetic energy related?

Potential and kinetic energy are related through the principle of conservation of energy. Potential energy can be converted into kinetic energy and vice versa. For example, when the box is released from a height, its potential energy is converted into kinetic energy as it falls.

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