Homework Help: Physics- Elena please help!

Posted by Anonymous on Wednesday, October 10, 2012 at 12:47pm.

A sled loaded with bricks has a total mass of 18.0 kg and is pulled at constant speed by a rope inclined at 40.0o above the horizontal. The sledge moves a distance of 16.0 m on a horizontal surface. The coefficient of kinetic friction between the sledge and surface is 0.510 .
(a) What is the tension in the rope?
b) How much work is done by the rope on the sled?
c)What is the magnitude of the force of friction?
(d) What is the mechanical energy lost due to friction?

Physics- Elena please help! - Damon, Wednesday, October 10, 2012 at 1:08pm
force pulling sled = T cos 40
force lifting sled = T sin 30

normal force = mg - T sin 40
friction force = 0.51(mg - T sin 40)

acceleration = 0
so
friction force = pulling force
so
0.51(mg - T sin 40) = T cos 40
solve for T

then work = 16 * T cos 40

etc
Physics- Elena please help! - Anonymous, Wednesday, October 10, 2012 at 1:14pm
Can you tell me the values you are getting so I can make sure I did the algebra right?
Physics- Elena please help! - Damon, Wednesday, October 10, 2012 at 1:17pm
How about the other way around? You post the values and I check. I already took the course (in 1955).
Physics- Elena please help! - Anonymous, Wednesday, October 10, 2012 at 1:17pm
For Tension I am getting 117.4
Physics- Elena please help! - Damon, Wednesday, October 10, 2012 at 1:23pm
m g = 177 N

.51 (177 - .643 T) = .766 T

90.3 - .328 T = .766 T

T = 82.5 N

T cos 40 = 63.2 = pull force = friction force
63.2 * 16 = 1012 Joules = work done (also energy lost to friction)
Physics- Elena please help! - Anonymous, Wednesday, October 10, 2012 at 1:29pm
What is the label for the work? I tried Newtons and that is wrong.
Physics- Elena please help! - Damon, Wednesday, October 10, 2012 at 1:30pm
Work is in Joules !!!!
Physics- Elena please help! - Damon, Wednesday, October 10, 2012 at 1:31pm
work done against friction = ENERGY lost
That is JOULES !
Physics- Elena please help! - Anonymous, Wednesday, October 10, 2012 at 1:31pm
Oh ya, I understand part a and b now, but I am still confused on part c. How would I solve this?
Physics- Elena please help! - Damon, Wednesday, October 10, 2012 at 1:36pm
We did part C immediately

T cos 40 = 63.2 = pull force = friction force
Physics- Elena please help! - Damon, Wednesday, October 10, 2012 at 1:37pm
It is not accelerating so friction force is equal and opposite to pulling force.
Physics- Elena please help! - Damon, Wednesday, October 10, 2012 at 1:39pm
Now slow down and draw a good picture and follow what I did carefully step by step. I have to go coach a high school sailing team now.

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