Quick question about work and forces(incline plane)

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Homework Statement



A block of mass is pushed up an incline
of width d and height h with coefficient of friction μk
up to the top of the incline by a constant applied force of
magnitude F parallel to the ground. The block begins
at rest.



Homework Equations



f=ma

w = f . d



The Attempt at a Solution



if i choose a coordinate system with an x axis Parrnell to the incline plane, then would it go like this:

normal force work= 0

applied force work = Fd/cos(60) <-- not sure about this one, my prof just wrote Fd in class, but i didnt understand why

work by fric = -ukmgd

work done by gravity = -mgh

is this right?
 

Answers and Replies

  • #2
PhanthomJay
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Homework Statement



A block of mass is pushed up an incline
of width d and height h with coefficient of friction μk
up to the top of the incline by a constant applied force of
magnitude F parallel to the ground. The block begins
at rest.
What is the question?

Homework Equations



f=ma

w = f . d



The Attempt at a Solution



if i choose a coordinate system with an x axis Parrnell to the incline plane, then would it go like this:

normal force work= 0
this is correct:approve:
applied force work = Fd/cos(60) <-- not sure about this one, my prof just wrote Fd in class, but i didnt understand why
where did the 60 come from? And work is not fd/cos theta. Work is force times displacement times the cos of the angle between the 2. And if d represents the base width of the incline, the distance along the incline is from heel to toe is greater than d.
work by fric = -ukmgd
if you are calling d the distance traveled along the incline, then yes, correct.
work done by gravity = -mgh
yes, correct, that is the work done by gravity from the bottom to top of the incline
is this right?
Please post the question exactly as written and perhaps as illustrated.
 
  • #3
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the question is just to find the work done by each of the forces, just really to find the expression for each individual force. i modified the applied force (F), because it is not parallel to my chosen coordinate system, it is parallel to the ground.(picture on its way)

http://s861.photobucket.com/albums/ab174/alkaline262/?action=view&current=inclineplanejpg.jpg

i actually know think work done by Force applied = Fsin(theta)d

:D
 
Last edited:
  • #4
PhanthomJay
Science Advisor
Homework Helper
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the question is just to find the work done by each of the forces, just really to find the expression for each individual force. i modified the applied force (F), because it is not parallel to my chosen coordinate system, it is parallel to the ground.(picture on its way)

http://s861.photobucket.com/albums/ab174/alkaline262/?action=view&current=inclineplanejpg.jpg

i actually know think work done by Force applied = Fsin(theta)d

:D
This is not correct. Firstly, you seem to have broken up the applied force into its components parallel and perpendicular to the incline.. Your trig is wrong. The component of the applied force along the incline is F(cos theta).

Secondly, the wording indicates or seems to indicate that 'd' is the length of the base of the triangle, and not the length of the incline. So work done by the applied force F is, by definition, F(L) cos theta, where L is the length of the incline; and since from trig L =d/cos theta, the equation for work simplifies to W =Fd, which is actually an alternate definition for work: W = force times distance travelled in the direction of the force, which is positive if the displacement is in the same direction of the force, and negative if the didsplacement is in the opposite direction of the force.
 

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