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Dynamics block problem

  • Thread starter srh
  • Start date
  • #1
srh
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Homework Statement


A 40-kg package is at rest on an inlcine when a force P is applied to it. Determine the magnitude of P if 4s is required for the package to travel 10 m up the incline. The static (us) and kinetic (uk) coefficients of friction between the package and the incline are 0.3 and 0.25 respectively.


Homework Equations


Newton's 2nd Law : sum of forces = mass x acceleration
x=x0+v0t+0.5at^2

The Attempt at a Solution


First I changed my coordinate system to make the x plane parallel w/ the incline. I then summed the forces in the y direction in equilibrium to find the value of the N, normal force. I found this to be 368.73 N. (N-mgcos(20)=0)

THen I summed the x forces and set equal to mass x acceleration:
Pcos(50)-(uk)(N)=ma

To find a I used the relation x=x0+v0t+0.5at^2:
x-x0 = 10m
v0=0
t=4s
I found a to be 1.25 m/s^2.

Plugging in all the known values I got P = 355.35 N. I know that P is supposed to be 612 N. I was wondering if it was obvious where I was going wrong?

Homework Statement





Homework Equations





The Attempt at a Solution

 

Attachments

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Answers and Replies

  • #2
berkeman
Mentor
56,455
6,369
Thread moved from Engineering to Intro Physics.

Could you list all of the forces acting normal to the plane and parallel to it? I think you might be missing a term or two from the force pushing down and sideways on the block.
 

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