Box on an inclined plane

  • Thread starter rumblef1sh
  • Start date
  • #1
I am stuck on a problem from the 5 Steps to a 5: AP Physics B&C book and would really appreciate anyone's help.

Homework Statement


Determine (a) the acceleration of the block down the plane and (b) the time for the block to slide to the bottom of the plane.

http://02.edu-cdn.com/files/static/mcgrawhillprof/9780071623209/INCLINED_PLANES_08.GIF [Broken]

Homework Equations


F=ma
F, gravity = mg
F, friction = umg
Kinematic equation ???

The Attempt at a Solution



Fx, up the plane = mgsin25 + umgcos25 = 204 N
a = F/m = 204/30 = 6.8 m/s^2
Fx, down the plane = mgsin25 - umgcos25 = 44 N
a = F/m = 44/30 = 1.5 m/s^2

I tried using x = Vot + 1/2 at^2 (x = 20 m, Vo = 3m/s) and v = Vo + at (v = 0, Vo = 3 m/s) to solve for time, but my answers were way off on both attempts.
 
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Answers and Replies

  • #2
435
13
If the question says the mass is going 3 m/s up the plane, and the a) part asks for the acceleration DOWN the plane this means that x != 20, but it is going to be 20 added to the distance travelled by the mass until it changes direction. I hope that helped.
 

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