Question

A block weighing 67.5 N rests on a plane inclined at 25.0° to the horizontal. A force F is applied to the object at 35.0° to the horizontal, pushing it upward on the plane. The coefficients of static and kinetic friction between the block and the plane are, respectively, 0.391 and 0.156.

Answer 1

(b) What is the minimum value of F that will start the block moving up the plane?

Answer 2

do you know how to find the normal force?

Answer 3

yep thats easy

Answer 4

N = m*g*Cos(25)

Answer 5

so then that makes static friction easy to find...
can you also find the force of gravity acting along the incline then?

Answer 6

yep

Answer 7

Wx = m*g*Sin(25)

Answer 8

Yes i get the static friction

Answer 9

okay, those are the two forces opposing motion of the block up the incline
so the sum of those forces (or rather, infinitesimally over that) is the force required to overcome those two and push the block up the plane.

Answer 10

So this is what i have:
F -(Fs + Wx) = m*a

Answer 11

here i dont have the acceleration

Answer 12

i have 2 unknowns

Answer 13

you just asked for F, nobody said anything about acceleration...

Answer 14

sorry its Fr instead of just F
so, Fr - (Fs+ Wx) = m*a

Answer 15

if we had a then it would have been easier

Answer 16

so where do i go from here

Answer 17

whatever you call it, the point is that the block will start moving just past the point of maximum static friction, at which point the block is still not moving, so the forces sum to zero!

Answer 18

I think i need to figure out another equation

Answer 19

Fr - (Fs+ Wx)=?
just before the block gets enough force to start moving, it's not moving. Therefore it's not accelerating. Therefore just befor the block moves the sum of the forces is...?

Answer 20

oh so its 0

Answer 21

Fr = (Fs + Wx)

Answer 22

yep... at least that is right before it starts moving

Answer 23

In a way I think the question is flawed because it would be infinitesimally little *more* force than that to actually get it to move, but I think we can overlook that here.

Answer 24

and if it starts moving with constant velocity
then --> Fr =(Fk+Wx)
right

Answer 25

Fk is the kinetic friction force

Answer 26

...but it would have to accelerate from rest to get a velocity; it can't go from not moving to moving without accelerating, ya know what I mean?
but again, I think they want you to see it the way you just did, so I would go with that.
Fr =(Fk+Wx)

Answer 27

you r right

Answer 28

yeah, it's subtle
if the forces are equal it could /keep/ moving with some constant velocity, but it can never change
but the heck with that, the answer is Fr=(Fk+Wx)+dF (where dF is a differential force that gets it moving, so we can ignore it)

Answer 29

Alright i will be back but in the time can you give me the right force for (b)

Answer 30

just go with what you have...

Answer 31

Cuz I got 53.24 N and it says its wrong

Answer 32

this is what i had

Answer 33

you can't use the kinetic force, it isn't moving yet!
jeez this question has all kinds of subtleties
it asks for the force to get it to *start* moving

Answer 34

---> F*Cos(10) = Fs+Wx

Answer 35

why cos(10) ?

Answer 36

cuz the force is in an angle of 35 and the incline is in an angle of 25 with the horizontal

Answer 37

U can post ur comments please .
I willl be back in couple of minutes, alright.

Answer 38

sure, and yeah, you are right about the cos(10) thing i missed that part

Answer 39

I still cant get this right