Homework Help: Motion and Forces for a block held against a wall

1. Sep 14, 2016

maria1998

1. The problem statement, all variables and given/known data

Hello, I have attached a physics problem that I solved yet some answers (in red+the first answer) are wrong and I can't find out why...
2. Relevant equations

3. The attempt at a solution

2. Sep 14, 2016

BvU

Hello Maria,

Your question is clear enough, but PF works a little different (see guidelines). The picture can serve as a problem statement, but you still need to post the relevant equations and your attempt at solution (not just the outcome). So, show what you did to calculate the numbers and we'll help you further ...

3. Sep 14, 2016

maria1998

the problem is: In the figure, a block weighing 23.0 N is held at rest against a vertical wall by a horizontal force https://edugen.wileyplus.com/edugen/shared/assignment/test/session.quest2528958entrance1_N10036.mml?size=14&ver=1473845185939 [Broken] of magnitude 61 N. The coefficient of static friction between the wall and the block is 0.55, and the coefficient of kinetic friction between them is 0.38. In six experiments, a second force https://edugen.wileyplus.com/edugen/shared/assignment/test/session.quest2528958entrance1_N1005C.mml?size=14&ver=1473845185939 [Broken] is applied to the block and directed parallel to the wall with these magnitudes and directions: (a)35 N, up, (b)13 N, up, (c)49 N, up, (d)61 N, up, (e)9.0 N, down, and (f)18 N, down. In each experiment, what is the frictional force on the block, including sign? Take the direction up the wall as positive, and down the wall as negative. Next, calculate the acceleration, including sign, of the block in each case. Note that acceleration is zero if the block does not move.
(g) What is the acceleration in (a)?
(h) What is the acceleration in (b)?
(i) What is the acceleration in (c)?
(j) What is the acceleration in (d)?
(k) What is the acceleration in (e)?
(l) What is the acceleration in (f)?

I got a wrong answer for c and i here's what i did:
for the c : sum of forces=0 which means P-W-f=0 ; 49-23=26 so f=26 but it is wrong
and for the i: a=sum of forces/m and m=W/9.8 I got -27.35 but it is wrong

Last edited by a moderator: May 8, 2017
4. Sep 14, 2016

BvU

I didn't intend for you to retype the whole problem statement, sorry.

The correct sign for up and down follows from b) which is counted as correct answer: + 10 N means 10 N up. Hence the cross at answer a, I assume.

What is the direction of the friction in (c) ? Which way is your f positive ?

I expect the cross at c Units to disappear once the numerical value is right ?

5. Sep 14, 2016

maria1998

it's okay i just copy pasted it
anyway i found the right answers but i still don t understand when f is positive or negative...

6. Sep 14, 2016

BvU

In (b) 23 N is down, 13 N is up, so the sum is zero with 10 N up. +10 is counted correct, so up = positive in this exercise.

For (c) you would get 49 N - 23 N (down is negative) + f = 0 leaves -26 N for f. Apparently downwards. |-26| N is still below the max friction force, so -26 N should be the right answer.