# Motion and Forces

learningphysics
Homework Helper
why is it - friction and not +?
Friction acts opposite the direction of motion... ie it tries to stop motion.

Ty - Wy + Ny = may
ok so

$$Tsin(\theta) - mg + N = 0$$

$$Tcos(\theta) - friction = ma_x$$
$$Tsin(\theta) - mg + N = 0$$

I also know that friction is uk*N, and I can plug in T and theta so:

$$10.6cos(15) - \mu_k*N = ma_x$$
$$10.6sin(15) - mg + N = 0$$

The unknowns in this equation are N and ax... you have two equations 2 unknowns... you should be able to solve a...

quick question...why is Fynet = 0?

for finding I have N I have
10.6 sin - 34.73N = -N
can I rewrite that as
10.6 sin + 34.73N = N
so I don't have that -N?

learningphysics
Homework Helper
quick question...why is Fynet = 0?
because the block is sliding horizontally... if the tension in the cord is large enough then Fynet won't be zero... but then there won't be a normal force or friction either... the block will lift off the ground... I'll put it this way... the vertical tension in the cord isn't large enough to lift the block... so the block stays on the ground.

As long as the block is on the ground vertical acceleration is 0, hence Fynet = 0.

learningphysics
Homework Helper
for finding I have N I have
10.6 sin - 34.73N = -N
can I rewrite that as
10.6 sin + 34.73N = N
so I don't have that -N?
If you multiply both sides by -1... then you get

-10.6sin(15) + 34.73 = N

Ahhh...I am getting the wrong answer...
okay I used Vf = Vo + at
Vf = 0 + (2.169 m/s^2)(3.98s) = 8.63 m/s

Sorry nevermind I don't know why I used 3.98! I have been doing this stuff all day and there are so many numbers in my head I am getting confused lol!