physics problem... PLEASE HELP ME!!!!

[piper210_355]

ok so would the normal force still be mass times gravity in this problem or no.

[big314mp]

No. Normal force is the force perpendicular to the plane. It is the green line (or the unlabeled blue, same thing) in the picture I posted.

 

Mass times gravity is the red line.

 

I'll list the relevant equations we have come up with, before I go to dinner:

 

F(f) = u*F(n)

 

F(f) = F(x)

 

tan(theta) = F(x)/F(n)

 

Rearrange those, and you should figure out theta. You don't actually need to find F(x), F(f), or F(n).

[piper210_355]

ok so I rearranged and I got this:

 

tan(theta)=F(x)/F(n)

tan(theta)=F(f)/[F(f)/u)]

 

Here is where I'm not sure if i'm over simplifying(since in the other one i am dividing by a fraction i then change it by multipling by the reciprical)

tan(theta)=F(f)*[u/F(f)]

 

This cancels out the F(f) leaving...

tan(theta)=u

 

so then

(theta)=tan^(-1) .45

(theta)=24.23

 

 

is that right? I really hope it is(I think it is). I'm pretty sure i know how to find the acceleration from here. All i need to know is if the angle is right.

[Klaynos]

Looks ok to me

[piper210_355]

:DYES:D

 

Ok I got the rest of these steps from a video on youtube, but I just want to make sure it's right.

 

First it said to get the force of gravity:

F(g)=(gravity)(mass)

F(g)=(9.81)(2.5)

F(g)=24.5N

 

Then I found the Force of the parallel (that's what they called the line in between F(n) and F(g) in the video).

F(p)= F(g)sin(24.23)

F(p)=(24.5)(sin 24.23)

F(p)= 10.05

 

After that I used newton's second law to find the acceleration.

a=F/m

a=(10.05)/2.5

a=4.02m/s^2

[big314mp]

Don't forget the coefficient of kinetic friction. You need to take that into account. Just calculate the force perpendicular (the normal force) using your trig, multiply by the coefficient of kinetic friction, and subtract from the force parallel. Then use that final force value for your last calculation.

 

Other than that, excellent work!

[piper210_355]

ok so i'm going to put what i did just to make sure i understood/did it right

tan 24.23=F(n)

.45=F(n)

 

 

F(n)F(k)=F(?what exactly am i calculating here?)

(.45)(.28)=F(?)

.126=F(?)

 

 

F(p)-F(?)=F

10.05-.126=F

9.92=F

 

 

a=F/m

a=9.92/2.5

a=3.97m/s^2

 

I hope that's right

thank you sooooooooooo much for your help

[big314mp]

Use the same method you used to calculate the force parallel.

 

F(perp) = F(g)*cos(24.23)

 

The F(perp) is your normal force. You calculate the kinetic friction the same way you calculate the static friction.

 

F(f) = F(perp)*u(kinetic)

 

This force "takes away" from the force accelerating the box, so you subtract it from the force accelerating the box. In physics terms, this force is acting in an opposite direction from the force parallel.

 

So subtract the kinetic friction force from the force parallel. The number that this gives you is the net force on the box. The net force, divided by the mass, will give you the acceleration.