Forces AND Friction AND Motion O MY

[kza62]

Homework Statement

A cart with a mass of 285 kg is pushed horizontally with a force of 1400 N. The acceleration of gravity is 9.8 m/s/s. The coefficient of friction (u) with the crate and the surface is 0.29. Calculate the acceleration of the crate.

Sooo given:

Fa: 1400 N
m: 285 kg
Fg: -9.8
u: 0.29
Ff: ?
Fn: ?
a: ?

Homework Equations

Fnet = ma
Ff = u * Fn

The Attempt at a Solution

Ok...
so if Fg is -9.8 then Fn is 9.8?
and then Ff = -(0.29)(9.8) which is -2.842?
so would Fnet = 1397.158 N?
and then do I use F = ma to find a?
so a = F/m which i got 4.9023
but the answer in the back of my textbook is 2.07028!
help please!

 

[tiny-tim]

Hi kza62!

so if Fg is -9.8

no no no no noooo …

-9.8 is the acceleration, isn't it? ​

 

[kza62]

not really dude, its moving horizontally, its not falling

 

[tiny-tim]

not really dude, its moving horizontally, its not falling

Yes, but I meant that when you wrote F_g = -9.8, I assumed that your F meant a force, so that would be -9.8m

 

[kza62]

ooooo okokokok but how do u get Fn?? that's the only thing i need to plug in everything else

 

[tiny-tim]

ooooo okokokok but how do u get Fn?? that's the only thing i need to plug in everything else

(just got up :zzz: …)

The ground is horizontal, so F_n (the normal force) is the same as F_g, and then F_friction = µF_n.

You know that, don't you? ​

 

[kza62]

so Ff = 0.29 x 9.8 = 2.842
and then Fnet = 1400 - 2.842 = 1397.158
a = 1397.158/285 = 4.902308... ?

what did i do wrong now?!

 

[tiny-tim]

I've told you … you have to use 9.8m.