find the total force?

Question

we got this question in physics and i was wondering if you could explain to me how to do it. i started(work below) but not sure if i'm doing it right.

A 438kg car is accelerating at 2.55 m/s^2. if the coefficient of friction felt by the car is .0500, what is the total force acting east on the car? (hint: 4 calculations needed)

1st i did F=ma
438kg*2.55m/s^2 and got 1120 N

then i did Fsubf=uFsubn
.0500*1120N=56N

then i did 1120N -56 N=1064N
i thought this was the net force. but it says i need 4 calculations and i only have 4.

Could you explain what i'm doing wrong?

Thank you so much

Answer 1

The mistake you made is that you caculated the force of friction incorrectly. You're supposed to mulitpy the coefficient of friction by the normal force, NOT the force that causes the acceleration. Remember that, when an object is not accelerating in the vertical direction, the normal force is equal to the force of gravity. Now i'll do it the right way for you:

(1) You did the first step right, but I found the force to be 1116 N, not 1120 N (maybe you just rounded up too much).

(2) The second step was wrong. I'll show you how it's done:

Ff = u*Fn = u*m*g
Ff = (.0500)(438)(9.80) = 214 N

(3) F = 1116 - 214 = 902 N


* Don't listen to "Mad Scientist." You're supposed to subtract the forces. All you did wrong was miscalculate the force of friction. I know what I'm talking about. I got a 5 on the physics AP test.

** Let me explain to you why the normal force is equal to the force of gravity (mg). I will do this by summing up the forces in the y direction:

Fy = may

Fn - mg = may (we know that "ay" is 0, since the car is staying on the ground, and NOT accelerating in the y-direction)

Fn - mg = 0

Fn = mg


*** To "Mad Scientist:" I hope I didn't offend you. Also, AP Physics is not a degree. It's a test I took last year in high school.

Answer 2

yea, what the guy with the fancy "test he took" and the great intelligence below me says. I removed my answer. I am sorry I totally forgot that you used "a" in place of "g" in the second step.

Dumb me.

*Even dumber me, sorry Abdullah and yea, I am not offended... I am a Gandhian... hard to offend.