# Homework Help: Initial acceleration

1. Oct 31, 2013

### rysezhae

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

I need to find the initial acceleration of a steel object that weighs .89 kg, that is pulled along a steel a rail by a cord. The cord is at an 45 degree angle and has a tension of 21 N. The friction coefficents(spelled right?) of steel on steel are: .8 for static friction, .6 for kinetic friction and .002 for for rolling friction.

2. Relevant equations
F=ma
Fp=21 N

3. The attempt at a solution
What i did was seperate the net forces into the x direction and the y direction. In the x direction i got Fpcos(theta)-Fk=max (x = x direction). I then solved for ax giving me (Fpcos(theta)-Fk)/m.
Since Fk =ukFn (kinetic friction coefficent * normal force), i substituted that in for Fk in ax. In the y direction i got, Fpsin(theta)+Fn-Fe=0. I solved for Fn then substituted that back in for Fn in the ax equation getting: (Fpcos(theta)-uk(Fe-Fpsin(theta)))/m. No sig figs required, since the problem doesnt say anything about it

2. Oct 31, 2013

### Simon Bridge

$\cos(45^\circ)=\sin(45^\circ)=1/\sqrt{2}$

I'd have wanted to see you check that static friction is overcome first.

You require sig figs whenever you have measurements in your calculations - or do you think the values given are infinitely accurate?

Did you have a question?

3. Oct 31, 2013

### haruspex

What's rolling here? If the object can roll then you might need to worry about moment of inertia. (I dislike the term 'rolling friction'. Sometimes people confuse it with static friction. It is often not a form of friction, but rather of imperfect elasticity. 'Rolling resistance' is better.)