# Finding acceleration in spring situations

1. Nov 14, 2012

### bunk66

1. The problem statement, all variables and given/known data
A duck of mass 40kg is being pulled by a spring with k = 900N/m. If μk = 0.6 between the duck and the road, find the acceleration of the duck given that the spring stretches 0.4m.

2. Relevant equations
Fspring = kx, where k = spring constant and x = stretch of spring
Fnet = ma

3. The attempt at a solution
Fspring = kx
F = 900N/m(0.6m)
F = 540N

That's about as far as I got before not knowing what to do next.

Last edited: Nov 14, 2012
2. Nov 14, 2012

### haruspex

Those are two different forces, so give them different names.
In which directions do they act?
How do they relate to the acceleration of the duck?

3. Nov 14, 2012

### bunk66

What exactly do you mean that they are two different forces? In the section that you quoted, isn't there only one force being calculated: the spring force?

And p.s, I made a typing error. Where it says 0.4, it should be 0.6

4. Nov 14, 2012

### haruspex

OK, that's what made me think you were referring to two different forces.
So, what is the frictional force acting on the duck (direction and magnitude)?

5. Nov 14, 2012

### bunk66

I mixed some of the numbers apparently.

Fspring = kx
F = 900N/m(0.4m)
F = 360N

40kg x 9.8 = 392N

Ff = μk(FN)
Ff = 0.6(392N)
Ff = 236N

Fnet = Fa - Ff
Fnet = 360N - 236N
Fnet = 124N

Fnet = ma
Fnet/m = a
124N / 40kg = a
3.1 m/s2 = a

Evidently that was a lot easier than I made it out to be