# Homework Help: Oscillations car suspension help

1. Oct 14, 2009

### physics_geek

1. The problem statement, all variables and given/known data
Four people, each with a mass of 71.7 kg, are in a car with a mass of 1150 kg. An earthquake strikes. The driver manages to pull of the road and stop, as the vertical oscillations of the ground surface make the car bounce up and down on its suspension springs. When the frequency of the shaking is 1.60 Hz, the car exhibits a maximum amplitude of vibration. The earthquake ends and the four people leave the car as fast as they can. By what distance does the car's undamaged suspension lift the car's body as the people get out?

2. Relevant equations

3. The attempt at a solution

so i used (2pif)^2 = k/(M+m) m = the weight of all four men

so (M+m)g-mg = k(x2-x1)

which gives delta_x = Mg/[(M+m)(2pif)^2)]

and i got .0776 m

but this is wrong? any help please

2. Oct 14, 2009

### Delphi51

Re: oscillations

Seems to me it should be (M+m)g-Mg = k(x2-x1)
since you want the difference between the car alone (M) and the car with people (M+m). Might be worth stopping to figure out k so we can compare answers at that point.

3. Oct 14, 2009

### physics_geek

Re: oscillations

well k wouldnt it just be (2pif)^2(M+m)

so at the end we would get delta_x = mg/((M+m)(2pif)^2) = .016m ??

4. Oct 14, 2009

### Delphi51

Re: oscillations

This is delta x = mg/k
Yes, looks good. But I don't get .016. What did you get for k?
Looks like we differ in the numbers entered or calculated.

5. Oct 14, 2009

### physics_geek

Re: oscillations

for k i got 1.742 x 10^5

what did you get?