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Old Jul 10th 2014, 11:26 PM   #1
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Frequency calculation- need help!

Hi there,

I'm really struggling with the below question.

The mass of a car is 1250kg and the mass of the driver is 80kg. When the driver sits on the seat, the suspension system of the car compresses by 0.02m. The car (with the drier sitting in it) is then pushed down by another person and let go, causing the car to oscillate up and down. Show that the frequency of the oscillation of the car is 0.864 Hz (3 s.f)

Okay, so what I did was calculate K from F=ky, using m x g for force (I combined the mass so it was 1330kg). Then I used the T equation (using mass and K). Then I thought frequency could just be calculated from f= 1/T, but I get a completely different answer to what I need to show.

Any help is really appreciated!
Thanks so much.
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Old Jul 11th 2014, 02:43 PM   #2
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Originally Posted by Fizzle View Post
Hi there,

I'm really struggling with the below question.

The mass of a car is 1250kg and the mass of the driver is 80kg. When the driver sits on the seat, the suspension system of the car compresses by 0.02m. The car (with the drier sitting in it) is then pushed down by another person and let go, causing the car to oscillate up and down. Show that the frequency of the oscillation of the car is 0.864 Hz (3 s.f)

Okay, so what I did was calculate K from F=ky, using m x g for force (I combined the mass so it was 1330kg). Then I used the T equation (using mass and K). Then I thought frequency could just be calculated from f= 1/T, but I get a completely different answer to what I need to show.

Any help is really appreciated!
Thanks so much.
I'm not getting the book answer, either. Can you show what you are doing?

-Dan
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Old Jul 11th 2014, 03:37 PM   #3
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Ok so this is my working:

F=ky => k= Fg/ y= 9.81x 1330/ 0.02= 652365 Nm-1

T= twopi square m/k= twopi square 1330/ 652365=0.2837 s

f=1/T= 1/ 0.2837= 3.52 Hz (3 s.f)

Is this the same answer you get?

Thanks for your help.
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Old Jul 11th 2014, 06:35 PM   #4
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Originally Posted by Fizzle View Post
Ok so this is my working:

F=ky => k= Fg/ y= 9.81x 1330/ 0.02= 652365 Nm-1

T= twopi square m/k= twopi square 1330/ 652365=0.2837 s

f=1/T= 1/ 0.2837= 3.52 Hz (3 s.f)

Is this the same answer you get?

Thanks for your help.
Yup. We're on the same page.

-Dan
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Old Jul 12th 2014, 03:09 AM   #5
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Thanks for taking the time to help- it must just be a misprint in my textbook then.
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