A physics question I already asked but was given wrong answer.?

Question

What should be the spring constant k of a spring designed to bring a 1200 kg car to rest from a speed of 109 km/h so that the occupants undergo a maximum acceleration of 5.0 g?

Any help here would be great!

Answer 1

I saw your question yesterday and agree that the answer you were provided was wrong. The solution you were given assumed a constant force from the spring which is incorrect since the force depends on the compression distance.

I think I solved it overnight thanks for re-posting.:

The maximum acceleration will be the instant before the car comes to a stop because that is when the spring will have maximum force.

this gives you the equation
F=K*x=m*a
K*x=1200*5*9.81

Then you can set up another equation where you solve for when the car comes to a stop using conservation of energy:

.5*m*v^2=.5*m*K*x^2
v^2=K*x^2
(109*1000/3600)^2=K*x^2

now you have two simultaneous equations and two unknowns K and X. I would solve for x first and use the positive root (it is a quadratic). Then solve for K.



j

Answer 2

A decellaration of 109kmph and the spring to weigh at 1200 kg or more to make the car come to a halt.

Answer 3

How do you know the first answer was wrong?

That said, what makes you think there are any physics majors here?

Oh, and you don't get acceleration when you bring a car to rest.