You are riding in an automobile of mass 3000 kg. Assuming that you are examining the oscillation characteristics of its suspension system, the suspension hangs 15 cm when the entire automobile is placed on it. Also, the amplitude of oscillation decreases by 50% during one complete oscillation. The values of the spring constant k and the damping constant b for the spring and shock absorber system of one wheel (assuming that each wheel supports 750 kg) are:
5×104 Nm−1, 1344 kg s−1
Give, mass of the automobile m = 3000 kg
Since the suspension sags by 15 cm when the entire automobile is placed on it, we must remember there are four wheels hence four suspensions.
We have weight of entire vehicle = spring force due to four suspensions
3000×g=k×(15100)×4
⇒3000×10=k×15100×4⇒k=5×104 Nm−1
We know the time dependence of amplitude during oscillation
A(t)=A0e−bt2m
In one time period A(T)=A02 (is what is given)
A02=A0e−bt2m
⇒−In2=−bt2m
Also ω′=√km−b24m2⇒T=2π√frackm−b24m2
⇒In2=b×2π2m√km−b24m2
m2 (ln2)2=b2π2km−b24m2
(ln2)2 m2(km−b24m2)=b2π2
(ln2)2mk(ln2)24b2=b2π2
(ln2)2mkπ2+(ln2)24=b2
b≈1344 kg/s