Question 268: Work, Energy and Power & Circular Motion

Question

An engine is attached to a wagon through a shock absorber of length 1.5 m. The system with a total mass of 40,000 kg is moving with a speed of 72 kmh$$-$$1 when the brakes are applied to bring it to rest. In the process of the system being brought to rest, the spring of the shock absorber gets compressed by 1.0 m. If 90% of energy of the wagon is lost due to friction, the spring constant is ____________ $$	imes$$ 105 N/m.

Accepted Answer

Given, the length of the shock absorber, l = 1.5 m
The total mass of the system, M = 40000 kg
The speed of the wagon, v = 72 km/h
When brakes are applied, the final velocity, v
f
= 0
The compressed spring of the shock absorber, x = 1 m
Applying the work-energy theorem,
Work done by the system = Change in kinetic energy
$$W = \Delta KE$$
$${W_{friction}} + {W_{spring}} = {1 \over 2}mv_f^2 + {1 \over 2}mv_i^2$$
$$ - {{90} \over {100}}\left( {{1 \over 2}m{v^2}} ight) + {W_{spring}} = 0 - {1 \over 2}mv_i^2$$ ($$\because$$ 90% energy lost due to friction)
$${W_{spring}} =  - {{10} \over {100}} 	imes {1 \over 2}m{v^2}$$
$$ - {1 \over 2}k{x^2} = {1 \over {20}}m{v^2}$$
$$k = {{m{v^2}} \over {10 	imes {x^2}}}$$
Substituting the values in the above equation, we get
$$k = {{40000 	imes {{\left( {72 	imes {5 \over {18}}} ight)}^2}} \over {10{{(1)}^2}}}$$
= 16 $$	imes$$ 10
5
N/m
Comparing the spring constant, k = x $$	imes$$ 10
5
The value of the x = 16.

Work, Energy and Power & Circular Motion

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