Question

Peter who weighs 80 kg drops into the valley below from the cliff top in a bungee jumping adventure. If it takes 7.5 s for him to drop down to the full length of the bungee cord before it pulls him back up, his kinetic energy at the moment is / (Take g = 10 ms ^-2)
1. 225 kJ
2. 375 kJ
3. 600 kJ
4. 750 kJ

Answer 1

Correct Answer - Option 1 : 225 kJ

CONCEPT:

• When a body moves in a straight line under constant acceleration, it follows three equations of motion.
• All unknowns are found by these equations.

v = u + at

s = ut + (1/2) at2 

v2 = u2 + 2as

where u is initial velocity, v is final velocity, a is constant acceleration, t is time and s is displacement.​

CALCULATION:

Given that t =  7.5 sec; m = 80kg; g = 10 ms-2

velocity of Peter after t = 7.5 sec

the initial speed of the ball

u = 0

using v = u + gt;

v = 0 + 10 (7.5)

v = 75 m/s

So Kinetic energy at this moment \(K= {1 \over 2}mv^2\)

\(K= {1 \over 2} \times 80 \times 75^2=225000J\)

K = 225 KJ

So the correct answer is option 1.