Question

8. A girl weighing 45 kg skips 0.075 m high 10 times. If
the energy thus spent is just sufficient to stop'a thief
weighing 65 kg, find the speed at which the thief is
running
[Ans. 3.19 m s-l1​

Answer 1

Answer:

Energy spent by girl to skip 10 times to height 0.075 m is:

mgh × 10 = 45 × 10 ×10 × 0.075 = 337.5 J

Energy required to stop the thief can be calculated as:

mv22=65v22

Given,

mv22=65v22=337.5 v2=10.38⇒v=3.22 m/s approx

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Answer 2

Answer:

3.8... m/s (approx)

Explanation:

Given:

• Weight of the girl = 45 kg
• Height the girl skips too = 0.075 m
• No.of times she skips = 10
• Gravitational force = g = 10 m/s²

To find:

• Speed at which the thief is running (v)

Height acquired = no.of steps × height acquired = 0.075 m

Height acquired = 10×0.075m = 0.75 m

First let us find the enrgy acquired due to potential energy:

Potential energy = m×g×h

Potential energy = 45×10×0.75

Potential energy = 337.5 Joules

Potential energy here will be equal to the kinetic energy

$\frac{1}{2} mv^{2}$=337.5

$mv^{2} = 675$

$45v^{2} =675$

$v^{2} = \frac{675}{45}$

$v^{2} =15$

$v=\sqrt{15}$

v = 3.8.. m/s (approx)

The velocity of the thief is 3.8 m/s (approx)