Question

A car is moving with a velocity of 54 km/h.the kinetic energy of a boy of mass 40 kg sitting in the car is ​

Answer 1

Given

• Velocity of the car = 54 km/h
• Mass of the boy = 40 kg

To find

• Kinetic energy of the boy

Solution

Firstly, we will convert the velocity of the car from km/h to m/s.

For converting it to m/s, multiply the value by 5/18.

⟼ 54 × 5/18

⟼ 15 m/s

❖ Velocity of the car = 15 m/s

Formula to be used :-

❖ KE = 1/2 mv²

Where,

• m = mass of the boy
• v = velocity of the car

Substituting the given values,

⟼ KE = 1/2 × 40 × (15)²

⟼ KE = 20 × 15 × 15

⟼ KE = 4500 J

❖ Kinetic energy of the boy = 4500 J

Answer 2

Velocity = 54 km/h = 15 m/s

Mass of the car = 40 kg

We know,

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

⇒ $KE = \frac{1}{\cancel{2}} \times \cancel{40 \ kg} \times (15 \ ms^{-1})^2$

⇒ $KE = 20 \times 225 \ kg \ m^2 s^{-2}$

⇒ $KE = 4500 \ Nm = 4500 \ J$

⇒ $KE = 4.5 \ kJ$.

∴ Kinetic energy of the bog sitting inside the car is 4.5 kJ.