Question

A 76 kg student traveling in a car with a constant velocity has a kinetic energy of 1.1 104 J. What is the speedometer reading of the car in km/h?

Answer 1

A 76 kg student traveling in a car with a constant velocity has a kinetic energy of $1.1 \cdot 10^{4} \, \text{J}$. What is the speedometer reading of the car in km/h?

Solution.

The formula for kinetic energy in general form is:

$$E = \frac{m v^{2}}{2},$$

where $m$ is the mass of an object, $v$ is the velocity of an object, $E$ is the kinetic energy of an object.

So we have from the formula above:

$$v = \sqrt{\frac{2E}{m}} = \sqrt{\frac{2 \cdot 1.1 \cdot 10^{4}}{76}} = 17.01 \, \text{m/s} = \frac{17.014 \cdot 3600}{1000} \, \text{km/h} = 61,3 \, \text{km/h},$$

and it is the speedometer reading of the car in km/h.

**Answer:** $61,3 \, \text{km/h}$.