Calculate the kinetic energy of an object with mass 500 g if it covers a distance of 20 m in 4 s

Answer

Assuming the object moves at constant speed, its speed is $v = \frac{20\,\text{m}}{4\,\text{s}} = 5\,\text{m/s}$. Converting $500\,\text{g}$ to kilograms gives $m = 0.5\,\text{kg}$. The kinetic energy is $KE = \tfrac{1}{2}mv^2 = \tfrac{1}{2}(0.5)(5^2) = 6.25\,\text{J}$.

Explanation

What the question is really asking

You are given a mass and information that lets you find the object’s speed from distance and time. Once you know the speed, you can use the kinetic energy formula.

Convert the mass to SI units

Kinetic energy is usually calculated in SI units (kg, m, s).

$$500\,\text{g} = 500\times 10^{-3}\,\text{kg} = 0.5\,\text{kg}$$

Find the speed from distance and time

Assuming constant speed,

$$v = \frac{d}{t} = \frac{20\,\text{m}}{4\,\text{s}} = 5\,\text{m/s}$$

Compute kinetic energy

Use

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

Substitute $m = 0.5\,\text{kg}$ and $v = 5\,\text{m/s}$:

$$KE = \tfrac{1}{2}(0.5)(5^2) = 0.25\times 25 = 6.25\,\text{J}$$

Quick check

A heavier mass or a higher speed would increase $KE$, and because $v$ is squared, speed changes have a big effect. The value $6.25\,\text{J}$ is reasonable for $0.5\,\text{kg}$ moving at $5\,\text{m/s}$.