A supertanker of mass 4.0 × 10^8 kg, cruising at an initial speed of 4.5 m s^(–1), takes one hour to come to rest. Assume the force slowing down the tanker is constant.

From newton's first law, an object remains in its inertial frame until a force acts upon it. This means that according to a stationary observer, the object will remain at rest or continue moving at the same velocity in the same direction until a force acts upon it. From Newton's second law we know that the force is equal to the mass (not weight) times the acceleration. The supertanker therefor goes from an initial velocity v0 = 4.5m/s to vf = 0 in one hour (3600s). The acceleration is defined as the change in velocity over time a = (v0 - vf)/t. As we all the variables on the right hand side of the equation we can solve for a = 4.5/(3600) = 0.00125 m/s2. We then use this value to calculate the braking force: F = m*a = 1.25 x (10^-3) x 4 x (10^+8) = 5 x (10^5) N.