A 2,000-kilogram car uses a braking force of 12,000 Newton to stop in 5 seconds. What is its initial speed of the car?

First, solve for the acceleration of the car. You know the mass of the car and the braking force, so you can use the equation Force = Mass x Acceleration. This gives you 12,000 = 2,000 x A. Divide 12,000 by 2,000 to find the acceleration equal to 6 m/s^2. This is the rate that the car is slowing down at. Velocity is equal to accleration x time (rate x time), so you multiply 6 by the time of 5 seconds. This leaves you with a velocity of 30 m/s or about 67.1 mph.