1. The problem statement, all variables and given/known data A car with mass 2500 kilograms travels along a straight level road at 90 km/h. A headwind begins to blow with a force of 200 N causing the car to slow down. Calculate the car’s deceleration. 2. Relevant equations The book uses the formula F =ma so 3. Book's solution = 0.08 m/s2 My confusion with the calculation comes from not knowing why the speed of the car is not considered. It’s a 2500 kg car driving at 90km/h so must have a force or a momentum that is important. Why is the car's speed not considered? I need to ask a bit more about the question here please: The second part of the question says The car travelling at a steady speed collides with a stationary traffic cone of mass 10 kg. The traffic on accelerates in the direction of the car’s motion at 29 m/s2. Calculate the force applied to the traffic cone by the car. Book says: Calculation is F =ma = 10 x 29 = 290N Book asks: What is the force applied by the cone to the car in the collision (hint in the paper is: Newton’s third law of motion). Answer in the book is 290N. But shouldn’t the answer be -290N because “to every action is an equal and opposite reaction”.