Which force primarily accounts for a vehicle's speed increase on a downhill grade?

Which force primarily accounts for a vehicle's speed increase on a downhill grade?

• A. Gravity
• B. Friction
• C. Momentum
• D. Normal force

Answer: (A)

When a vehicle moves downhill, gravity provides the driving pull. Gravity acts downward, but the slope splits this pull into a component that runs along the incline. This along-slope component pulls the vehicle downward and causes it to accelerate, making it speed up. The steeper the grade, the larger this component and the greater the acceleration. The normal force acts perpendicular to the surface and doesn’t push the vehicle along the slope, so it doesn’t cause the speeding up. Friction generally resists motion, so it would counteract acceleration rather than drive it, and momentum is not a force at all. So gravity is the primary force increasing speed downhill.

When a vehicle moves downhill, gravity provides the driving pull. Gravity acts downward, but the slope splits this pull into a component that runs along the incline. This along-slope component pulls the vehicle downward and causes it to accelerate, making it speed up. The steeper the grade, the larger this component and the greater the acceleration. The normal force acts perpendicular to the surface and doesn’t push the vehicle along the slope, so it doesn’t cause the speeding up. Friction generally resists motion, so it would counteract acceleration rather than drive it, and momentum is not a force at all. So gravity is the primary force increasing speed downhill.