Drag forces
- Drag forces are forces acting in the opposite direction to an object moving through a fluid (either gas or liquid)
- Examples of drag forces are friction and air resistance
- A key component of drag forces is that the magnitude of the drag force increases with the speed of the object
- As an object speeds up, the drag force increases
- As an object slows down, the drag force decreases
- This will be explored further in Terminal Velocity
- Therefore, drag forces have the greatest effect at high speeds
- Consider a car traveling forward on a straight road
- When the car is accelerating:
- The driving force is greater than the frictional force
- The resultant force is acting in the same direction as the direction of motion
- Therefore, the car speeds up
- When the car is traveling at a constant velocity:
- The driving force is equal to the frictional force
- There is no resultant force acting on the car
- Therefore, the motion of the car remains the same (continues traveling at a constant velocity)
- When the car is decelerating:
- The driving force is less than the frictional force
- The resultant force is acting in the direction that opposes the motion of the car
- Therefore, the car slows down
Driving force and frictional force acting on a car at different accelerations
For the car to accelerate, the driving force must be larger than the frictional force; for the car to travel at a constant velocity, the driving force and frictional force must be balanced; for the car to decelerate, the driving force must be less than the frictional force
Worked example
A car of mass 800 kg has a horizontal forward driving force of 3 kN acting on it. The car accelerates at 2.0 m s-2.
Determine the magnitude of the frictional force acting on the car.
Answer:
Step 1: List the known quantities and convert to SI units
- Mass, m = 800 kg
- Driving force, FD = 3000 N
Step 2: Calculate the resultant force acting on the car using Newton's Second Law
Step 3: Equate the resultant force to the driving force and frictional force
Resultant force = driving force − frictional force
Step 4: Calculate the magnitude of the frictional force
Frictional force = driving force − resultant force
Examiner Tip
The key to answering this question correctly is remembering to consider drag forces in your calculation for the resultant force.