Drag Forces
- Drag forces are forces that oppose the motion of an object moving through a fluid (gas or liquid)
- Examples of drag forces are friction and air resistance
- Drags forces:
- Are always in the opposite direction to the motion of the object
- Never speed an object up or start them moving
- Slow down an object or keeps them moving at a constant speed
- Convert kinetic energy into heat and sound
- Lift is an upwards force on an object moving through a fluid. It is perpendicular to the fluid flow
- For example, as an aeroplane moves through the air, it pushes down on the air to change its direction
- This causes an equal and opposite reaction upwards on the wings (lift) due to Newton's third law
Drag forces are always in the opposite direction to the thrust (direction of motion). Lift is always in the opposite direction to the weight
- A key component of drag forces is it increases with the speed of the object
- This is shown in the diagram below:
Frictional forces on a car increase with speed
Worked example
A car of mass 800 kg has a horizontal driving force of 3 kN acting on it.Its acceleration is 2.0 m s-2.What is the frictional force acting on the car?