3.4.1.3
Drag forces oppose motion and increase with speed
Motion Along a Straight Line — AQA A-Level Physics
Key Definition
Drag force — A force that opposes the motion of an object moving through a fluid (gas or liquid).
- Friction and air resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohms (Ω). are examples of drag.
- Drag always acts opposite to the direction of motion.
- Drag forces convert kinetic energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). an object possesses due to its motion. into thermal energyThe capacity to do work. Measured in joules (J). and sound.
- Drag increases with the speed of the object.
- Factors affecting drag: speed, cross-sectional area, shape, densityMass per unit volume of a material. Measured in kg m⁻³. of the fluid.
- Lift is a perpendicular upward force on an object moving through a fluid (e.g. aeroplane wings).
Worked Example
A car of mass 800 kg has a driving force of 3 kN and an accelerationThe rate of change of velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.. A vector quantity. Measured in m s⁻². of 2.0 m s^-2. What is the frictional force?
Show Solution
1
Calculate the resultant force
$$F_{\text{res}} = ma = 800 \times 2.0 = 1600 \text{ N}$$
2
Find the frictional force
$$F_{\text{res}} = F_{\text{drive}} - F_{\text{friction}}$$
$$1600 = 3000 - F_{\text{friction}}$$
$$F_{\text{friction}} = 3000 - 1600 = 1400 \text{ N}$$Answer
Frictional $force = 1400 N$