Fluid Resistance (DP IB Physics): Revision Note

Fluid Resistance

• Fluid resistance refers to the effects of gases and liquids on the motion of a body

• When an object moves through a fluid (a gas or a liquid), there are resistive forces for that movement

  • These forces are known as viscous drag 

  • Viscous drag, also known as air resistance, is a type of friction

• Frictional forces:

  • Always act in the opposite direction to the motion of the object

  • Never speed an object up or start them moving

  • Always slow down an object or keep them moving at a constant speed

  • Always transfer energy away from the object to the surroundings 

• Lift is an upward force on an object moving through a fluid. It is perpendicular to the fluid flow

  • For example, as an aeroplane moves through the air, the aeroplane pushes down on the air to change its direction

  • This causes an equal and opposite reaction as the air pushes upwards on the wings of the aeroplane (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 that they increase with the speed of the object

• This is shown in the diagram below:

Frictional forces on a car increase with speed

Fluid Resistance in Projectile Motion

• In projectile motion, the factors that are affected by fluid resistance are:

  • Time of flight

  • Horizontal velocity

  • Horizontal acceleration

  • Range

  • Shape of trajectory

• Air resistance is the frictional force which has the most significant effect on a projectile

• Air resistance decreases the horizontal component of the velocity of a projectile

  • This means both its range and maximum height will decrease compared to an identical situation with no air resistance (like a vacuum)

A projectile with air resistance travels a smaller distance and has a lower maximum height than one without air resistance

• When air resistance is applied, the path of the projectile no longer follows a parabola shape

  • Its path is now steeper on the way down than it is up

• The flight time will also decrease as the projectile is in the air for a shorter period of time

  • This is due to having a smaller range and lower maximum height

• In summary:

• The angle and launch speed of a projectile can be varied to cover a longer range or reach a greater maximum height, depending on the situation

  • For sports, such as the long jump or javelin, an optimum angle against air resistance is used to produce the greatest range (distance)

  • For gymnastics or ski jumper, the initial vertical velocity is made as large as possible to reach a greater maximum height and longer flight path