1. Lift
The dynamic fluid force component that acts perpendicular to the relative motion of the object.

2. Lift Force
Changes the direction of the relative motion of an object through a fluid.
Lift can be directed in any direction. Not just up. Lift is perpendicular to the motion of the object relative to the fluid, or the fluid relative to the object.
Lift is caused by the deflection of fluid molecules as they pass the object.

3. Deflection of Fluid Molecules by a Wing
Direction of fluid molecules before being deflected
Direction of fluid molecules after being deflected
Lift Force
The wing exerts a force on the molecules causing them to be deflected downwards.
Newton’s 3rd Law states that the molecules exert an equal but opposite force on the object. A component of that force generates lift.

4. Lift from a Spoiler Vf Vi Dynamic Fluid Force Vf V Drag Force Vi
Lift Force
The lift force produced by a spoiler is applied in the same direction as mg, thus increasing the normal force and the potential for friction.

5. Bernoulli’s Principle
Faster moving fluids exert less pressure laterally than do slower moving fluids.
Wing
Low pressure
The air molecules on both the top and bottom reach the end of the wing at the same time. The top molecules have further to travel and therefore must be moving with a greater speed.
High pressure

6. Spin and Magnus Effect
A lift force that results from a spin is called a Magnus force. Vi  Vf V
Drag
Dynamic
Fluid Force
Lift
Air molecules attempt to cling to the surface of the ball and their direction changes based on the spin of the ball.

7. Explain the practice of treading water using Lift and Drag

8. How are Lift and Drag Force used in Swimming
How are Lift and Drag Force used in Swimming? Are they different for the Front Crawl compared to the Breast Stroke?

9. Applying Lift and Drag Forces
In sailing, the lift force from the wind is used to propel the boat in the intended direction.

10. Sailboat Example
Would the sailboat reach the Island faster with the wind coming from the North or coming from the West?
15 km/h N W E S
15 km/h

11. Wind From The West (top view)
sail
Drag from water
15 km/h
Two dynamic fluid forces will act on the sailboat.
Wind Dynamic Fluid Force on sail
Water Dynamic Fluid Force on boat from front
Notes:
Forces cause accelerations which cause changes in velocity
Magnitude of dynamic fluid force depends on relative velocity

12. Wind From The West (top view)
With the wind pushing from behind, can the sailboat reach a velocity of 15 km/h? NO
As the sailboat’s velocity increase we have to look at the factors affecting the dynamic fluid forces F  ½CA v2
sail
15 km/h
Water Drag
As boat velocity increases, the relative force of the wind decreases, and the drag force from the water increases.
sail
With zero net force acting on the boat, terminal velocity is reached at 10 km/h.
Water Drag
5 km/h

13. Wind from the North (top view)
15 km/h
Water Drag
Water Drag
Three dynamic fluid forces will act on the sailboat.
Wind Dynamic Fluid Force on sail
Water Dynamic Fluid Force on boat from front
Water Dynamic Fluid Force on boat from side

14. Wind from the North (top view)
Using Newton’s model we must break down the wind force into Lift and Drag components to determine the boat’s motion Vi Vf
2nd Law tells us that the force of the sail on the wind is in the same direction as the wind’s change in velocity Vi v Vf
Sail force on wind
3rd Law states that the wind pushes back with an equal but opposite force.
Lift
Drag
Dynamic
Dynamic Force from Wind

15. Wind from the North (top view)
Water Drag
Drag
Lift
Side profile of hull
Front profile of hull
As the boat’s velocity increases, the relative force of the wind does not change. This is because the water drag from the side gets very large very fast due to the side profile of the hull in the water. This means the Lift force propelling the boat also does not change. The boat must be moving very fast, before the sleek front profile produces enough drag to balance out the Lift force of the wind. Terminal velocity is reached well beyond 15 km/h