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

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.

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 3 rd Law states that the molecules exert an equal but opposite force on the object. A component of that force generates lift.

Lift from a Spoiler ViVi VfVf ViVi VfVf VV Dynamic Fluid Force Lift Force Drag 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. Dynamic Fluid Force

Bernoulli’s Principle Faster moving fluids exert less pressure laterally than do slower moving fluids. Wing 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. Low pressure High pressure

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

Explain the practice of treading water using Lift and Drag

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

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

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

Wind From The West (top view) sail 15 km/h Two dynamic fluid forces will act on the sailboat. 1.Wind Dynamic Fluid Force on sail 2.Water Dynamic Fluid Force on boat from front Drag from water Notes: Forces cause accelerations which cause changes in velocity Magnitude of dynamic fluid force depends on relative velocity

Wind From The West (top view) sail 15 km/h Water Drag 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 v 2 sail 5 km/h Water Drag As boat velocity increases, the relative force of the wind decreases, and the drag force from the water increases. With zero net force acting on the boat, terminal velocity is reached at 10 km/h.

Wind from the North (top view) 15 km/h Three dynamic fluid forces will act on the sailboat. 1.Wind Dynamic Fluid Force on sail 2.Water Dynamic Fluid Force on boat from front 3.Water Dynamic Fluid Force on boat from side Water Drag

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 Vi Vf vv 2 nd Law tells us that the force of the sail on the wind is in the same direction as the wind’s change in velocity Sail force on wind 3 rd Law states that the wind pushes back with an equal but opposite force. Dynamic Force from Wind Drag Lift Dynamic

Wind from the North (top view) Water Drag Drag Water Drag Lift 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 Side profile of hull Front profile of hull