Chapter 13 Forces in Fluids It’s a bit of a review…

Pressure Pressure is force exerted over an area Pressure is force exerted over an area P = Force P = Force Area Area Pressure is measured in kilopascals (kPa), Remember? Pressure is measured in kilopascals (kPa), Remember?

Pressure If a weightlifter exerts 30,000 Newtons of force on a barbell 3.5m 2, what is the resultant Pressure? If a weightlifter exerts 30,000 Newtons of force on a barbell 3.5m 2, what is the resultant Pressure? P = F / A P = F / A = 30,000N / 3.5m 2 = kPa

Water Pressure Water pressure increases with depth Water pressure increases with depth Pressure in a fluid at any given depth is constant, and is exerted equally in all directions Pressure in a fluid at any given depth is constant, and is exerted equally in all directions

Air Pressure Air is a mixture of gases that has mass Air is a mixture of gases that has mass The weight of the earth’s atmosphere exerts a pressure of 101kpa at sea level The weight of the earth’s atmosphere exerts a pressure of 101kpa at sea level You cannot feel this pressure because your body is used to it, but if you were on another planet, you’d notice a change You cannot feel this pressure because your body is used to it, but if you were on another planet, you’d notice a change

Planetary Atmospheric Pressures

Air Pressure Air pressure decreases with altitude: the higher up you go, the less air there is, and the less pressure it exerts Air pressure decreases with altitude: the higher up you go, the less air there is, and the less pressure it exerts This is why mountain climbers need oxygen – there is not enough air to breath at that altitude This is why mountain climbers need oxygen – there is not enough air to breath at that altitude

Air Pressure The weight of the atmosphere above you is exerting a force of 1000N at all times The weight of the atmosphere above you is exerting a force of 1000N at all times Your body has a similar pressure which balances out the force of the atmosphere Your body has a similar pressure which balances out the force of the atmosphere Ever have your ears pop? Ever have your ears pop?

The pressure applied to a fluid is transmitted unchanged throughout the fluid. The pressure applied to a fluid is transmitted unchanged throughout the fluid. When you squeeze one end of the tube, the pressure is transmitted through the entire fluid, and the paste will pushed out of all openings. When you squeeze one end of the tube, the pressure is transmitted through the entire fluid, and the paste will pushed out of all openings. Pascal’s Principle

States that as the velocity of a fluid increases, the pressure exerted by that fluid decreases. States that as the velocity of a fluid increases, the pressure exerted by that fluid decreases. Airplane wing Bernoulli’s Principle

Is the ability of a fluid (liquid or gas) to exert an upward force on an object immersed in it. Is the ability of a fluid (liquid or gas) to exert an upward force on an object immersed in it. This upward force is called the buoyant force. Force of the object pushing down on the water. This upward force is called the buoyant force. Force of the object pushing down on the water. Buoyant force of water pushing up on the object. Buoyancy

The amount of buoyant force determines if an object will sink or float. The amount of buoyant force determines if an object will sink or float. If the buoyant force is less than the object’s weight, it will sink. If the buoyant force is greater than the weight, it will float. If the buoyant force is less than the object’s weight, it will sink. If the buoyant force is greater than the weight, it will float. Buoyant Force

States that the buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object. States that the buoyant force on an object in a fluid is equal to the weight of the fluid displaced by the object. Ship hull Same mass of steel Since hull displaces more water, the buoyant force is greater. It floats. Archimedes's Principle