FORCES AND FLUIDS

1. FORCES AND FLUIDS Chapter 4

2. Pressure and Density of fluids • A fluid is any substance that can flow and take on the shape of the container that holds it. • Any liquid, such as water, is a fluid; although a liquid changes shape when it flows, its volume stays the same. • Any gas, such as air, is a fluid. -When gas flows, its shape becomes the shape of its container. -Gases have different volumes in different containers. When a gas flows, it fills the volume of its new container.

3. Pressure of fluids • Pressure is the amount of force per unit area applied to the object’s surface. • All fluids apply pressure. -Pressure applies to an object by a fluid is related to the weight of the fluid. -Pressure can be calculated by dividing the force applied to a surface by the area over which the force is applied.

4. The unit used when pressure is measured is the pascal (Pa). • Pressure is applied by a fluid to all sides of an object in contact with In the fluid. • Pressure decreases when the surface area over which it is applied increases. • Pressure increases when the surface area over which it is applied decreases.

5. Pressure applied by a fluid increases with depth. • Atmospheric pressure is the ratio of the weight of all the air above you to your surface area. • Atmospheric pressure decreases as elevation increases because the weight above you is less. • The pressure of an underwater object depends somewhat on the weight of the air above the object, but mostly the weights of the water above the object.

6. Density of fluids • For the same volume of two different liquids, the weight of the denser fluid results in more pressure. • Density is mass divided by volume. • A common unit of density is g/cm3 • The density of the a material is determined by masses of the atoms or molecules in the material and the distances between them. • Solids are usually denser than liquids or gases.

7. What is a buoyant force? • A buoyant force is an upward force applied by a force on an object in the fluid. • The upward buoyant force on an object is balanced by the force of gravity. • When the buoyant force on an object in a fluid equals to the total weight of the objects, the object will float.

8. A buoyant force acts on any object in a liquid, such as water, and in a gas, such as air. • Forces from water pressure on an object within a fluid are in horizontal or vertical directions. • Horizontal forces on the object are equal on all sides of the object, but the vertical forces are not. • The difference between the upward and downward forces from the pressure on an object in the fluid is the buoyant force. • The buoyant force on an object is always in the upward direction. • The buoyant force on an object in a fluid stays the same when the object goes deeper in the fluid.

9. ARCHIMEDES PRINCIPLE • Archimedes principle states that the weight of a fluid that an object displaces is equal to the buoyant force on an object. • As more water is displaced by an object that is pushed under water, the buoyant force acting on the object increases.

10. Engineer a well designed sturdy boat. Draw your design and describe what its made of.

11. Sinking and Floating • When an object weighs more than the weight of the water it displaces, the object sinks. • When the weight of an object is equal to the weight of the water it displaces, the object floats. • If an object has greater density than the fluid in which it is placed, the object sinks.

12. Propose your theory of the sinking of Titanic.

13. A metal boat floats because the overall density of the metal and the air in the boat is less dense than the density of the water. • If a metal boat fills with water, then the density of the metal and the water within the boat becomes greater than the density of water outside of the boat, and the boat sinks. • A helium balloon rises in the air because the density of the helium within the balloon is less than the density of the air outside of the balloon. • As the volume of helium in the balloon decreases, the balloon’s density increases and the balloon falls to the ground.

14. Fluid Forces- Benefits and Challenges • When you squeeze a ketchup bottle, you use a fluid force. • When strong winds damage property during a storm, fluid forces can be dangerous.

15. PASCAL’S PRINCIPLE • Pascal’s Principle states that when pressure is applied to a fluid in a closed container, the pressure increases by the same amount everywhere in the container.

16. A piston uses a smaller force in one area to apply a larger force in another area. • The input work by the piston is equal to the output work done by the piston. • If the force is larger on one part of the piston, then that part moves less than the part where the force is less. • A hydraulic lift uses Pascal’s principle. • An hydraulic lift, a narrow tube connects to a wider tube that fits under a car. • Applying a small force to the narrow tube generates enough force to lift a car.

17. Bernoulli’s Principle • Bernoulli’s Principle states that the pressure of a fluid decreases when the speed of the fluid increases.

18. For example, when a hose is pinched, the speed of water through the pinched area increases and the pressure is less than it is unpinched portions of the hose. • When the wind is strong, the air pressure is less than it is in still air. • When strong winds blow over a house, the air outside the house has a high speed, and the air inside the house has no or low speed. • When this happens, the force of air pressure pushing down on the roof is less than the force of the air pressure pushing up. • When the upward force inside becomes greater than the combined downward forces, including the force of gravity, the roof can rise.

19. When a soccer ball is kicked so it spins , the speed of one part of the ball is greater than it is on the opposite part. • The part that has lower speed has greater air pressure acting on it. • Because of the pressure difference, the ball curves toward the part that has lower pressure

20. Drag Forces • Drag Force is a force that opposes motion of an object through a fluid. • As the speed of an object through a fluid increases, the drag force on the object increases. • Drag force also depends on the size and shape of the object. -If two objects move in the same direction, the object that has greater surface area toward the direction of the motion has greater drag force on it. -A larger parachute will have greater drag force on it than a smaller parachute. The drag force when you move through air is less than it is when you move through water because the density of water is greater.