Forces and Fluids Chapter 12

BIG IDEAS Newton’s laws apply to all forces Gravity is a force exerted by all masses Friction is a force that opposes motion Pressure depends on force and area Fluids exert a force on objects

Pressure

What is Pressure? Related to the word press Earth’s gravity pulls downward Due to gravity, your feet exert a force on the surface of Earth over an area the size of your feet

Pressure and Area The amount of pressure you exert depends on the area over which you exert force. – Larger area = less pressure exerted

Calculating Pressure Pressure = Force Area Measured in Pascals – 1 N/m²

Fluid Pressure Fluid is a material that can easily flow – Liquids – Gases Air and helium Viscosity – a liquid’s resistance to flow -the slower the flow the more viscous ex. Syrup is more viscous than water

What Causes Fluid Pressure In a fluid, all of the forces exerted by the individual particles combine to make up the pressure exerted by the fluid.

Air Pressure Air exerts pressure because it has mass Because the force of gravity pulls down on the mass of air, the air has weight Weight of the air is the force that produces air pressure or atmospheric pressure

Balanced Pressure Hold out your hand – Holding up air – 1,000 N About the same weight as that of a large washing machine

Unbalanced Pressure What happens when air pressure becomes unbalanced?

Variations in Fluid Pressure Atmospheric Pressure and Elevation – Popping in ear is caused by changing air pressure. – Higher elevations – less air above you = less air pressure *Atmospheric pressure decreases as your elevation increases

Variations in Fluid Pressure

Water Pressure and Depth – Water pressure increases as depth increases Deepest part of the ocean 1,000 times greater air pressure than we experience every day Volume of water does not effect amount of pressure only depth

Measuring Pressure Barometer measures atmospheric pressure

Floating and Sinking

Density Comparing densities, you can predict whether an object will float or sink in a fluid. – If object is more dense than fluid – it sinks – If object is less dense than fluid - it floats – An object with a density equal to that of the fluid floats at a constant depth.

Density Density = Mass Volume Density of water = 1.0

Density

Changing Density Changing density can explain why an object floats or sinks. – Submarine

Buoyancy Buoyancy is the ability to float.

Buoyant Force Buoyant force is when water and other fluids exert an upward force. The buoyant force acts in the direction opposite to the force of gravity, so it makes an object feel lighter. The less dense the object is, the greater the buoyant force it experiences.

Buoyant Force

Archimedes’ Principle The buoyant force acting on a submerged object is equal to the weight of the volume of fluid displaced by the object.

Pascal’s Principle

Transmitting Pressure in a Fluid In the 1600s, Blaise Pascal developed a principle to explain how pressure is transmitted in a fluid – Fluid exerts pressure on any surface it touches.

Pascal’s Principle Pascal’s principle states that pressure increases by the same amount throughout an enclosed or confined fluid – When force is applied to a confined fluid, the change in pressure is transmitted equally to all parts of the fluid.

Pascal’s Principle at Work

Hydraulics and the use of liquid pressure

Hydraulic Systems Hydraulic system uses liquids to transmit pressure and multiply force in a confined fluid. – A hydraulic system multiplies force by applying the force to a small surface area. The increase in pressure is then transmitted to another part of the confined fluid, which pushes on a larger surface area. Because hydraulic systems use fluids to transmit pressure, hydraulic systems have few moving parts that can jam, break or wear down.

The important features of pressure in liquids Liquids will transmit pressure throughout the liquid, regardless of the shape of the container A force therefore can be applied wherever you want it The force can be multiplied according the the area of the pistons used to push down on the water

Examples of the use of hydraulics Car brakes Car jacks Fork lift trucks Lifts bulldozers Television cameras

Hydraulic Lifts

How car brakes use hydraulics A small force on the brake pedal will act on a piston with a small area The small piston pushes against the brake fluid The pressure exerted is transmitted to all parts of the liquid The pressure acts against four bigger pistons with larger areas The large pistons will multiple the force and cause the brake pads to push against the wheels As pressure is transmitted equally throughout the liquid, each brake works evenly too

Hydraulic brake The diagram shows a simple brake where a pedal will push down on the smaller master piston The force pushing down will put the liquid under pressure The pressure is transmitted through the liquid and against the slave piston The pressure causes the slave piston to exert a force where it is needed

Using the formula P=F/A What is the pressure exerted on the liquid by the slave piston? Answer: Pressure = 10N / 5cm2 = 2 N/cm 2 Calculate the area of the slave piston that will lift a 100N load Answer: Area = force / pressure = 100N / 2 N/cm 2 Master Piston Is 5 cm 2 10N Slave piston

More questions on hydraulics 1.What will the force (F1) need to be to exert a pressure of 20 N/cm2 ? 2.Calculate the weight (F2) that can be lifted by the slave piston. Answers: 1. Force = 20 N/cm 2 x 10cm 2 = 200N 2. Force = 20 N/cm 2 x 100cm 3 = 2000N Master piston = 10cm 2 Slave piston = 100cm 2

Hydraulic Lifts

Hydraulic Brakes

Summary of the principles of hydraulics Pressure is transmitted throughout a liquid A force may be applied anywhere to the liquid All hydraulic systems use a small master piston and a large slave piston A small force is used to create a very big force The small master piston is used to apply a force The slave piston is always bigger than the master piston The larger slave piston will multiply the original force put on the master piston