12-4 Notes

Pressure and Fluids

Fluids can exert an upward force on objects.

Buoyancy- upward force on objects.1. Objects seem lighter in water because the

net force is reduced. This is why ice floats!

2. Increasing the volume of object increases volume of liquid it displaces, which

increases the buoyant force.

Density and Buoyancy

1. Density measures the amount of matter packed into a unit volume.

Formula: D=m/V Units for Denisty = g/cm3

HOW TO FIND VOLUME ??

Volume = Length x Width x Height

So if we had an object that was 5 cm long, 5 cm wide, and 5 cm tall, we have:

5cm x 5cm x 5cm = 125 cm3

If an object is less dense than fluid it’s in, object floats.

- Buoyant force = Weight.

If an object is MORE dense than fluid it’s in, object will sink.

- Weight > Buoyant force.

Review

• 1. What is the formula for Density?

• 2. What are the units for Density?

• 3. What is the upward force on an object in a fluid called?

The motion of a fluid affectsits pressure.

A faster moving fluid exerts less pressure as it flows over the surface of an object than a slower

moving fluid.

Example: Wind blowing over a chimney top DECREASES the pressure at the top of the chimney.

The faster air has less pressure than the slower-moving air in the fireplace. This effectively pulls the smoke

from the fire out of the fireplace.

Bernoulli’s Principle- increase in speed of motion of a fluid decreases the pressure in the fluid; faster a fluid moves, the less pressure it exerts on surfaces or openings it flows over.

1. Airplane wings can be shaped to cause air flowing over the top to move faster than air flowing under the wing.

This improves lifting force on a flying airplane.

2. Racecars have a device on the rear of the car that has the reverse effect of airplane wings. It is like an upside-down airplane wing. It increases pressure on top of the car so that as it presses downward on the road, friction is increased between the tries and the road. This helps minimize skidding around curves at

high speeds.

Forces can be transmitted through fluids.

Pascal’s Principle- when an outside pressure is applied at any point to a fluid in a container, the pressure is transmitted throughout the fluid with equal strength.

Using a carjack is an example of this. The jack contains liquids that transmit and increase the force you apply. Pushing down on the piston (handle) increases pressure on the liquid. It is transmitted throughout the liquid to push upward ion another piston to lift the car.

Hydraulic Machines- use liquids to transmit or increase a

force.

Squeezing liquids doesn’t change volume as much as it would a gas. In a

liquid, particles are closer together than in a gas.

Example: garbage truck