3.4 Fluids can exert a force on objects

BIG IDEA: Newton’s laws apply to all forces.

Now learn:

· How fluids apply forces to objects

· How the motion of a fluid affects the pressure it exerts

· How forces are transmitted through fluids

I. Buoyant Force

· Buoyant Force is the upward force on objects in a fluid.

· Consider it a reaction force opposing gravity.

· It is why ice floats in water. It is also why objects seem lighter in water.

II. Buoyancy

· Remember that in a fluid pressure increases with depth.

· This means that there is greater pressure acting on the bottom of the balloon than on the top.

· This pressure difference produces a net force that pushes the balloon upward.

III. Density and Buoyancy

· Whether or not an object floats in a fluid depends on the densities of both the object and the fluid.

· If an object is less dense than the fluid that it is in, then it will float.

· If an object is denser than the fluid that it is in, then it will sink.

o If an object sinks, then its weight is greater than the buoyant force on it.

IV. Bernoulli’s Principle

· Named after Daniel Bernoulli, a Swiss mathematician who lived in the 1700’s.

· Understand first, that the motion of a fluid affects the amount of pressure it exerts.

· Bernoulli’s Principle says that the faster a fluid moves, the less pressure it puts on surfaces or openings it flows over.

V. Applying the Principle

The layers of air on the top surface of the wing have to travel a longer distance than those at the bottom. Because the distance is longer, the air flow on the top of the wing travels at a higher velocity. Because the air under the wings travels slower than the air on top, the pressure there is greater, and thus, generates a lift force.

If the lift force is greater than the plane’s weight, the plane will fly.

For the plane to go forward, its engines and propellers must produce enough thrust to overcome the drag force. The drag force is produced by the friction of the plane with the air.

Spoilers on cars work in the opposite way. They are designed like upside-down airplane wings. The shape increases pressure on the top of the car which increases friction with the road. The more the friction, the more the control.

VI. Pascal’s principle

· In the 1600’s a French scientist named Blaise Pascal experimented with fluids in containers. One of his key discoveries is called Pascal’s principle.

· This principle states that when an outside pressure is applied at any point to a fluid in a container, that pressure is transmitted throughout the fluid with equal strength.

· Visit http://science.howstuffworks.com/hydraulic1.htm to see an animation.

· Machines that use liquids to transmit or increase a force are called hydraulic machines.

· Hydraulic systems are used in large machines such as garbage trucks.

REVIEW QUESTIONS

1. Why is there an upward force on objects in water?

2. How does changing the speed of water coming out of a garden hose affect its pressure?

3. If you drop a marble into a glass of water it will sink. Explain why this happens.

4. How could you apply Newton’s third law to an airplane’s ability to fly?

5. What types of machines may use Pascal’s principle to operate? Explain.