# Gravity and Friction By Dr. Lee

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Gravity and Friction By Dr. Lee
Chapter 12

Describe how friction affects motion.
List the factors that affect friction

Gravity Masses attract each other
Gravity is the force that objects exert on each other because of their masses.

Gravity Gravity is a universal force because it acts between any two masses anywhere in the universe. For example, there is a gravitational pull between the Sun and the Moon.

Force of Gravity If there is a force between all masses, why are you not pulled toward your desk by the desk’s gravity when you walk away from it? The force of gravity between your desk and you is extremely small The strength of gravitational force is important when determining what will be attracted.

The Mass of Objects Roll Tide!
The more mass two objects have, the greater the force of gravity the masses exert on each other. Roll Tide!

The Distance Between Objects
As distance between the objects increases, the force of gravity decreases. If the distance is doubled, the force of gravity is ¼ as strong as before.

Gravity on Earth Page 381

Weight and Mass They are not the same, but similar
Mass is a measure of how much matter an object contains. Weight is the force of gravity on an object. Balance=mass Scale= weight

Gravity keeps objects in orbit
Sir Isaac Newton hypothesized that the force that pulls objects to the ground-gravity- also pulls the Moon in its orbit around Earth.

Orbit An Orbit is the elliptical path one body, such as the Moon, follows around another body, such as the Earth, due to the influence of gravity

Centripetal Force Centripetal Force is keeping one object in orbit around another object is due to the gravitational pull between the two objects. Page 358

Spacecraft in Orbit The minimum speed needed to send an object into orbit is approximately 8000 meters per second. If a spacecraft is launched with a speed of 11,000 meters per second or more, it is moving too fast to go into an orbit. This allows it to escape Earths gravity.

People in Space While astronauts are in orbit, their weight does not press against the floor of the spacecraft. This is known as a microgravity environment, in which objects behave as if there were no gravity.

Friction A force that resists the motion between two surfaces in contact. Friction between your feet is what provides the action and reaction forces that enable you to walk. Like this penguin……

Friction and Surfaces Friction changes depending on surfaces
Example: Hockey Puck sliding across an ice floor and a wooden floor.

Friction and motion on surfaces
You need a larger force to start something moving than you do to keep something moving.

Friction and pressing surfaces together
The harder two surfaces are pushed together, the more difficult it is for the surfaces to slide over each other. Page 391

Friction and Heat Friction between surfaces produces heat.

Friction in Fluid Objects falling through air have different accelerations. This is because air is fluid Fluid is a substance that can flow easily. The friction due to air is called Air resistance.

Skydiving Page 393

Chapter 12.3 & 12.4 Explain how pressure is determined
Describe how forces act on objects in fluids. Describe pressure changes in fluids. Explain how fluids apply forces to objects. Describe how forces are transmitted through fluids.

Pressure depends on force and area
Pressure is a measure of how much force is acting on a certain area. In other words, pressure describes how concentrated a force is.

Pressure While increased pressure may make you feel as if there is more force on you, the force is actually the same!

Pressure One way to increase pressure is to increase force.
Example: If you press your finger on the desk you increase pressure. Formula: Pressure= Force Area P- Pressure, also known as Pascal’s F-Force, A-area over with force is exerted

Pascal’s One pascal is the pressure exerted by one newton of force on an area of one square meter.

Knowing Pressure Important
Sometimes, knowing pressure is more useful than knowing force. For example, many surfaces will break or crack if the pressure on them is too great. A person with snowshoes can walk on top of snow, while a person in hiking boots will sink into the snow.

Pressure acts in all directions in fluids
Randomly moving water molecules collide with a diver. The net force from the many collisions produces the pressure on the diver. Page 397

Pressure in Air Although you do not notice the weight of the air, air exerts pressure on you at all times! Change in Elevation Change in Density Effects on Pressure Page 398

Pressure in Water Just as air pressure increases at lower elevations, water pressure increases with greater water depth. Water exerts more pressure on you than air does because water has a greater density than air.

How does water affect weight?
Do you feel bigger or smaller in water? You should feel like you weight less because water exerts an upward force on objects. This upward force on objects in a fluid is known as buoyant forces.

Buoyancy This is why it is easier to lift people or a heavy rock in water than on land. Density and Buoyancy are similar—this is why ice floats on water

Bernoulli’s Principle
Daniel Bernoulli, a Swiss mathematician who lived in the 1700’s described the effects of fluid in motion on pressure. Bernoulli’s Principle- says that an increase in speed of the motion of a fluid decreases the pressure within the fluid. The faster a fluid moves, the less pressure it exerts on surfaces or openings it flows over. Page 405

Pascal’s Principle In the 1600’s Blaise Pascal, a French scientist for whom the unit of measure called the pascal was named, experimented with fluids in containers. Pascal’s 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. Page 406