Gravity and FrictionBy 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.

http://www.amnh.org/ology/astronomy/gravity/index.htm

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

• The more mass two objects have, the greater the force of gravity the masses exert on each other.

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