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
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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.
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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.
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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
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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.
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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
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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.
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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.
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