Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Gravity and Friction 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.h tm 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 382 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 groundgravity- 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 385 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