Chapter 3 Gravity

Gravity The force that objects exert on each other because of their masses. Causes the downward pull on an object on Earth. Depends on two factors: mass and distance.

Gravity and Mass The more mass two objects have, the greater the forces of gravity the masses exert on each other. If mass is doubled, gravity is doubled.

Gravity and Distance The closer two objects are, the greater the force of gravity. The further apart they are, the weaker the force of gravity.

Gravity on Earth Gravity acts on both objects/masses equally. The acceleration of a falling object due to Earth’s gravity is called g and is equal to 9.8 m/s2.

Gravity on Earth Recall Newton’s 2nd Law of Motion: F=ma If we use g in place of a, we can find the force due to gravity on a mass close to Earth’s surface by using: Force = (m)(g)

Gravity in a Vacuum There is no air; all falling objects have the same acceleration. If you dropped a penny and a quarter at the same time, they would fall at the same acceleration. Horizontal velocity does not affect acceleration due to gravity.

Weight in Mass Remember mass is a measure of matter in an object; weight is the force of gravity exerted on that object. We use a balance to measure mass, and a scale/spring scale to measure weight. Gravity on the moon is 1/6 the gravity on Earth.

Gravity and Orbit Orbit: elliptical path one body follows around another body due to gravity. Centripetal force keeps these objects in orbit.

Gravity and Orbit If an object is thrown at a fast enough speed, it will begin to orbit the Earth instead of falling to the ground. This is the technique used to launch satellites into orbit. The spacecraft must move between 8,000 and 11,000 meters per second to stay in orbit; anything higher will escape gravitational pull.

People in Orbit A space craft in orbit is in free fall. Their weight does not press against the floor of the spacecraft. Results in a microgravity environment; people simply float as if they were weightless.

Chapter 3 Friction

Friction Friction: is a force that resists the motion between two surfaces in contact. Depends on the surfaces (bumpy, smooth). Bumpy surfaces produce more friction than smooth surfacs.

Motion of the Surfaces You need a larger force to start something moving than you do to keep it moving. Nonmoving objects have a frictional force that keeps them from moving. Once your force is stronger than the frictional force, the object will move.

Surfaces Pressing Together The harder two surfaces push together, the greater the friction between them. A rectangular box can rest on a larger side or a smaller side; the force of the box is the same regardless of the side, but the weight is spread out more on the larger side.

Friction and Heat Friction produces heat. Rub your hands together. What happens? For example, striking a match on its box causes fire. Substances like oil are used to reduce heat and friction.

Fluids Fluid: a substance that can flow easily; gas or liquid. When an object moves through a fluid, the molecules are pushed out of the way, which creates drag. This depends on the shape of the object.

Air Resistance Friction due to air Depends on surface area and speed: An object with more surface area has a greater air resistance. The faster an object moves, the greater the air resistance.

Air Resistance Eventually, air resistance balances gravity when an object falls. The object will reach its terminal velocity, which is the final maximum speed of the falling object.

Chapter 3 Pressure

Pressure Pressure: a measure of how much force is acting on a certain area. How concentrated a force is. Pressure = Force / Area Pressure is measured in pascals.

What does the formula mean? Pressure depends on force and area. If you increase the force or decrease the area, then the pressure will increase. If you decrease the force or increase the area, then the pressure will decrease. Pressure = Force / Area

Changing the formula Pressure = Force / Area Area = Force / Pressure Force = Pressure x Area

Practice If a winder hiker weighing 500 N is wearing boots that have an area of 0.075 m2, how much pressure is exerted on the snow? A pressure of 2000 Pa is exerted on a surface with an area of 20 m2. What is the total force exerted on the surface?

Practice Suppose there is a square box with a side length of 2 m. The mass of the box is 5 kg, and is moving at 3 m/s2. How can you use this information to find the pressure it is exerting on the floor? First, find area of the bottom of the box. 2 times 2 = 4 m2. Then, find the force of the box using F = ma. Force = 15 N. Pressure = 15 / 4 = 3.75 Pa

Pressure in fluids Fluids are made of particles loosely connected and too small to see. Fluids are in constant, rapid movement – colliding into each other and into any object in it. As they collide, they apply a force to the surface of the objects.

Pressure in fluids Fluids are made of particles loosely connected and too small to see. Fluids are in constant, rapid movement – colliding into each other and into any object in it. As they collide, they apply a force to the surface of the objects from all directions.

Pressure in air Pressure in fluids depends on depth and density. Air exerts a constant pressure on you; at sea level, it is about 100,000 Pa. This is known as atmospheric pressure; 100,000 Pa = 1 atmosphere

Pressure in air Changing Elevation: as you go higher, the air pressure decreases. Changing Density: as you go higher, air becomes less dense. Effects on Pressure: in less dense air, there are more particles to collide together. Therefore, there is less pressure on an object.

Pressure in water Water pressure increases as you go deeper. Water has a greater density than air, so it applies a higher pressure.

Pressure in water If you dive 1000 m below the surface, the pressure is almost 100 times stronger than 1 atmosphere. Without special equipment, your lungs would be crushed.

Chapter 3 Buoyancy

Buoyant Force/Buoyancy The upward force on objects in a fluid Makes objects seem lighter in water. Remember, pressure increases with depth – so the bottom of an object is being acted on more strongly, causing it to move upward. The buoyant force acting on an object is equal to the water it displaces.

Density and Buoyancy density = mass / volume; how much matter is packed in a unit volume; g/cm3 If an object is less dense than the fluid it is in, the object will float. The buoyant force is balancing its weight. If a object is more dense than the fluid, it will sink. The weight is greater than the buoyant force acting on it.

Motion of a Fluid and Pressure Faster moving = less pressure Bernoulli’s Principle: an increase in the speed of a fluid decreases the pressure within the fluid.

Using Bernoulli’s Principle Airplanes  wing shape can cause air to move faster/slower to improve lifting force. Racecars  device increases pressure on the top of the car, allowing it to have greater friction between the tires and the road. Burrowing animals, such as prairie dogs, build mounds of dirt to create diff. in pressure that support ventilation underground.

Pascal’s Principle Pascal’s Principle: when an outside pressure is applied at any point to a fluid in a container, that pressure is transmitter throughout the fluid at equal strength. Used in mechanics frequently; example on page 102.

Hydraulics Hydraulic machines: machines that use liquids to transmit or increase a force. Advantage: when you squeeze a liquid, the volume does not change much; gases spread out and decrease in volume. Researchers are working to use hydraulics on a much smaller scale.