Chapter 13 Forces and Motion

Section 1 Gravity Gravity is the force of attraction between two objects –Depends on size and distance –All matter is affected by gravity Law of Universal Gravity –All objects in the universe are attracted to each other through gravitational force –The magnitude of the force depends on the masses of the objects and the distance between them Isaac Newton

Gravity depends on distance and mass Gravity increases as mass increases Gravity increases as distance decreases Large objects have lots of gravity Small objects have little gravity Close objects have lots of gravity Far objects have little gravity

Mass vs Weight Weight is a measure of gravitational force –Weight changes as gravity changes More gravity means more weight Less gravity means less weight Differences –Weight is measured in newtons (N), mass is in grams (g) –Weight changes when gravity is different, mass remains constant –Weight is measured on a scale, mass is measure using a triple-beam balance On the Earth On the Moon

13.2 Gravity and Motion Gravity- A force of attraction between all objects due to their masses. Motion- An objects change in position over time when compared to a reference point. Reference point

All objects drop at the same rate due to gravity. –Doesn’t matter if objects are large or small –Proven by Galileo more than 400 years ago

Air Resistance Acceleration due to gravity can be affected by a fluid friction called air resistance. The amount of air resistance depends on the size and shape of the object. Acceleration due to gravity Air resistance Acceleration due to gravity Air resistance

Speed due to Gravity All objects accelerate toward the Earth at a constant rate. –9.8 m/s 2 Remember, we are ignoring air friction and the size of the object doesn’t matter –Only time is a factor (how long is the object falling) Velocity of a falling object V g = g x t Time (seconds) Accel. due to gravity (9.8 m/s 2 )

Example #1 –A box falls from an airplane. The plane is traveling 35,000 feet up at 540 miles per hour. The box takes one minute to hit the ground. What is its speed when it hits the ground? What do you need? V g = g x t V g = 9.8 m/s/s x 60 s V g = 588 m/s

What is a penny’s velocity if it falls for 2 seconds? V g = g x t V g = 9.8 m/s 2 x 2 s V g = 19.6 m/s What is a penny’s velocity if it falls for 10 seconds? V g = g x t V g = 9.8 m/s 2 x 10 s V g = 98 m/s

An object falls… …for 1 second (9.8 m/s) …for 2 seconds (19.6 m/s) …for 3 seconds (29.4 m/s) … for 4 seconds (39.2 m/s) Air resistance (air friction) slows down acceleration –As objects fall faster air resistance increases. Without air resistance objects get faster and faster due to gravity –Free fall is when there is no air resistance.

Terminal Velocity The point at which the downward pull of gravity and the upward push of air resistance become equal. The object no longer accelerates, but falls at a constant rate the rest of the way down..

Free Fall Free fall occurs when gravity is the only force acting on the object. It can only occur in a vacuum or in space. Sky divers are not actually in free fall before they release their parachute, they are still being affected by air resistance. Objects in orbit are in free fall because there is no air resistance.

Projectile Motion The curved path an object follows when propelled near the Earth’s surface –Involves… the downward pull of gravity Forward motion Gravity Forward motion

When gravity is the only force acting on the shuttle…

When forward motion is the only force acting on the shuttle…

When both gravity and forward motion forces act on the shuttle… Forward motion Pull of Gravity Actual Path Shuttle Travels

Section 3 Newton’s Laws Newton’s 1 st Law –An object at rest remains at rest and an object in motion remains in motion at a constant speed and in a straight line until acted on by an unbalanced force. Objects at rest –Objects at rest will not move unless forced to Objects in motion –Objects that are moving will continue to move unless forced to turn or stop

Forces Forces have both size and direction. 40 N 10 N 50 N No Motion Balanced forces create no motion Unbalanced forces create motion

Friction’s Effect on Newton’s 1 st Law –Due to the unavoidable friction experienced on Earth (air friction, friction with other surfaces) objects cannot be seen continuing to move. Examples –A soccer ball rolling across a field –A skateboard (you don’t have to push all of the time) –Key word Inertia- the tendency of an object to resist any change in motion.

Newton’s 2 nd Law –The acceleration of an object depends on the mass of the object and the amount of force applied. Small objects need only a small force to move, Large objects need a large force F = m x a mass forceacceleration F m a

What force is necessary to accelerate a 1,250 kg car at a rate of 40 m/s 2 ? F = m x a F = 1,250 x 40 F = 50,000 N What is the acceleration of a 7 kg mass if a force 56 N is used to move it? F mama F

What is the mass of an object if a force of 34N produces an acceleration of 4 m/s/s? F ma

Newton’s 3 rd Law –Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. Action/reaction force pairs can result in motion or no motion –The forces do not act on the same object. Otherwise, they would always cancel out. Examples –Swimming Action- pushing on the water Reaction- water pushing on the hands/feet move you forward –Kicking a ball Action- foot striking a ball pushing the ball forward Reaction- ball striking the foot trying to push the foot back

Since the downward pull of gravity is constant on all objects (regardless of size or forward velocity) even objects moving forward will fall at the same rate as objects with no forward velocity.

Key word –Momentum- the property of a moving object that depends on mass and velocity. The more momentum an object has the harder it is to stop. –Examples »A full grocery cart verses and empty one. »A loaded train verses a single car. »An 8 th grader verses a 6 th grader.

Newton’s First Law of Motion Newton’s Second Law of Motion Newton’s Third Law of Motion

Fluid and Pressure Pressure is the amount of Force exerted on a given Area. UNITS Force = Newtons (N) Area = square meters (m 2 ) Pressure = Pascals (Pa) 1 Pa = 1N/m 2 F A P

Deepest ocean depth = 110,000,000 Pa Atmospheric pressure - The weight of the Earth’s atmosphere exerts 101,300 N on each square meter (or 101,300 Pa) at sea level. This is equal to 10 N per sq centimeter. We aren’t crushed because our body fluids push out with an equal amount of force. Mt. Everest= 30,000 Pa Sea level= 101,300 Pa Deepest ocean depth = 110,000,000 Pa

Principles of Flight Flight requires some fluid friction to hold the plane up Thrust Lift Drag (air friction) Gravity (weight)

A wing (for a bird or a plane) creates LIFT because the air pressure surrounding it is not the same on each side. Air over the wing moves faster creating less pressure on top of the wing. If the difference between the pressures above and below the wing (lift) becomes greater than the weight of the plane, it will lift off the ground.

A helicopter creates lift by moving its wings around in a circle very fast. A fast plane creates lift even with tiny wings (very little drag) by going extremely fast. A huge cargo plane moves slower and weighs a lot using giant wings to create lift.

Chapter 14, section 2 Buoyancy is the upward force a fluid exerts on all matter –If the upward force of buoyancy is greater than the downward force of gravity (weight) the object will float

Determining Buoyant Force Archimedes Principle: “the buoyant force on an object in a fluid is an upward force equal to the weight of the volume of fluid that object displaces” Weight of an object has nothing to do with the buoyant force. The weight of the displaced water that matters. Weight of Displaced Water= 3.0N Buoyant Force= 3.0N

A hot-air balloon must displace a large amount of heavier cool air to float. A boat will float if it displaces the proper amount of water. Amount of air displaced This cruise ship is made of steel and weighs 100,000 tons (200 million pounds). It floats because the bottom of the ship pushes 100,000 tons (200 million pounds) of water out of the way pound balloon 1000 pounds air displaced

Negative Buoyancy Object weight 100 N Weight of displaced water = 90 N Object will sink. What happens if you change the volume (increase)?

Neutral Buoyancy Object weight 100 N Weight of displaced water = 100 N Object will not move up or down (Neutral buoyancy). What happens if you change the volume (decrease)?

Positive Buoyancy Object weight 100 N Weight of displaced water = 200 N Object will float up.

To begin preparing for the Chapter 12, 13 & 14 Test… –Create a page that has all of the necessary formula that we need to do the following calculations 1.Speed2. Acceleration 3.Speed due to gravity4. Newton’s 2 nd Law 5. Buoyancy - Include examples, all needed units, etc