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Transcript
Safety First! That’s what My Daddy always says! EGS Physics Unit Describing and Measuring Motion • What is motion and are you really moving? • Is your table moving? • Is the building moving? • Can you be sure if something is really moving? Describing and Measuring Motion What is all this talk about motion? Are we really moving in space at 30 km per second? Describing and Measuring Motion • A reference point is a place or object used for comparison to determine if something is in motion. • An object is in motion if it changes position relative to the reference point. Describing and Measuring Motion What is the basic unit used to describe motion? Describing and Measuring Motion This one is easy, METER Constant Speed • If you know the distance an object travels in a given amount of time, you can use the formula for speed to calculate the object's constant speed. • Speed = distance divided by time •S=d/t time distance 0 0 1 2 2 4 3 6 4 8 5 10 6 12 7 14 8 16 9 18 10 20 distance calculating slope 25 20 15 10 5 0 time distance 1 3 5 time 7 9 11 time distance 0 0 1 2 2 4 3 6 4 8 5 8 6 8 7 8 8 12 9 16 10 20 25 20 15 10 5 0 time time 11 9 7 5 3 distance 1 distance calculating slope I think I am getting this slope stuff. Velocity • When you know both the speed and direction of an object’s motion, you know the velocity of the object. • Speed in a given direction is called velocity. Velocity • Speed – 197 mph – 88.07 m/s – 317.05 km/hr • Velocity – 197 mph North – 88.07 m/s to the left – 317.05 km/hr downward We’re trying our best, but even after two three four five championships, We still can’t teach Junior how to drive! ACCELERATION • Acceleration is the rate at which velocity changes. • Recall that velocity has both speed and direction. • So, acceleration refers to increasing speed, decreasing speed and changing direction. CALCULATING ACCELERATION • Acceleration is equal to the final velocity minus the initial velocity divided by the time. final velocity – initial velocity • Acceleration= ------------------------------- time CALCULATING ACCELERATION • A roller coaster car rapidly picks up speed as it rolls down a slope. As it starts down the slope, its speed is 4 m/s. But 3 seconds later, at the bottom of the slope, its speed is 22 m/s. What is its average acceleration? CALCULATING ACCELERATION • • • • • • Final velocity = 22 m/s Initial velocity = 4 m/s Time = 3 seconds 22 m/s- 4 m/s Acceleration = ----------------------- = 3 sec CALCULATING ACCELERATION • • • • • • Final velocity = 22 m/s Initial velocity = 4 m/s Time = 3 seconds 22 m/s- 4 m/s Acceleration = ----------------------- = 6 m/s2 3 sec CALCULATING ACCELERATION • The same roller coaster car goes through its final loop before the ride ends. Its speed is 40 m/s and slows down to a complete stop in 8 seconds. What is the roller coaster’s acceleration (deceleration)? CALCULATING ACCELERATION • Final velocity = 0 m/s • Initial velocity = 40 m/s • Time = 8 seconds • 0 m/s- 40 m/s • Acceleration = ------------------- = -5.0 m/s2 • 8 sec Newton’s First Law of Motion • INERTIA –the tendency of an object to resist change in its motion (even if it is at rest) Newton’s First Law of Motion • The amount of inertia an object has depends on its mass. • The greater the mass of an object, the greater its inertia. • Mass, then, can also be defined as a measure of the inertia of an object. Newton’s Second Law of Motion • The net force on an object is equal to the product of its acceleration and its mass. • • • • Force = mass X acceleration Units for mass is kg. Units for acceleration is m/s/s or m/s2 Unit of force- Newton (N) or (kg X m/s2) Newton’s Second Law of Motion • What net force is needed to accelerate a 55 kg cart at 15 m/s2 •Force = mass X acceleration •Mass = 55 kg •Acceleration = 15m/s2 Newton’s Second Law of Motion • Force = 55 kg X 15 m/s2 • Force = 825 (kg X m/s2) • Force = 825 N Newton’s Second Law • If you push on a shopping cart, it will accelerate. • If you push 5 times harder, what happens to the acceleration? • Acceleration is 5 times greater. Newton’s Second Law • If you push the same, but the cart is loaded so it has five times as much mass, what happens to its acceleration? • Acceleration will be less - 1/5 as much. Where’s my Cart? Newton’s Second Law • If you push five times harder when it is loaded with five times as much mass, what happens to the acceleration? • It will have the same acceleration as it had to begin with. Newton’s Second Law • Gravity is actually an acceleration equal to 9.8 m/s2 • So the force of gravity is the mass of an object times its acceleration due to gravity • F = mg Newton’s Third Law of Motion • Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. • To every action there is always an opposed equal reaction. Newton’s Third Law of Motion • Examples – Walking across the floor – Your car moving (burning rubber??) – Swimming – A person on ice – Leaning on a wall – Using a pen to write Newton’s Third Law of Motion • Concept check • Which exerts more force, the Earth pulling on the moon, or the moon pulling on the Earth? • The force is the same, only in opposite directions. • Which is farther, the distance from New York to S.F. or S.F. to N.Y.? Hmm??? Newton’s Third Law of Motion • Concept Check • A high speed bus and an innocent bug have a head on collision. The force of the bus on the bug splatters the poor bug all over the wind-shield. Is the corresponding force of the bug on the bus greater, less, or the same? Is the resulting deceleration of the bus greater, less, or the same as that of the bug? Momentum • Momentum = mass X velocity • Which has more momentum, a 3 kg sledgehammer swung at 1.5 m/s or a 4 kg sledgehammer swung at 0.9 m/s? • 3 kg X 1.5 m/s = 4.5 kg *m/s • 4 kg X 0.9 m/s = 3.6 kg * m/s Momentum • A golf ball travels 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is 0.045 kg and the mass of the baseball is 0.14 kg. Which ball has the greater momentum? • 0.045 kg X 16 m/s = 0.72 kg *m/s • .14 kg X 7 m/s = 0.98 kg * m/s Conservation of Momentum • Law of Conservation of Momentum states that the total momentum of the objects that interact does not change. Work and Power • Work- a transfer of energy that occurs when a force makes an object move • Work = Force (N) x Distance (m) • Units for work are N∙m or joules (J) Work and Power • Power- the amount of work done in a certain amount of time • Calculating Power – Power = Work (J) / Time (s) – Units for power are J/s or watts (W) • Horsepower (hp) is equal to 746 watts Machines • Any device that makes doing work easier – Increase applied force – Change direction of applied force – Increase the distance over which a force is applied Machines • Machines help move things that resist being moved • Input force- the force that you exert on the machine • Output force- the force exerted by the machine on some object Machines • You know what input force and output force are, so what do you think input distance and output distance are? • How could we calculate Input Work and Output Work? Energy • Energy is the ability to cause change or do work • Many forms of energy can be classified into two general types: – Kinetic – Potential Energy • Kinetic Energy (KE) – Energy in the form of motion – KE of an object is dependent on the object’s mass and velocity (speed) – KE = ½mv2 – Measured in joules (J) Energy • Potential Energy (PE) – Energy stored in a stationary object • Elastic PE- energy stored in things that stretch or compress • Chemical PE- energy stored in chemical bonds • Gravitational PE- energy stored by things that are above a reference level, for example the Earth’s surface, the room floor, a table top, etc… Energy • Gravitational Potential Energy (GPE) – Depends on an objects mass, its acceleration due to gravity, and its height above a reference level – GPE = mgh – Units for GPE are also in joules (J) – ‘g’ on Earth is 9.8 m/s2 – What is ‘mg’? Energy • Energy can change from one form to another • For example, radiant energy from the Sun can be converted into chemical energy by plants, which in turn which in turn were buried millions of years ago and converted into fossil fuels. Energy • Mechanical Energy (ME) – ME = KE + PE • Law of Conservation of Energy – Energy may change from one form to another, but the total amount of energy never changes Energy • Example – A moving swing is constantly converting GPE into KE and back. – Eventually the swing stops because it loses energy due to friction. As the swing loses an amount of energy, the frame of the swing must gain that same amount of energy. – Friction converts the ME into thermal (heat) energy Energy • Einstein tells us that we must think of mass and energy as being equivalent and that one can be converted into the other • This is especially important in nuclear physics. During a nuclear reaction, as atoms are split or fused to release energy, the total amount of mass and energy is conserved. Energy • The food we eat has chemical potential energy that can be converted into kinetic energy enabling you to breathe, think, move, live, etc… • Energy in food is measured in Calories. One Calorie is equivalent to 4,180 J.