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EOCT REVIEW

EOCT REVIEW. Domains 3 and 4 Physics. Energy. A measure of the ability to do work Measured in joules Whenever work is done, energy is transferred or transformed to another system. Potential Energy. Stored energy “potential” to do work

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EOCT REVIEW

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  1. EOCT REVIEW Domains 3 and 4 Physics

  2. Energy • A measure of the ability to do work • Measured in joules • Whenever work is done, energy is transferred or transformed to another system

  3. Potential Energy • Stored energy • “potential” to do work • Elastic potential energy – energy stored in stretched or compressed elastic material • Gravitational potential energy – energy of two or more objects separated by a distance

  4. Gravitational Potential Energy • GPE = mass x free-fall acceleration x height • GPE = mgh • M x g = Weight, so actually same as Work = Force x distance • Height is usually measured from the ground

  5. GPE Problems • 1. Calculate GPE in each: • A. car with a mass of 1200 kg at top of 42 m hill • PE = mgh = 1200 kg x 9.8 m/s2 x 42 m = 493920 J • B. 65 kg climber on top of Mt Everest (8800 m high) • PE = mgh = 65kg x 9.8 m/s2 x 8800m = 5605600 J • C. 0.52 kg bird flying at altitude of 550 m • PE = mgh = 0.52 kg x 9.8 m/s2 x 550 m = 2800 J

  6. Kinetic Energy • Energy an object has because it is in motion • Depends on mass and speed • Larger and/or faster objects have ability to do more work • Depends on speed more than mass • Explains car crashes at higher speed

  7. Kinetic Energy Formula • Kinetic energy = ½ x mass x speed squared • KE = ½ mv2

  8. Other Forms of Energy • Mechanical energy = sum of PE + KE in a system • Chemical energy = breaking and forming bonds • Living things get chemical energy from the sun • Nuclear energy = changes in nucleus, powers the sun • Electricity = flow of charged particles • Light energy = form of electromagnetic waves

  9. Conservation of Energy • Law of Conservation of Energy • Energy may change forms, but it cannot be created or destroyed under ordinary conditions. • EX: • PE  KE • mechanical  thermal • chemical  thermal

  10. Work • What is work? • Work is the transfer of energy that occurs when a force makes an object move • In order to do work two conditions MUST be met: • 1. the applied force must make the object move • 2. the movement must be in the same direction as the applied force

  11. Work • Decide if work is being done or not! • Pushing against a wall • Lifting a box • Holding a box • Walking while holding a box • Pushing a shopping cart

  12. Work and Energy • How are work and energy related? • When work is done energy is always transferred • When you do work on an object you increase it’s energy • If you push an object across the floor you increase its velocity and so its KE increases • If you lift a box you increase its height so its PE increases

  13. Calculating Work • Work (in joules)= applied force (newtons) x distance (meters) • W=Fd • What work is done if you lift a box 2m using 5N of force? • W=5N x 2m= 10J

  14. Power • Power is the rate at which work is done • Power= the amount of work done in a period of time • Power (watts) = Work (joules)/ time (seconds) • P=W/t • How much power is used if you do 15J of work in 3 seconds while lifting a box? • P=15J / 3sec = 5 watts

  15. Power and Energy • Power is also the rate at which energy is transferred • Power (watts)= energy transferred (joules)/ time (seconds) • P=E/t • If 25J are transferred from you to a cart in 5 seconds as you push it, how much power was used? • P=25J / 5sec = 5watts

  16. Motion

  17. Motion Diagrams • Motion diagrams--- a series of images showing the positions of a moving object at equal time intervals • -Particle Motion--- a simplified version of a motion diagram in which the object is replaced by a series of single points -size of the object must be much less than the distance it moves

  18. Motion Diagrams

  19. Frame of Reference • After a reference point is chosen, a frame of reference can be created • A frame of reference is a coordinate system in which the position of the objects is measured • The x-axis and y-axis of the reference frame are drawn so that they intersect the reference point

  20. Distance vs Displacement • Distance= how far an object has moved • Example: a person runs 100m • The SI unit of measure =meter (m) • Displacement= the distance and direction of an object’s change in position from the starting point • Displacement = final distance-initial distance • Example: a person runs 100m north • The SI unit of measure=meter (m)

  21. Speed • The distance an object travels per unit of time • Speed(meter/second)=distance(meters) time (seconds) • s=d/t • Average speed=total distance divided by total time of travel • Instantaneous speed= speed at a given point in time

  22. Graphing Motion • Distance-Time graph • The distance is plotted on the vertical axis and the time on the horizontal axis • The slope of a line on a distance-time graph gives the speed of an object in motion

  23. Velocity • Velocity is the speed and direction of an object • Velocity=displacement/time • V=(df-di)/(tf-ti) • Soooooo, how are speed and velocity different?

  24. Acceleration Acceleration, Speed and Velocity Acceleration = rate of change of velocity. A change in velocity can be either a change in how fast something is moving, or a change in the direction it is moving. Acceleration occurs when an object changes its speed, it's direction, or both.

  25. Acceleration If the acceleration is in the same direction as the velocity, the speed increases and the acceleration is positive. Speeding Up and Slowing Down

  26. Acceleration If the speed decreases, the acceleration is in the opposite direction from the velocity, and the acceleration is negative. Speeding Up and Slowing Down

  27. Acceleration Changing Direction The speed of the horses in this carousel is constant, but the horses are accelerating because their direction is changing constantly.

  28. Acceleration Calculating Acceleration To calculate the acceleration of an object, the change in velocity is divided by the length of time interval over which the change occurred.

  29. Acceleration Calculating Acceleration So, the formula for acceleration is:

  30. Acceleration Calculating Positive Acceleration • How is the acceleration for an object that is speeding up different from that of an object that is slowing down? • Positive acceleration = “speeding up” • Negative acceleration = “slowing down” Suppose a jet airliner starts at rest at the end of a runway and reaches a speed of 80 m/s in 20 s.

  31. Acceleration Calculating Positive Acceleration Its acceleration can be calculated as follows:

  32. Acceleration Calculating Positive Acceleration The airliner is speeding up, so the final speed is greater than the initial speed and the acceleration is positive.

  33. Acceleration Now imagine that a skateboarder is moving in a straight line at a constant speed of 3 m/s and comes to a stop in 2 s. Calculating Negative Acceleration The final speed is zero and the initial speed was 3 m/s.

  34. Acceleration Calculating Negative Acceleration The skateboarder's acceleration is calculated as follows: a=(vf-vi)/t a=(0-3)/2 a= -1.5m/s2

  35. Acceleration The skateboarder is slowing down, so the final speed is less than the initial speed and the acceleration is negative. Calculating Negative Acceleration

  36. Forces!!!! Chapter 3

  37. Force • A force is a push or a pull • A force can cause the motion of an object to change • Force does not always change velocity • Balanced forces=forces that are equal in size and opposite in direction • When two or more forces act on an object at the same time, the forces combine to form the net force

  38. Inertia and Mass Inertia is the tendency of an object to resist change in its motion Objects have different inertias based on their mass The greater the mass of an object, the greater the inertia of that object Ex: A bowling ball takes much more force to move, change directions or slow down, the bowling ball has great inertia. Ex: A tennis ball can be stopped or started by blowing on it, the tennis ball has small inertia!

  39. Newton---the motion guy Sit Isaac Newton, a British scientist, was able to state the rules that describe the effects of forces on the motion of objects (known as Newton’s law of motion) 1st law=inertia 2nd law=F=ma 3rd law=action/reaction

  40. Newton’s 1st law An object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force acts on it Think----An object in motion stays in motion and an object at rest stays at rest unless acted on by an outside force Refers to the idea of inertia

  41. Factors Affecting Force Acceleration The more force, the greater the acceleration Mass The more mass, the greater the force

  42. Newton’s 2nd Law • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. • a = FNET/m • FNET=ma • Force is measured in Newtons (N) • Mass is measured in kilograms (kg) • Acceleration is measured in meters/second2 (m/s2)

  43. Practice Probelm • What force would be required to accelerate a 40 kg mass by 4 m/s2? GIVEN: F = ? m = 40 kg a = 4 m/s2 WORK: F = ma F = (40 kg)(4 m/s2) F = 160 N

  44. Friction Think back to Newton’s 1st law Why does a ball rolling on the desk eventually slow down and stop? Friction=the force that opposes the sliding motion of two surfaces that are touching each other

  45. Friction • The amount of friction between two surfaces depends on two factors • The kinds of surfaces • The force pressing the surfaces together • Friction is caused by the rough surfaces of objects • ALL objects have a rough surface, even if it’s only on a microscopic level

  46. Types of Friction Static Friction= the frictional force that prevents two surfaces from sliding past each other The reason its harder to start pushing than to continue pushing the same object Sliding Friction=the frictional force that opposes the option of two objects sliding past each other Rolling Friction=frictional force between a rolling object and the surface it rolls on

  47. 3.2 GravityA Basic Force Gravity is one of the four basic forces. The other basic forces are the electromagnetic force, the strong nuclear force, and the weak nuclear force.

  48. 3.2 Weight The gravitational force exerted on an object is called the object’s weight. Because the weight of an object on Earth is equal to the force of Earth’s gravity on the object, weight can be calculated from this equation:

  49. 3.2 Weight and Mass Weight and mass are not the same. Weight is a force and mass is a measure of the amount of matter an object contains. Weight and mass are related. Weight increases as mass increases.

  50. 3.2 Weight and Mass The weight of an object usually is the gravitational force between the object and Earth. The weight of an object can change, depending on the gravitational force on the object.

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