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Physics!

Physics!. Motion and Forces Chapter 2 & 3 (Write down wHite writing). Distance, Direction and Velocity. Distance vs. Displacement. Distance: describes how far an object has moved, regardless of its direction ex: 40km east + 25km west = 65 total km traveled

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Physics!

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  1. Physics! Motion and Forces Chapter 2 & 3 (Write down wHite writing)

  2. Distance, Direction and Velocity

  3. Distance vs. Displacement Distance: describes how far an object has moved, regardless of its direction ex: 40km east + 25km west = 65 total km traveled Displacement: describes both distance and direction ex: 40 km east + 25 km west = 15 km east ex: 40 km east + 25 km east = 65 km east

  4. speed • How fast an object is traveling regardless of direction • Unit: m or km sec hour Described in two ways: 1. Instantaneous speed: measured at a specific instant • initial speed and final speed are examples • this is what speedometers measure 2. Average speed: total distance traveled per unit of time • if an object travels at a constant speed, then the instantaneous and average speed will be equal • if the object is traveling at varying speeds, then the average speed is calculated as total distance traveled over total time

  5. velocity • Describes both the speed of an object AND its direction of motion • Units: speed + direction: 24 m north sec Measured in two ways: 1. Instantaneous velocity: velocity at a specific instant 2. Average velocity: the total displacement per unit of time Velocity can change if: 1. the object’s speed is changed (increased or decreased) 2. the object’s direction can change

  6. The math… dun dun dun  v = d t V = either velocity or speed d = distance or displacement t = time Units count!!!!!!!!!! Let’s practice!!!!

  7. Sample problem #1 An elevator travels from the 1st to the 60th floor, a distance of 210 meters in 35 seconds. What is the elevator’s speed?

  8. Sample problem #2 How Far does a car travel in 1.5 hours if it is moving at a constant speed of 88 km/hr?

  9. Homework tonight!!!! Complete problems #1-5 from “Practice problems” worksheet

  10. FLASHBACK • What is the difference between distance and displacement? • What is the difference between speed and velocity? • Your average walking speed is 0.90 m/sec. What is it in km/hour?

  11. Calculating the speed of a toy car… how fast can it go????

  12. FLASHBACK • EOC WORKBOOK - pg. 48 [1,2,4,5] • Why do you think some people confuse speed with velocity? • Does a speedometer show both speed and velocity? • You travel to and from RHHS, which is located 5 mi to the west of your house. What is your distance & displacement?

  13. acceleration and gravity PS 5.3 and PS 5.4

  14. Acceleration • The rate of change of an object’s velocity over the time it takes for that change to occur • Equation: a = (vf – vi) t • Unit: m/s/s or m/s2 (most common) However, the unit used for velocity (m/s) and the unit used for time (s) do not necessarily need to be the same: km/hr per second

  15. Acceleration The change in acceleration may involve a change in speed or direction Can acceleration be negative??? Yes!! When you are slowing down at a stoplight, speed is decreasing, so acceleration is opposite of velocity... = negative acceleration

  16. three ways to describe acceleration: 1. constant acceleration: acceleration is zero because the velocity does not change ex: walking 2 meters every 15 seconds 2. positive acceleration: the object is speeding up ex: getting on the interstate in your car 3. negative acceleration:the object is slowing down ex: pulling into a parking lot to park your car

  17. Gravity’s affect on acceleration • All objects accelerate as they fall because the Earth exerts gravitational force on them • All objects when released accelerate in the direction of the force (downward) • At initial release, the object has an initial velocity of 0.0 m/sec • As it falls, the object accelerates at a constant rate of 9.8 m/s2 • This means the object will travel 9.8 m/sec every second it is falling  as long as there is no air resistance • The value 9.8 m/sec2 is called the acceleration of gravity (ag).

  18. Sample problem #1 a = (vf – vi) t Positive Acceleration: (speeding up) A jet airliner starts at rest and reached a speed of 80 m/s in 20 seconds. What is it’s acceleration?

  19. Sample problem #2 a = (vf – vi) t Negative Acceleration: (slowing down) A skateboarder is moving at a constant speed of 3 m/s and comes to a stop in 2 seconds. What is his acceleration?

  20. Assignments! • Right Now: pg 51 “Self Check” # 1-4

  21. Marble Lab!!! Acceleration

  22. FLASHBACK • What needs to occur for an object to have acceleration? • Which of the following is a proper unit of acceleration? • m/km2 b. km/h c. m/s2 d. m2/s • 3. When describing the rate of a race car going around a track, should you use the term speed or velocity, why? • 4. How long does it take for the sound of thunder to travel 1485 m, if sound travels at 330 m/s?

  23. FLASHBACK • If you ride your bike down a straight road for 500 m then ride back, your distance is ____ your displacement. • a. greater than b. less than c. equal to • 2. Acceleration is rate of change of ____. • a. position b. time c. velocity d. force • 3. If you ride your bike up a hill, then ride down the other side, your acceleration is ____. • a. all positive b. first positive, then negative • c. all negative d. first negative, then positive

  24. Tennis Ball Activity What changes when a tennis ball bounces??? • Draw and label for two bounces: • Positive acceleration • Negative acceleration • Increasing and decreasing speed

  25. FLASHBACK • How long will it take to reach Bi-Lo 15 km away if traveling at 50 km/hr? • 2. How long will it take a whale to travel 200 km swimming 4 m/s? • What is the value of 9.8 m/sec2 called… • What is the formula for acceleration?

  26. Newton’s laws: PS 5.7

  27. Newton’s 1st law of motion • “The velocity of an object will remain constant unless a net force acts on it.” • This is called the Law of Inertia. • Inertia: is the tendency of an object to remain at rest or in motion • Inertia is dependent upon an object’s mass. • Force: a push or pull that one object exerts on another • It is measured in Newtons (N). • If one object has a greater force, then net force must be calculated

  28. How does this relate to Newton’s First Law?????

  29. Newton’s 2nd law of motion • “When a net force acts on an object, it will accelerate in the direction of the net force” • The larger the force, the greater the acceleration • The larger the mass, the smaller the rate of acceleration • Force = Mass (acceleration) • Friction and air resistance can affect net force; they act as opposing forces.

  30. III. Newton’s Third Law • Newton’s Third Law : When one object exerts a force on a second object, the second object exerts a force on the first that is equal in magnitude and opposite in direction. 1. a swimmer pushes on the water, and the water pushes back 2. a ball thrown on a wall, bounces off

  31. Force, Mass, Weight, and Gravity PS 5.8 – 5.10

  32. I. The Math Behind the Force • Force is a derived unit; it involves multiplying the mass of an object by its acceleration: F = mass * acceleration B. For us, the mass is measured in Kilograms (Kg) and the acceleration in m/sec2. C. This means the final answer (Force) is measured as Kg * m/sec2 or 1 Newton D. 1 Newton is the amount of force required to accelerate a 1 Kg object at a rate of 1 m/sec2

  33. E. Look at these examples: What is the force required to move a 2400Kg car with an acceleration of 4 m/sec2? If a force of 4.2 Newtons is applied to an object with a mass of 75 Kg, what is the rate of acceleration?

  34. II. Newton’s Law of Gravitation • States that there is a force of attraction between all objects in the universe. • The object with the larger mass will exert the greater force. This is why objects weigh more on the earth than on the moon. The earth is larger so it has a greater force of attraction (gravity). • We do not notice the forces between most objects on earth because the force of the earth is so much greater than the force between the two objects.

  35. D. The amount of force the earth exerts on an object is dependent upon the object’s mass & distance. The greater the mass, the greater the gravitational force. E. Weight is a measure of an object’s gravitational force and it can be found using the formula: Fw = mass (acceleration of gravity) F. The acceleration of gravity is what? 9.8 m/sec2

  36. G. Try these examples: What is the weight of an object with a mass of 10 Kg? What is the mass of an object with a weight of 25 N?

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