1 Describing Motion Displacement and Distance Chapter 2

2 How do You Describe Motion? Use a Frame of Reference – A system of objects that are not moving with respect with each other. Reference Point – The zero point for your system, what you make your measurements from. Think about it: –If you were just floating in space could you tell you were moving if you couldn’t see anything else? Answer: No, it would be impossible. You might be able to tell if you were speeding up or down but not now fast you were moving, if you were moving or in what direction. *All Motion is Relative to some reference

3 Who is Moving Where? The girl sees the boy moving to the right The boy sees the girl moving to the left Which is correct? It is all a matter of what frame of reference you use. The girl sees the boy moving to the right The boy sees the girl moving to the left Which is correct? It is all a matter of what frame of reference you use.

4 The most commonly used frame of Reference is the surface of the Earth.

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8 Earth rotates on its axis at 1,100 mph Earth orbits the Sun at 67,000 mph

9 The whole galaxy rotates at 480,000 mph So how fast are you going? It’s all relative So how fast are you going? It’s all relative

10 So How Fast are You Really Moving? Earth rotates at 1100 miles / hr You Earth revolve around the Sun at 30 km / sec or 67,000 miles / hr Earth revolve around the Sun at 30 km / sec or 67,000 miles / hr Our solar system revolves around the galaxy at 220 km / sec or 490,000 miles / hr Our solar system revolves around the galaxy at 220 km / sec or 490,000 miles / hr 225 million years to make one (1) rotation around the galaxy 225 million years to make one (1) rotation around the galaxy You The Milky Way relative to other galaxies moves at 1,300,000 miles / hr The Milky Way relative to other galaxies moves at 1,300,000 miles / hr

11 Most all Physics quantities can be described as either a vector or scalar How Much How Much How Much & in Which Direction How Much & in Which Direction

12 Scalar A quantity that takes one piece of information to describe In Physics, a quantity that consists of a magnitude or amount only (number and units) Examples – distance, speed, liters,

13 Vectors A number that takes two pieces of information to describe In Physics, this is a magnitude (scalar) and a direction of action –Direction can be –Positive (+) or negative (-) –Up, down, left or right –N, S,E or W –Degree or angle heading Examples – displacement, velocity, acceleration, force

14 Distance v.s. Displacement Distance –The total length of the path taken. –Scalar –Always positive –Measured in meters (m), kilometers (km), or centimeters (cm) Displacement –The difference between the final position and the initial position –Or, how far from the starting point in a straight line and what direction –Vector –May be positive or negative –Measured in meters (m), kilometers (km), or centimeters (cm)

15 An Ant Takes a Trip Which represents the ants distance traveled? Blue path Which represents the ants displacement? Red Path When can displacement and distance traveled be the same? If you travel in a straight line. Which represents the ants distance traveled? Blue path Which represents the ants displacement? Red Path When can displacement and distance traveled be the same? If you travel in a straight line.

16 Speed and Velocity Chapter 2

17 Speed The rate at which something is moving. The ratio of how far something moves to the time it takes to move –Scalar –Always positive –Measured in meters/second (m/s) or kilometers/hour (km/hr) Average speed = distance traveled total time

18 Equation for Speed v = speed d = distance t = time

19 Velocity How fast an object is moving in a particular direction Describes both rate and direction The ratio of an objects displacement to the time it takes to be displaced. –Vector –Can be positive or negative –Measured in meters/second (m/s) or kilometers/hour (km/hr) Average velocity = displacement total time

20 Farmer Jones drives 6 miles east down a straight road. She turns around and drives 4 miles back. What was her average speed and velocity for this trip if it took 1 hour? Speed = ? 1 st Distance = 6mi + 4mi = 10 miles 2 nd Speed = distance time = 10 miles 1 hr = 10 miles/hr Velocity = ? 1 st Displacement = 6mi - 4mi = 2 miles [east] 2 nd Velocity = displacement time = 2 miles [east] 1 hr = 2 miles/hr [east] Q: Why is Ms. Jones’ speed and velocity different for her trip? A: She changed direction, backtracked for part of her trip. Knowns: 1 st leg = 6 mi [east] 2 nd leg = 4 mi [west] t = 1 hr

21 Example Problem A toy train moves along a circular path of radius 20 cm as shown in Fig What is the distance and displacement traveled when the train (a)moves from O to P? (b)moves from O to P and then back to O? Answer: (a) distance: 62.8 cm, displacement: 40 cm, to the North; b) distance: 126 cm, displacement: 0 cm.