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Chapter 1: Describing Motion

Chapter 1: Describing Motion. Units: how quantities in the physical world are described Basic Units: A set of units which can be used to express all physical quantities. Système International (S.I. = metric system). Common prefixes. Converting units treat units as algebraic quantities

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Chapter 1: Describing Motion

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  1. Chapter 1: Describing Motion • Units: how quantities in the physical world are described • Basic Units: A set of units which can be used to express all physical quantities. • Système International (S.I. = metric system)

  2. Common prefixes

  3. Converting units • treat units as algebraic quantities • Multiplying or dividing a quantity by 1 does not affect its value

  4. Speed displacement Dx Dt time

  5. Constant Speed • average speed = instantaneous speed (at all times) displacement time Example 1.3: A woman standing in front of a cliff claps her hands, and 2.50 s later she hears an echo. How far away is the cliff? (speed of sound in air = 343 m/s) Example 1.4: If the speed limit is raised from 55 mph to 65 mph, how much time is saved on a 100 mile trip?

  6. Velocity: the true measure of motion • = how fast, in what direction • = speed (how fast) + direction (velocity is a vector) • Vector: a quantity which has magnitude (how big) and direction • => the magnitude of velocity is speed • Displacement = change in position • velocity is the rate of change of displacement • average velocity vs. instantaneous velocity

  7. Acceleration: changes in motion • = how fast,and in what direction velocity is changing • acceleration is a vector • for motion in one dimension: speed changes with time • increasing/decreasing speed • changes in direction of velocity • usually assumed constant

  8. at at at at Velocity and Acceleration final velocity = initial velocity + change in velocity vf v v vf t v0 v0 t v v t v0 v0 vf t vf

  9. Example 1.7: A car has an initial velocity of 20 m/s and and acceleration of -1.0 m/s2. Find its velocity after 10s and after 50 s.

  10. Displacement, Time and Acceleration Example 1.8: A car has an initial velocity of 20 m/s and and acceleration of -1.0 m/s2. Find its displacement after 10s and after 50 s.

  11. Displacement, Velocity and Acceleration Example 1.9: A car has an initial velocity of 20 m/s and and acceleration of -1.0 m/s2. How far will it go before (momentarily) coming to rest?

  12. Problem Solving Tips • Draw a diagram • Write down all known quantities, identify on diagram. • Write down all unknown quantities, identify on diagram. • Find basic equation which relates each unknown quantity to known quantities. • Solve equation for the desired unknown. • “plug and chug”

  13. Acceleration of Gravity • Historical Perspective: • Attempts to describe motion (Aristotle, Galileo, ...) • Experiment and observation as the foundation of science • idealizations and simplifications: ignoring air resistance • g = 9.81 m/s2 (9.78 at equator, 9.83 at poles) • = 32 ft/s2 • a = g, acceleration is directed downwards

  14. Example 1.12 (variation) • A penny is dropped from a building 300m high. How long does it take the penny to reach the ground? What is its velocity when it reaches the ground? • Example 1.13 • A stone is thrown upward with an initial speed of 16 m/s. • What will its maximum height be? • When will it return to the ground? • Where will it be in .8 s? In 2.4 s?

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