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Physic² 121: Fundament°ls of Phy²ics I

Physic² 121: Fundament°ls of Phy²ics I. September 22, 2006. Outline. Worked Example Problem More Newton’s Laws of Motion. Heads-Up on Problem 9.

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Physic² 121: Fundament°ls of Phy²ics I

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  1. Physic² 121:Fundament°ls of Phy²ics I September 22, 2006 University of Maryland

  2. Outline • Worked Example Problem • More Newton’s Laws of Motion University of Maryland

  3. Heads-Up on Problem 9 • For the part of the problem where you are asked to determine the two times, it is possible for WebAssign to mark on time as correct and the other time incorrect even though the two times add up to 90s. • This is not a bug in WebAssign: Issue is that WebAssign will count a numerical answer as correct as long as it is within 1~2% of the correct answer. So, the time marked as correct isn’t REALLY correct, and that throws off the other time University of Maryland

  4. Chapter 2, Problem 58 • A ranger in a national park is driving at 35.0 mi/h when a deer jumps into the road 200 ft ahead of the vehicle. After a reaction time t the ranger applies the brakes to produce an acceleration a = -9.00 ft/s2. What is the maximum reaction time allowed if she is avoid hitting the deer? University of Maryland

  5. Newton’s First Law • An object at that is at rest will remain at rest, or an object this is moving will continue to move in a straight line with constant velocity, if and only if the net force acting on the object is zero • New ideas we need to understand: • Force • Net Force • This Law basically says no cause is needed for an object to move. • Uniform (constant) motion is an object’s natural state Translation from original Latin: “Every body perseveres in its state of rest, or of uniform motion in a right (straight) line, unless it is compelled to change that state by forces impressed thereon” University of Maryland

  6. Newton’s Second Law • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. • F and a are both vectors • Can also be applied three-dimensionally University of Maryland

  7. Newton’s Third Law • If object 1 and object 2 interact, the force exerted by object 1 on object 2 is equal in magnitude but opposite in direction to the force exerted by object 2 on object 1. • Equivalent to saying a single isolated force cannot exist University of Maryland

  8. Newton’s Third Law cont. • F12 may be called the action force and F21 the reaction force • Actually, either force can be the action or the reaction force • The action and reaction forces act on different objects University of Maryland

  9. Gravity as a Force • Question: Does gravity affect all objects the same? • How do you know? • If not the same, what does it depend on? • (book, paper) • Experimentally, it has been observed that all objects experience the same acceleration due to gravity near the surface of the earth, when gravity is the only force involved (i.e. neglecting air resistance) • Galileo dropped objects of different mass from the Leaning Tower of Pisa to show this • Test… • The acceleration is: • g = 9.8 m/s2 • Note: units of acceleration (not force) • If we call “up” positive, then the acceleration is: • a = -g University of Maryland

  10. Gravity as Force • Gravitational Force • Fgrav = -mg (from N2, F = ma with a = -g) • Be careful of sign! g is a positive number! • Value of g would change if you weren’t on surface of earth (on the moon, for example) • Value of mass doesn’t change • Weight is magnitude (absolute value) of grav force, mg • Unit of weight is Newton (just like force) University of Maryland

  11. How smart is your table? Consider a book sitting calmly on a table. Does the table (an inanimate object) exert a force on a book that is resting on it? • Yes, an upward force exactly equal to the weight of the book. • No, an inanimate object cannot exert a force. It’s just “in the way.” • Yes, a frictional force. • Yes, a Newton-3 force – a reaction to the action of gravity on the block. • No, there doesn’t need to be a force for the table to hold it up. • None of the above

  12. Some Action-Reaction Pairs • is the normal force, the force the table exerts on the TV • is always perpendicular to the surface • is the reaction – the TV on the table University of Maryland

  13. More Action-Reaction pairs • is the force the Earth exerts on the object • is the force the object exerts on the earth University of Maryland

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