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

2. D. Roberts PHYS 121 University of Maryland Outline Worked Example Problem More Newton’s Laws of Motion

3. D. Roberts PHYS 121 University of Maryland 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

4. D. Roberts PHYS 121 University of Maryland 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/s 2. What is the maximum reaction time allowed if she is avoid hitting the deer?

5. D. Roberts PHYS 121 University of Maryland 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”

6. D. Roberts PHYS 121 University of Maryland 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

7. D. Roberts PHYS 121 University of Maryland 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

8. D. Roberts PHYS 121 University of Maryland Newton’s Third Law cont. F 12 may be called the action force and F 21 the reaction force –Actually, either force can be the action or the reaction force The action and reaction forces act on different objects

9. D. Roberts PHYS 121 University of Maryland 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/s 2 –Note: units of acceleration (not force) –If we call “up” positive, then the acceleration is: a  g

10. D. Roberts PHYS 121 University of Maryland Gravity as Force Gravitational Force –F grav = -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)

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? 12345 1.Yes, an upward force exactly equal to the weight of the book. 2.No, an inanimate object cannot exert a force. It’s just “in the way.” 3.Yes, a frictional force. 4.Yes, a Newton-3 force – a reaction to the action of gravity on the block. 5.No, there doesn’t need to be a force for the table to hold it up. 6.None of the above

12. D. Roberts PHYS 121 University of Maryland 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 –

13. D. Roberts PHYS 121 University of Maryland More Action-Reaction pairs – is the force the Earth exerts on the object – is the force the object exerts on the earth –