Workshop Lecture From Physics 101 Newtons Laws Buoyancy

Workshop Lecture (From Physics 101) (Newton's Laws & Buoyancy) l Assumptions: èYou, as “students”, have done the preflights on the web. » 16/24 people did (thanks!). èYou, as “students”, have read the textbook. » I know you haven’t, however for this lesson it won’t matter much. l l This material would usually be spread over several lectures. Lectures usually have quantitative examples (but not today) 4/10/01 Workshop, Pg 1

Newton's Laws 1. An object moving with constant velocity will keep moving with that same velocity (both speed and direction) unless a force acts on it. (demo) a 2. Ftot = ma (demo) Ftot m Both magnitude & direction FF-C 3. If object 1 exerts a force F on object 2, then object 2 exerts an equal but opposite force (-F) on object 1. (demo) FC-F 4/10/01 Workshop, Pg 2

Act 1 (Pre-Flights 1 -2) Driving your car on I-57 you encounter a bug which (sadly) splatters on your windshield. During the collision between the car and the bug: 1. The force exerted by the car on the bug is BIGGER than the force exerted by the bug on the car. VOTE 2. The force exerted by the car on the bug is SMALLER than the force exerted by the bug on the car. 3. The force exerted by the car on the bug is THE SAME AS than the force exerted by the bug on the car. Preflight responses correct The car has greater mass. I think force is something like a product of mass and change in speed. The car had higher speed. For every action there is an equal and opposite reaction. But a bug can't withstand the same amount of force as a windshield, so it squishes. 4/10/01 Workshop, Pg 3

Act 1 (Pre-Flights 1 -2) This is a beautiful response: I seem to remember a phrase stating that for every force, there is an equal and opposite force. For example, if I push against the wall, there is an equal and opposite force pushing back. But, if I push against a door and it closes, I have to reason that the opposite force cannot be equal because the door is moving. I guess I feel that way about the bug. Can anyone see the very subtle flaw in this argument ? ? This was part of the same answer: - it reminds me of a line from the Man of La Mancha, whether the stone hits the pitcher or the pitcher hits the stone, it's going to be bad for the pitcher. 4/10/01 Workshop, Pg 4

Act 2 Follow-up: During the collision between the car and the bug, which one experiences the greatest acceleration? 1. The car has a greater acceleration. 2. The bug has a greater acceleration. correct VOTE 3. The accelerations will be the same. F = m a bug car = bug car x bug car 4/10/01 Workshop, Pg 5

Act 3 (Pre-Flights 3 -4) In Case 1 shown below, a weight is hung from a rope (over a pulley) and is attached to one side of a spring. The other side of the spring is attached to a wall using a second rope. In Case 2, instead of being attached to a wall, the second rope is attached to a second identical weight. down wall Case 1 Case 2 In which case is the spring stretched the most? 1. Case 1 2. Case 2 3. Same in both cases 4/10/01 Workshop, Pg 6

down In which case is the spring stretched the most? 1. Case 1 2. Case 2 3. Same in both cases wall Case 1 correct VOTE Case 2 The net force on the spring is zero in each case. In both cases the weight on the right exerts the same force W. So there must be an equal force on the left side of each spring Tension exerted on the string attached to the wall is equal to the downward force of the weight. When the wall is replaced by a weight of equal mass, then like the wall it exerts tension on the string that is equal to the downward force of the opposite weight. 4/10/01 Workshop, Pg 7

down In which case is the spring stretched the most? 1. Case 1 2. Case 2 3. Same in both cases wall Case 1 correct Case 2 Case 1 -fixed on one end therefore all of weight acts to distract coils of spring. Case 2 -opposite distractive forces but ends not fixed so can move to pint where opposite forces "cancel" each other out and eliminate distractive forces on spring The stretched spring must counter 1 times weight in case 1, and 2 times weight in case 2. There seems to be more total weight pulling on the spring. 4/10/01 Workshop, Pg 8

Buoyancy This is why an object floats if it is less dense than the liquid. Upward (buoyant) force FB = weight of displaced liquid. FB FB mg DEMOS mg 4/10/01 Workshop, Pg 9

Act 4 (Pre-Flights 5 -6) An ice cube floats in a full glass of water as shown below. When the ice melts, the level of the water will: 1. Go up, causing the water to spill out of the glass. 2. Go down. 3. Stay the same. CORRECT VOTE The amount of ice that projects above the water line is exactly equal to the extra volume that is created when the liquid water is expanded by freezing. The volume of the liquid water from the melted ice cube will equal the volume of the ice cube that is below the water. The amount of liquid displaced is equal to the weight of the object it is supporting. Even though the density of ice is less than water, the mass, and therefore the volume in the liquid form, is the same. 4/10/01 Workshop, Pg 10

Act 4 (Pre-Flights 5 -6) An ice cube floats in a full glass of water as shown below. When the ice melts, the level of the water will: 1. Go up, causing the water to spill out of the glass. 2. Go down. 3. Stay the same. CORRECT VOTE The idea is that a volume of a liquid is increasing when some more liquid is added, and ice melts into water eventually and takes more space than the glass can hold Ice cubes take up a larger volume than water. 4/10/01 Workshop, Pg 11

My Favorite Answer: Archimedes's principle says that the weight of water displaced equals the weight of the body. When the ice melts, it provides exactly this amount of water to the glass. Now consider a boat floating on a lake, and what happens to the level of the lake when the boat's anchor is thrown overboard. ACT 5: Does the level of the lake: 1. Go up 2. Go down 3. Stay the same CORRECT VOTE 4/10/01 Workshop, Pg 12