Unformatted text preview: ConcepTest 5.1 Tetherball ConcepTest Tetherball
In the game of tetherball, In the struck ball whirls around a pole. In what direction does the net force on the ball point? force 1) toward the top of the pole toward 2) toward the ground toward 3) along the horizontal component of the tension force tension 4) along the vertical component of the tension force tension 5) tangential to the circle tangential T W ConcepTest 5.1 Tetherball Tetherball
In the game of tetherball, In the struck ball whirls around a pole. In what direction does the net force on the ball point? force 1) toward the top of the pole toward 2) toward the ground toward 3) along the horizontal component of the tension force tension 4) along the vertical component of the tension force tension 5) tangential to the circle tangential The vertical component of the tension balances the weight. The tension weight horizontal component of tension horizontal provides the centripetal force that centripetal points toward the center of the circle. W W T T ConcepTest 5.2a Around the Curve I Around
You are a passenger in a car, not wearing a seat belt. The car makes a sharp left turn. From your perspective in the car, what do you feel is happening to you? (1) you are thrown to the right (2) you feel no particular change (3) you are thrown to the left (4) you are thrown to the ceiling (5) you are thrown to the floor ConcepTest 5.2a Around the Curve I Around
You are a passenger in a car, not wearing a seat belt. The car makes a sharp left turn. From your perspective in the car, what do you feel is happening to you? (1) you are thrown to the right (2) you feel no particular change (3) you are thrown to the left (4) you are thrown to the ceiling (5) you are thrown to the floor The passenger has the tendency to continue moving in a straight line. From your perspective in the car, it feels like you are being thrown to the right, hitting the passenger door. ConcepTest 5.2b Around the Curve II Around
During that sharp left turn, you found yourself hitting the passenger door. What is the correct description of what is actually happening? (1) centrifugal force is pushing you into the door (2) the door is exerting a leftward force on you (3) both of the above (4) neither of the above ConcepTest 5.2b Around the Curve II Around
During that sharp left turn, you found yourself hitting the passenger door. What is the correct description of what is actually happening? (1) centrifugal force is pushing you into the door (2) the door is exerting a leftward force on you (3) both of the above (4) neither of the above The passenger has the tendency to continue moving in a straight line. There is a centripetal force, provided by the door, that forces the passenger into a circular path. ConcepTest 5.2c Around the Curve III Around
You drive your dad’s car You too fast around a curve and the car starts to skid. What is the correct description of this situation? situation? (1) car’s engine is not strong enough to keep the car from being pushed out keep (2) friction between tires and road is not (2) strong enough to keep car in a circle strong (3) car is too heavy to make the turn (4) a deer caused you to skid (5) none of the above ConcepTest 5.2c Around the Curve III Around
You drive your dad’s car You too fast around a curve and the car starts to skid. What is the correct description of this situation? situation? (1) car’s engine is not strong enough to (1) keep the car from being pushed out keep (2) friction between tires and road is not (2) strong enough to keep car in a circle strong (3) car is too heavy to make the turn (4) a deer caused you to skid (5) none of the above The friction force between tires and road provides the centripetal force that keeps the car moving in a circle. If this force is too small, the car continues in a straight line! Followup: What could be done to the road or car to prevent skidding? the ConcepTest 5.3 Missing Link ConcepTest Missing
A ping pong ball is shot into a circular tube that is lying flat (horizontal) on a tabletop. When the ping pong ball leaves the track, which path will it follow? ConcepTest 5.3 Missing Link ConcepTest Missing
A ping pong ball is shot into a circular tube that is lying flat (horizontal) on a tabletop. When the ping pong ball leaves the track, which path will it follow? q Once the ball leaves the tube, there is no longer a force to keep it going in a circle. Therefore, it simply continues in a straight line, as Newton’s First Law requires! Followup: What physical force provides the centripetal force? ConcepTest 5.4 Ball and String ConcepTest Ball
Two equalmass rocks tied to strings are Two whirled in horizontal circles. The radius of radius circle 2 is twice that of circle 1. If the period twice period of motion is the same for both rocks, what same is the tension in cord 2 compared to cord 1? is 1) T2 = 1/4 T1 2) T2 = 1/2 T1 3) T2 = T1 4) T2 = 2 T1 5) T2 = 4 T1 ConcepTest 5.4 Ball and String ConcepTest Ball
Two equalmass rocks tied to strings are Two whirled in horizontal circles. The radius of radius circle 2 is twice that of circle 1. If the period twice period of motion is the same for both rocks, what same is the tension in cord 2 compared to cord 1? is 1) T2 = 1/4 T1 2) T2 = 1/2 T1 3) T2 = T1 4) T2 = 2 T1 5) T2 = 4 T1 The centripetal force in this case is given by the ConcepTest 5.5 Barrel of Fun ConcepTest Barrel
A rider in a “barrel of fun” rider finds herself stuck with her back to the wall. Which diagram correctly shows the forces acting on her? on 1 2 3 4 5 ConcepTest 5.5 Barrel of Fun Barrel
A rider in a “barrel of fun” rider finds herself stuck with her back to the wall. Which diagram correctly shows the forces acting on her? on 1 2 3 4 5 The normal force of the wall on the normal rider provides the centripetal force centripetal needed to keep her going around in a circle. The downward force of gravity is balanced by the upward frictional force on her, so she does frictional not slip vertically. Followup: What happens if the rotation of the ride slows down? ConcepTest 5.6a Going in Circles I ConcepTest Going
You’re on a Ferris wheel moving in a You’re vertical circle. When the Ferris wheel is at rest, the normal force N exerted by your seat is equal to your weight mg. mg How does N change at the top of the Ferris wheel when you are in motion? Ferris 1) N remains equal to mg 1) mg 2) N is smaller than mg 2) mg 3) N is larger than mg 3) mg 4) None of the above ConcepTest 5.6a Going in Circles I ConcepTest Going
You’re on a Ferris wheel moving in a You’re vertical circle. When the Ferris wheel is at rest, the normal force N exerted by your seat is equal to your weight mg. mg How does N change at the top of the Ferris wheel when you are in motion? Ferris You are in circular motion, so there has to be a centripetal force pointing inward. At the top, the only two inward forces are mg (down) and N (up), so mg N must be smaller than mg. mg Followup: Where is N larger than mg? Followup: mg 1) N remains equal to mg 1) mg 2) N is smaller than mg 2) mg 3) N is larger than mg 3) mg 4) None of the above ConcepTest 5.6b Going in Circles II Going
A skier goes over a small round hill skier with radius R. Since she is in circular Since motion, there has to be a centripetal force. At the top of the hill, what is force. Fc of the skier equal to? 1) Fc = N + mg 1) mg 2) Fc = mg – N 2) mg 3) Fc = T + N – mg 3) mg 4) Fc = N 4) 5) Fc = mg 5) mg v R ConcepTest 5.6b Going in Circles II Going
A skier goes over a small round hill skier with radius R. Since she is in circular motion, there has to be a centripetal force. At the top of the hill, what is force. Fc of the skier equal to? 1) Fc = N + mg 1) mg 2) Fc = mg – N 2) mg 3) Fc = T + N – mg 3) mg 4) Fc = N 4) 5) Fc = mg 5) mg v
mg N R
Followup: What happens when the skier goes into a small dip? the ConcepTest 5.7c Going in Circles III Going
You swing a ball at the end of string You in a vertical circle. Since the ball is in circular motion there has to be a centripetal force. At the top of the centripetal ball’s path, what is Fc equal to? 1) Fc = T – mg 1) mg 2) Fc = T + N – mg 2) mg 3) Fc = T + mg 3) mg 4) Fc = T 4) 5) Fc = mg mg v top R ConcepTest 5.7c Going in Circles III ConcepTest Going
You swing a ball at the end of string You in a vertical circle. Since the ball is in circular motion there has to be a centripetal force. At the top of the centripetal ball’s path, what is Fc equal to? 1) Fc = T – mg 1) mg 2) Fc = T + N – mg 2) mg 3) Fc = T + mg 3) mg 4) Fc = T 4) 5) Fc = mg mg mg
F points toward the center of the circle, i.e. downward in this case. The i.e weight vector points down and the weight down tension (exerted by cthe string) also tension points down. The magnitude of the down v T R ConcepTest 2.1
You and your dog go for a walk to the Walking the Dog park. On the way, your dog takes many side trips to chase squirrels or examine fire hydrants. When you arrive at the park, do you and your dog have the same displacement? displacement? 1) yes 2) no ConcepTest 2.1 ConcepTest
You and your dog go for a walk to the Walking the Dog park. On the way, your dog takes many side trips to chase squirrels or examine fire hydrants. When you arrive at the park, do you and your dog have the same displacement? displacement? 1) yes 2) no Yes, you have the same displacement. Since you and your dog had the same initial position and the same final position, then you have (by definition) the same displacement. Followup: Have you and your dog traveled the same distance? ConcepTest 2.3
If the position of a car is zero, does its speed have to be zero? to Position and Speed
1) yes 2) no 3) it depends on the position ConcepTest 2.3 ConcepTest
If the position of a car is zero, does its speed have to be zero? to Position and Speed
1) yes 2) no 3) it depends on the position No, the speed does not depend on position, it depends on the change of position. Since we know that the displacement does not depend on the origin of the coordinate system, an object can easily start at x = –3 and be moving by the time it gets to x = 0. ConcepTest 2.4
Does the odometer in a car measure distance or displacement? displacement? Odometer
1) distance 2) displacement 3) both ConcepTest 2.4 ConcepTest
Does the odometer in a car measure distance or displacement? displacement? Odometer
1) distance 2) displacement 3) both If you go on a long trip and then return home, your odometer does not measure zero, but it records the total miles that you traveled. That means the odometer records distance. Followup: How would you measure displacement in your car? ConcepTest 3.3 Vector Addition
You are adding vectors of length 20 and 40 units. What is the only possible resultant magnitude that you can obtain out of the following choices? 1) 0 2) 18 3) 37 4) 64 5) 100 ConcepTest 3.3
You are adding vectors of length 20 and 40 units. What is the only possible resultant magnitude that you can obtain out of the following choices? Vector Addition
1) 0 2) 18 3) 37 4) 64 5) 100 The minimum resultant occurs when the vectors minimum are opposite, giving 20 units. The maximum opposite 20 maximum resultant occurs when the vectors are aligned, aligned giving 60 units. Anything in between is also 60 possible, for angles between 0° and 180°. ConcepTest 3.4a Firing Balls I
A small cart is rolling at constant velocity on a flat track. It fires a ball straight up into the air as it moves. After it is fired, what happens to the ball? 1) it depends on how fast the cart is 1) moving moving 2) it falls behind the cart 3) it falls in front of the cart 4) it falls right back into the cart 5) it remains at rest ConcepTest 3.4a Firing Balls I
A small cart is rolling at constant velocity on a flat track. It fires a ball straight up into the air as it moves. After it is fired, what happens to the ball? 1) it depends on how fast the cart is 1) moving moving 2) it falls behind the cart 3) it falls in front of the cart 4) it falls right back into the cart 5) it remains at rest In the frame of reference of the cart, the ball only has a vertical component of vertical velocity. So it goes up and comes back down. To a ground observer, both the cart and the ball have the same horizontal velocity, same so the ball still returns into the cart. when viewed from train when viewed from ground ConcepTest 3.4b ConcepTest
Now the cart is being pulled along a horizontal track by an external force (a weight hanging over the table edge) and accelerating. It fires a ball straight out of the cannon as it moves. After it is fired, what happens to the ball? Firing Balls II Firing 1) it depends upon how much the track is tilted 2) it falls behind the cart 3) it falls in front of the cart 4) it falls right back into the cart 5) it remains at rest ConcepTest 3.4b ConcepTest
Now the cart is being pulled along a horizontal track by an external force (a weight hanging over the table edge) and accelerating. It fires a ball straight out of the cannon as it moves. After it is fired, what happens to the ball? Firing Balls II Firing 1) it depends upon how much the track is tilted 2) it falls behind the cart 3) it falls in front of the cart 4) it falls right back into the cart 5) it remains at rest Now the acceleration of the cart is completely unrelated to the ball. In fact, the ball does not have any horizontal acceleration at all (just like the first question), so it will lag behind the accelerating cart once it is shot out of the cannon. ConcepTest 4.1a Newton’s First Law I
A book is lying at rest on a table. The book will remain there at rest because: 1) there is a net force but the book has too much inertia 2) there are no forces acting on it at all 3) it does move, but too slowly to be seen 4) there is no net force on the book 5) there is a net force, but the book is too heavy to move ConcepTest 4.1a Newton’s First Law I
A book is lying at rest on a table. The book will remain there at rest because: 1) there is a net force but the book has too much inertia 2) there are no forces acting on it at all 3) it does move, but too slowly to be seen 4) there is no net force on the book 5) there is a net force, but the book is too heavy to move There are forces acting on the book, but the only There forces acting are in the ydirection. Gravity acts downward, but the table exerts an upward force that is equally strong, so the two forces cancel, leaving no net force. leaving ConcepTest 4.1b Newton’s First Law II
A hockey puck slides on ice at constant velocity. What is the net force acting on the puck? 1) more than its weight 2) equal to its weight 3) less than its weight but more than zero 4) depends on the speed of the puck 5) zero ConcepTest 4.1b Newton’s First Law II
A hockey puck slides on ice at constant velocity. What is the net force acting on the puck? 1) more than its weight 2) equal to its weight 3) less than its weight but more than zero 4) depends on the speed of the puck 5) zero The puck is moving at a constant velocity, and constant therefore it is not accelerating. Thus, there must not be no net force acting on the puck. no Followup: Are there any forces acting on the puck? What are they? Followup: ConcepTest 4.1c Newton’s First Law III
You put your book on the bus seat next to you. When the bus stops suddenly, the book slides forward off the seat. Why? 1) a net force acted on it 2) no net force acted on it 3) it remained at rest 4) it did not move, but only seemed to 5) gravity briefly stopped acting on it ConcepTest 4.1c Newton’s First Law III
You put your book on the bus seat next to you. When the bus stops suddenly, the book slides forward off the seat. Why? 1) a net force acted on it 2) no net force acted on it 3) it remained at rest 4) it did not move, but only seemed to 5) gravity briefly stopped acting on it The book was initially moving forward (since it was on a moving bus). When the bus stopped, the book continued moving forward, which was its initial state continued of motion, and therefore it slid forward off the seat. of Followup: What is the force that usually keeps the book on the seat? Followup: ConcepTest 6.1 To Work or Not to Work To
Is it possible to do work on an object that remains at rest? 1) yes 2) no ConcepTest 6.1 To Work or Not to Work ConcepTest To
Is it possible to do work on an object that remains at rest? 1) yes 2) no Work requires that a force acts over a distance. force If an object does not move at all, there is no displacement, and therefore no work done. displacement no ConcepTest 6.2a Friction and Work I ConcepTest Friction
A box is being pulled box across a rough floor at a constant speed. What can you say about the work done by friction? by 1) friction does no work at all 2) friction does negative work 3) friction does positive work ConcepTest 6.2a Friction and Work I Friction
A box is being pulled box across a rough floor at a constant speed. What can you say about the work done by friction? by 1) friction does no work at all 2) friction does negative work 3) friction does positive work Friction acts in the opposite opposite direction to the displacement, so the work is negative. Or using the negative definition of work: W = F d cos f N displacement Pull mg
o ConcepTest 6.2c Play Ball! Play
In a baseball game, the catcher stops a 90mph pitch. What can you say about the work done by the catcher on the ball? 1) catcher has done positive work 2) catcher has done negative work 3) catcher has done zero work ConcepTest 6.2c Play Ball! Play
In a baseball game, the catcher stops a 90mph pitch. What can you say about the work done by the catcher on the ball? 1) catcher has done positive work 2) catcher has done negative work 3) catcher has done zero work The force exerted by the catcher is opposite in direction to the opposite displacement of the ball, so the work is negative. Or using the displacement definition of work (W = F d cos θ ), since θ = 180o, then W < 0. 180 Note that because the work done on the ball is negative, its speed decreases. Followup: What about the work done by the ball on the catcher? ConcepTest 7.3a Momentum and Force ConcepTest Momentum
A net force of 200 N acts on a 100kg boulder, and a force of the same magnitude acts on a 130g pebble. How does the rate of change of the boulder’s momentum compare to the rate of change of the pebble’s momentum? 1) greater than 2) less than 3) equal to ConcepTest 7.3a Momentum and Force Momentum
A net force of 200 N acts on a 100kg boulder, and a force of the same magnitude acts on a 130g pebble. How does the rate of change of the boulder’s momentum compare to the rate of change of the pebble’s momentum? 1) greater than 2) less than 3) equal to The rate of change of momentum is, in fact, the force. Remember that F = ∆ p/∆ t. Since the force exerted on the boulder and the pebble is the same, then the rate of change of momentum is the same. ConcepTest 7.3b Velocity and Force ConcepTest Velocity
A net force of 200 N acts on a 100kg boulder, and a force of the same magnitude acts on a 130g pebble. How does the rate of change of the boulder’s velocity compare to the rate of change of the pebble’s velocity? 1) greater than 2) less than 3) equal to ConcepTest 7.3b Velocity and Force Velocity
A net force of 200 N acts on a 100 kg boulder, and a force of the same magnitude acts on a 130g pebble. How does the rate of change of the boulder’s velocity compare to the rate of change of the pebble’s velocity? 1) greater than 2) less than 3) equal to The rate of change of velocity is the acceleration. Remember that a = ∆ v/∆ t. The acceleration is related to the force by Newton’s 2nd Law (F = ma), so the acceleration of the boulder is less than that of the pebble (for the same applied force) because the boulder is much more massive. ConcepTest 7.4 Collision Course ConcepTest Collision
A small car and a large truck collide headon and stick together. Which one has the larger momentum change? 1) the ...
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 Fall '09
 ORDONEZ
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