ConcepTest 6.4Friction ConcepTest 6.4 Friction A) the force from the rushing air pushed it off B) the force of friction pushed it off C) no net force acted on the box D) truck went into reverse by accident E) none of the above A box sits in a pickup truck on a frictionless truck bed. When the truck accelerates forward, the box slides off the back of the truck because:

friction If there is no friction, there is no force to push the box along, and it remains at rest. Generally, the reason that the box in the truck bed would move with the truck is due to friction between the box and the bed. If there is no friction, there is no force to push the box along, and it remains at rest. The truck accelerated away, essentially leaving the box behind!! ConcepTest 6.4Friction ConcepTest 6.4 Friction A box sits in a pickup truck on a frictionless truck bed. When the truck accelerates forward, the box slides off the back of the truck because: A) the force from the rushing air pushed it off B) the force of friction pushed it off C) no net force acted on the box D) truck went into reverse by accident E) none of the above

Antilock brakes keep the car wheels from locking and skidding during a sudden stop. Why does this help slow the car down? A)  k >  s so sliding friction is better B)  k >  s so static friction is better C)  s >  k so sliding friction is better D)  s >  k so static friction is better E) none of the above ConcepTest 6.5Antilock Brakes ConcepTest 6.5 Antilock Brakes

Antilock brakes keep the car wheels from locking and skidding during a sudden stop. Why does this help slow the car down? Static friction is greater than sliding friction Static friction is greater than sliding friction, so by keeping the wheels from skidding, the static friction force will help slow the car down more efficiently than the sliding friction that occurs during a skid. ConcepTest 6.5Antilock Brakes ConcepTest 6.5 Antilock Brakes A)  k >  s so sliding friction is better B)  k >  s so static friction is better C)  s >  k so sliding friction is better D)  s >  k so static friction is better E) none of the above

ConcepTest 6.7Will it Budge? ConcepTest 6.7 Will it Budge? A) moves to the left B) moves to the right C) moves up D) moves down E) the box does not move A box of weight 100 N is at rest on a floor where  s = 0.5. A rope is attached to the box and pulled horizontally with tension T = 30 N. Which way does the box move? T m Static friction (  s  = 0.4  )

maximum  s N = 40 N 30 N The static friction force has a maximum of  s N = 40 N. The tension in the rope is only 30 N. So the pulling force is not big enough to overcome friction. ConcepTest 6.7Will it Budge? ConcepTest 6.7 Will it Budge? A box of weight 100 N is at rest on a floor where  s = 0.5. A rope is attached to the box and pulled horizontally with tension T = 30 N. Which way does the box move? T m Static friction (  s  = 0.4  ) Follow-up: What happens if the tension is 35 N? What about 45 N? A) moves to the left B) moves to the right C) moves up D) moves down E) the box does not move

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? (A) You are thrown to the right (B) You feel no particular change (C) You are thrown to the left (D) You are thrown to the ceiling (E) You are thrown to the floor ConcepTest 6.10aAround the Curve I ConcepTest 6.10a Around the Curve I

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? ConcepTest 6.10aAround the Curve I ConcepTest 6.10a Around the Curve I 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. (A) You are thrown to the right (B) You feel no particular change (C) You are thrown to the left (D) You are thrown to the ceiling (E) You are thrown to the floor

(A) centrifugal force is pushing you into the door (B) the door is exerting a leftward force on you (C) both of the above (D) neither of the above During that sharp left turn, you found yourself hitting the passenger door. What is the correct description of what is actually happening? ConcepTest 6.10bAround the Curve II ConcepTest 6.10b Around the Curve II

During that sharp left turn, you found yourself hitting the passenger door. What is the correct description of what is actually happening? 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 6.10bAround the Curve II ConcepTest 6.10b Around the Curve II (A) centrifugal force is pushing you into the door (B) the door is exerting a leftward force on you (C) both of the above (D) neither of the above

(A) car’s engine is not strong enough to keep the car from being pushed out (B) friction between tires and road is not strong enough to keep car in a circle (C) car is too heavy to make the turn (D) a deer caused you to skid (E) none of the above You drive your dad’s car too fast around a curve and the car starts to skid. What is the correct description of this situation? ConcepTest 6.10cAround the Curve III ConcepTest 6.10c Around the Curve III

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! You drive your dad’s car too fast around a curve and the car starts to skid. What is the correct description of this situation? ConcepTest 6.10cAround the Curve III ConcepTest 6.10c Around the Curve III Follow-up: What could be done to the road or car to prevent skidding? (A) car’s engine is not strong enough to keep the car from being pushed out (B) friction between tires and road is not strong enough to keep car in a circle (C) car is too heavy to make the turn (D) a deer caused you to skid (E) none of the above

ConcepTest 6.11Missing Link ConcepTest 6.11 Missing Link 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 6.11Missing Link ConcepTest 6.11 Missing Link l 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! 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? Follow-up: What physical force provides the centripetal force?

ConcepTest 6.13Barrel of Fun ConcepTest 6.13 Barrel of Fun A rider in a “barrel of fun” finds herself stuck with her back to the wall. Which diagram correctly shows the forces acting on her? A B C D E

normal force centripetal force downward force of gravity is balanced by the upward frictional force The normal force of the wall on the rider provides the centripetal force 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 not slip vertically. ConcepTest 6.13Barrel of Fun ConcepTest 6.13 Barrel of Fun A rider in a “barrel of fun” finds herself stuck with her back to the wall. Which diagram correctly shows the forces acting on her? A B C D E Follow-up: What happens if the rotation of the ride slows down?