9/20/11. Physics 101 Tuesday 9/20/11 Class 8" Chapter " Weight and Normal forces" Frictional Forces"

Transcription

1 Reading Quiz Physics 101 Tuesday 9/20/11 Class 8" Chapter " Weight and Normal forces" Frictional Forces" The force due to kinetic friction is usually larger than the force due to static friction. A) True B) False C) False because they are always equal. Weight! Consider a person standing in an elevator that is moving upward with constant velocity. The upward normal force N exerted by the elevator floor on the person is A: larger than B: identical to C: smaller than the downward weight W of the person. Weight! Consider a person standing in an elevator that is accelerating upward. The upward normal force N exerted by the elevator floor on the person is A: larger than B: identical to C: smaller than the downward weight W of the person. 1

2 Weight! Weight! In the demonstration that we had earlier in the semester, in the vacuum, the acceleration of a feather and coin were A) the same B) the acceleration of the feather was greater than the coin C) the acceleration of the coin was greater than the feather D) I don t remember the experiment In the demonstration that we had earlier in the semester, in the vacuum, the gravitational force acting on the feather and coin were A) the same B) the gravitational force acting on the feather was greater than the coin C) the gravitational force acting on the coin was greater than the feather D) I don t remember the experiment Weight! I weigh 850 N. I am standing on a scale in an elevator and see that the scale reads 850 N. From this observation I know for certain: A) the elevator is moving upwards at a constant velocity. B) the elevator is moving downwards at a constant velocity C) the elevator is sitting still D) the elevator is not accelerating. E) the elevator is accelerating The normal force on an object A) is always normal to the surface of the earth B) is perpendicular to the surface supporting the object C) is normally equal to g D) causes a large acceleration of the object University of Colorado,

3 A mass m is pulled along a frictionless table by a constant force external force F ext at some angle above the horizontal. Fext N F ext A mass m accelerates downward along a frictionless inclined plane. The magnitudes of the forces on the free-body diagram have not been drawn carefully, but the directions of the forces are correct. N a Which statement below must be true? A) N < mg B) N > mg C) N=mg mg Which statement below must be true? A) N < mg B) N > mg C) N = mg mg For the previous problem, a student chooses a tilted coordinate system as shown, and then proceeds to write down Newton's 2 nd Law in the form! F x = m a, F m a x! y = y. What is the correct equation for the y- direction! Fy = m a y?! mg N A) N! mgsin " = m a B) N! mg cos " = m a C) mgsin! = m a D) N! mg cos " = 0 E) N + mg = m a! y a x I want to move a book across the table. I can either push it (case 1) or pull it (case 2)[see figure below]. In which case is the normal force greater? Case 1 A) Case 1 B) Case 2 C) it is the same D) it depends on the magnitude of F E) depends on the surface Case 2 3

4 A 20 kg suitcase is pulled with a constant speed by the handle at an angle of 30 deg above the horizontal. If the normal force on the suitcase is 150 N, what is the Force applied to the handle? A) 46/cos(30 deg) N B) 46/sin(30 deg) N C) 46 cos(30 deg) N D) 46 sin(30 deg) N There are two identical blocks sitting on surfaces. One surface is tilted, the other is horizontal. The normal force is: A) the same B) greater for the block on the horizontal surface C) greater for the block on the tilted surface Inertial Mass! An astronaut on earth kicks a bowling ball and hurts his foot. A year later, the same astronaut kicks a bowling ball on the Moon with the same force. His foot hurts: A) more B) less C) the same Anti lock brakes keep car wheels from locking and skidding during a sudden stop. Why does this help slow the car down? A) µ k > µ s, so sliding friction is better B) µ k < µ s, so sliding friction is better C) µ k > µ s, so static friction is better D) µ k < µ s, so static friction is better 4

5 Your little sister wants you to give her a ride on her sled. On level ground, what is the easiest way to do this? A) push her from behind B) pull her from the front C) both are the same D) tell her to get out and walk 2 1 A box sits on a flat board. You lift one end of the board, making an angle with the floor. As you increase the angle, the box will begin to slide down. Why? A) component of gravity force parallel to the board increased B) Coefficient of static friction decreased C) the normal force exerted by the board decreased Normal D) A and C Net Force E) A, B, and C Weight Worked problem #1! A child accelerates down a slide at 1 m/s 2. The slide is at an angle of 35 deg to the horizontal. The coefficient of friction between the child and the slide is? Worked problem #2! I jump into my Miata and floor it to accelerate at 12 m/s 2 without spinning the tires. What is the minimum coefficient of friction between the tires and the road needed to make this happen. Why do I say minimum? 5

6 Skinny and Fatty are having a tug-of-war. So far, no one is winning. Skinny and Fatty are having a tug-of-war. So far, no one is winning. Q1: What is the direction of the force of friction on Skinny's feet! F S? A: Right! B: Left! Q2: How large is the force of friction on Skinny's feet! F S compared to the force of friction F! F on Fatty's feet?!!!!!! A: F > F B: FS = FF C: FS < FF S F Reading Assignment! Thursday 9/ Tuesday 9/

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