Friction Can Be Rough

Transcription

1 8.1 Observe and Find a Pattern Friction Can Be Rough Perform the following experiment: Rest a brick on a rough surface. Tie a string around the brick and attach a large spring scale to it. Pull the scale slowly, pulling harder and harder. Notice what happens to the scale reading while the brick does not move. Notice the reading right before the brick starts moving. a) Explain a time when you have experience this phenomenon before. b) Draw a force diagram for the brick (the system) for the following situations. 1. The brick is at rest sits on the surface with no scale pulling it. 2. The spring scale pulls lightly on the brick; the brick does not move. 3. The spring scale pulls harder on the brick; the brick still does not move. 4. The spring scale pulls even harder on the brick; the brick still does not move (but is just about to start moving). c) Summarize the pattern you found. d) Describe in words how the magnitude of each force changed as the spring scale exerted an increasing force on the block. What must have been done in order for the brick to remain stationary? 8.2 Observe and Find a Pattern Examine the data in the table that follows. Mass of the block Surface area Quality of Maximum surfaces static friction force 1 kg 0.1 m 2 Medium smooth 3.1 N 1 kg 0.2 m 2 Medium smooth 3.0 N 1 kg 0.3 m 2 Medium smooth 3.1 N 1 kg 0.1 m 2 A little rougher 4.2 N 1 kg 0.1 m 2 Even rougher 5.1 N 1 kg 0.1 m 2 Roughest 7.0 N a) Now decide how the maximum static friction force that the surface exerts on the block depends on the surface area of the block and on the roughness of the two surfaces.

2 You have rectangular blocks with different surface areas and different types of surfaces on which the block slides horizontally. A B C The force that the string exerts on the block (as measured by the spring scale reading) when the block just starts to slide is recorded in the table that follows. This force is equal in magnitude to the maximum static friction force (as we discovered in the previous activity). b) Consider scenarios A and B. Which scenario, if any, do you think will give a larger spring scale reading? a. A b. B c. They will give the same reading c) Consider scenarios A and C (the surface in C is rougher than the surface in A). Which scenario, if any, do you think will give a larger spring scale reading? b. A b. C c. They will give the same reading 8.3 Observe and Find a Pattern Take a textbook and drag it with your pinky finger. Repeat but this time, have your neighbor push down lightly on the book. Repeat 3 more times with your neighbor pushing down successively harder. a) Draw a force diagram for each case. b) What can you say about the maximum static friction force as you push down harder and harder?

3 8.4 Observe and Find a Pattern A spring scale pulls a 1 kg block over a medium smooth surface. The reading of the scale can be used to determine the magnitude of the maximum static friction force in this instance, the force when the block starts to slide. In some experiments, a compressible spring also pushes vertically down on the block (see the second block). Spring pushes down 1 kg 1 kg Mass of the block Extra downward force exerted on the 1-kg block Normal force exerted by the board on the block 1 kg 0 N 10 N 3 N 1 kg 5 N 15 N 4.5 N 1 kg 10 N 20 N 6 N 1 kg 20 N 30 N 9 N Maximum static friction force a) Use the data in the table to find the relationship between the maximum static friction force and the normal force the surface exerts on the block. b) Express mathematically a relationship between the normal force and the maximum static friction force. Write this relationship as an equation. 8.5 Reason a) Consider the previous activity. Why would we consider the normal force exerted on the object rather than the force of the Earth exerted on the object? b) A person is holding a book against a vertical wall, pushing on it horizontally. The book is at rest. Draw a force diagram for the book. Check if all forces balance. c) Which force prevents the book from falling down? d) Why, if you do not push on the book hard enough, does the book start falling? Draw a force diagram to support your reasoning.

4 8.6 Observe Repeat the previous friction experiments in 8.1 but instead pull the brick so that it moves at a constant velocity. Notice the reading right before the brick starts moving and right after, keeping it moving but not accelerating. a) Explain a time when you have experienced this phenomenon before. b) Draw a force diagram for the brick (the system) for the following situations. 1. The spring pulls on the brick, and the shoe is just about to start moving but hasn t moved yet. 2. The spring pulls the brick at a slow, constant velocity. c) Summarize the pattern you found. Compare the magnitude of the resistive force just before the shoe starts moving to the magnitude when it is moving at a constant velocity. What do you observe? Did You Know? The kinetic friction force is the resistive force exerted on a moving object. Did You Know? Normal force: When two objects touch each other, they exert a normal force on each other. The force of the one object on the other object points perpendicular to the surface of contact. Often one symbol N is used to denote this force (do not confuse with the Newton, N). There is no equation for calculating the normal force. Its magnitude must be determined for each situation by some other method. Static friction force: When two objects touch each other, they exert a friction force on each other. The friction force of the one object on the other object points parallel to the surfaces of contact. If the objects are not moving with respect to each other, the friction force that they exert on each other is static. The static friction force between two surfaces opposes the tendency of one surface to move across the other and provides flexible resistance (as much as is needed) to prevent motion up to some maximum value. This maximum static friction force depends on the relative roughness of the surfaces (on the coefficient of static friction µ s between the surfaces) and on the magnitude of the normal force N between the surfaces. The magnitude of the static friction force is always less than or equal to the product of these two quantities: f s max = µ s N 0 f s µ s N Kinetic friction force: The kinetic friction force between two surfaces is exerted parallel to the surfaces and opposes the motion of one surface relative to the other surface. The kinetic friction force depends on the relative roughness of the surfaces (on the coefficient of kinetic friction µ k ) and on the magnitude of the normal force N between the surfaces: f k = µ k N

5 8.7 Represent and Reason Some students are trying to move a heavy desk across the room. Diana pushes it across the floor at the same time that Omar and Jeff pull on it. Omar pulls on the desk, exerting a (-150) N force, and Jeff pulls exerting a (-125) N force. There is also a (-200) N friction force exerted by the floor on the desk. The sum of the forces exerted on the desk is 27 N. a) Make a sketch of the situation. b) Draw a force diagram for the desk. Draw a motion diagram. c) Write an algebraic statement that describes the force diagram you drew. d) How hard is Diana pushing? e) Is the desk moving with a constant velocity or is it speeding up? How do you know? f) What would happen if, after a few seconds, the boys stopped pulling? 8.8 Represent and Reason A 50 kg box rests on the floor. The coefficients of static and kinetic friction between the bottom of the box and the floor are 0.70 and 0.50, respectively. a) What is the minimum force a person needs to exert on it to start the box sliding? b) After the box starts sliding, the person continues to push it exerting the same force. What is the acceleration of the box? 8.9 Represent and Reason The Ford P2000 fuel cell car has a mass of 1520 kg. While it is traveling at 20 m/s, the driver applies the brakes to stop the car on a wet surface with a 0.40 coefficient of friction. a) How far does the car travel before stopping? b) If a different car with the mass 1.5 times as much as the mass of the Ford P2000 is on the road traveling at the same speed and the coefficient of friction between the road and the tires is the same, what will its stopping distance be? Does the answer make sense to you? 8.10 Represent and Reason A wooden pallet carrying a load of 600 kg rests on a wooden floor. The friction coefficients for static and kinetic friction are 0.28 and 0.17 respectively. a) A forklift driver decides to push it without lifting it. What force must be applied to just get the pallet moving? b) After a bit of tie, the pallet begins to slide. How fast is the pallet moving after 0.5 seconds of sliding under the same force you calculated in part a)? c) If the forklift stops pushing, how far does the pallet slide before coming to a stop? 8.11 Represent and Reason A 165 g hockey puck is sliding on the ice with an initial velocity of 10 m/s. The kinetic coefficient of friction between the ice and the puck is After 15 meters the puck exits the ice rink and starts to slide on a linoleum tile floor with a kinetic friction coefficient of How far does the puck travel on the linoleum floor before coming to a rest?