Chapter 5: Q17CQ (page 138)
Describe two examples in which the force of friction exerted on an object is in the direction of motion of the object.
Walking and driving a car are two classic examples.
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The manager of a department store is pushing horizontally with a force of magnitude 200 N on a box of shirts. The box is sliding across the horizontal floor with a forward acceleration. Nothing else touches the box. What must be true about the magnitude of the force of kinetic friction acting on the box (choose one)? (a) It is greater than 200 N. (b) It is less than 200 N. (c) It is equal to 200 N. (d) None of those statements is necessarily true.
The third graders are on one side of a schoolyard, and the fourth graders are on the other. They are throwing snowballs at each other. As a result, snowballs of various masses are moving with different velocities, as shown in Figure OQ5.3. Rank the snowballs (a) through (e) according to the magnitude of the total force exerted on each one. Ignore air resistance. If two snowballs rank together, make that fact clear.
Question: In Figure P5.92, the pulleys and the cord are light, all surfaces are frictionless, and the cord does not stretch. (a) How does the acceleration of block 1 compare with the acceleration of block 2 ? Explain your reasoning. (b) The mass of block 2 is 1.30 kg . Find its acceleration as it depends on the mass m1 of block 1. (c) What If? What does the result of part (b) predict if m1 is very much less than 1.30 kg ? (d) What does the result of part (b) predict if m1 approaches infinity? (e) In this last case, what is the tension in the cord? (f) Could you anticipate the answers to parts (c), (d), and (e) without first doing part (b)? Explain.
An experiment is performed on a puck on a level air hockey table,where friction is negligible . A constant horizontal force is applied to the puck ,and the puck’s acceleration is measured. Now the same puck is transported far into outer space, where both friction and gravity are negligible. The same constant force is applied to the puck is measured . What is the puck’s acceleration in outer space ?
A) It is somewhat greater than its acceleration on the earth.
B) It is the same as its acceleration on the earth
C) It is less then its acceleration on the earth
D) It is infinite because neither friction nor gravity constrains it.
E) It is very large because acceleration is inversely proportional to weight and the puck’s weight is very small but not zero .
As shown in figure CQ5.22, student A, a 55 kg girl, sits on one chair with metal runners, at rest on a classroom floor. Student B, an 80 kgboy, sits on an identical chair. Both students keep their feet off the floor. A rope runs from student A’s hands around a light pulley and then over her shoulder to the hands of a teacher standing on the floor behind her. The low-friction axle of the pulley is attached to a second rope held by student B, all ropes run parallel to the chair runners.
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