Resistance Exercises

Resistance Exercises 1. You need to push your lab table away from the wall. Why is it harder to start the table sliding than it is to keep the table sliding when it is moving?

7. The forces acting on four objects moving through a fluid are shown below. The force vectors are drawn to scale. Which object(s) moves at a constant velocity?

2. Suppose you drop an empty water bottle and an identical full water bottle from the same height. Explain which bottle will hit the ground first?

8. The volume flow rates and pressure drops for two pipes are graphed below. 3. When you push on the car brakes, friction between the wheels and the road create a force that pushes opposite the direction of the car. (a) Will the friction be greater when the tires continue to roll to a stop, or when the tires “lock” and start to skid?

(b) If more people were in the car, explain if that would increase or if it would decrease the maximum braking force.

____ (a) Which pipe has the highest resistance? ____ (b) If the pipes are the same length, which one has the larger diameter? ____ (c) If the pipes have the same diameter, which one is longer?

(c) If it starts raining, explain if that would increase or if it would decrease the maximum braking force.

9. The two water pipes shown below have the same diameter and length. The water flow rate through the pipes is the same, but the pressure drop is not the same. Explain what could cause the difference in pressure drop?

4. The drag on an object moving through a stationary fluid _________________ (increases or decreases) as the object’s speed increases. 5. If a fluid flows past a stationary object, the drag on the object _________________ (increases or decreases) as the fluid’s speed increases. 6. Fluid resistance increases as flow becomes ________________ (laminar or turbulent).

10. Electrical resistance in a wire is a measure of the opposition to the flow of ______________. 11. Are copper wires and metallic solder conductors, semiconductors, or insulators?

16. (a) What is the total current through the parallel circuit shown below? (b)What is the voltage drop across each resistor?

(c) What is the current through each resistor? 12. The resistance of a wire does not depend on which of the following? (a) wire material (b) wire mass (c) wire cross-sectional are (d) wire length 13. The resistivity of a wire depends on (a) wire material (b) wire mass (c) wire cross-sectional are (d) wire length 14. The graph below shows the relationship between current and voltage for two resistors. Which resistor has the greater resistance?

17. Thermal resistance is the opposition to the flow of _______________________. 18. Thermal resistance is the ratio of _______________ to ___________________. 19. If temperature is measured in ºC and heat flow is measured in cal/s, what are the units of thermal resistance?

15. (a) What is the current through each resistor in the series circuit shown below?

(b) What is the voltage drop across each resistor?

20. The thermal conductivity of a material depends on (a) thickness of the material (b) type of material (c) temperature difference across the material (d) surface area of the material 21. An architect designs an office building with large windows. Each window has length L, width W, and thickness Δx. How can the architect reduce the heat flow rate through each window? (a) us a material with a higher thermal conductivity (b) increase L and /or W (c) increase Δx (d) all of the above 22. A wall has three layers with resistances of R1, R2, and R3. How would you calculate the total resistance?