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Physics II workshop_Part_39

# Physics II workshop_Part_39 - 4 Multiple forces varying...

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80 4. Multiple forces, varying direction of motion. The situation is the same as in Problem 3, except that now we are going to account for friction. The coefficient of kinetic friction between the ice and the boat’s runners is μ k =0.03. (a) What is the work done by friction as the boat swings through 90°? (b) If the boat starts from rest, what is its speed at the end of this motion? Hint: the work done by the wind is the same as in Problem 3. 5. Using the dot product. A particle of dust of mass m = 2 × 10 -3 kg has an initial speed v = 2 m s -1 . An electric field inside the lab exerts a force r F = 12 ˆ i 6 ˆ j + 8 ˆ k (mN) on a small dust particle. The particle moves from initial position r r 1 = 1.2 ˆ i + 0.3 ˆ j + 0.6 ˆ k (m) to final position r r 2 = 3.5 ˆ i 1.2 ˆ j 0.1 ˆ k (m) (a) What is the work done on the dust particle by the electric force? (b) What is the work done on the dust particle by the gravitational force (“down” is direction ˆ j )?

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