Recit%20%204%2c%20Ch%203-4(1)

# Recit%20%204%2c%20Ch%203-4(1) - PHY2053 RECITATIONS SPRING...

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PHY2053 RECITATIONS, SPRING 2011 Recitation 4 (Chapter 3, 4) Feb 7 - 11 Ch. 3 # 42. In the javelin throw at a track-and-field event, the javelin is launched at a speed of 29 m/s at an angle of 36° above the horizontal. As the javelin travels upward, its velocity points above the horizontal at an angle that decreases as time passes. How much time is required for the angle to be reduced from 36° at launch to 18°? 42. REASONING As shown in the drawing, the angle that the velocity vector makes with the horizontal is given by tan y x v v θ = where, from Equation 3.3b, 0 0 0 sin y y y y v v a t v a t = + = + x y v y v x v and, from Equation 3.3a (since a x = 0 m/s 2 ), 0 0 0 cos x x v v v = = Therefore, 0 0 0 0 sin tan cos y y x v v a t v v + = = SOLUTION Solving for t , we find ( 29 ( 29 ( 29 0 0 0 2 cos tan – sin 29 m/s cos 36 tan 18 – sin 36 0.96 s –9.80 m/s y v t a ° ° ° = = = ____________________________________________________________________________________________ Ch. 3 # 78. Two boats are heading away from shore. Boat 1 heads due north at a speed of 3.00 m/s relative to the shore. Relative to boat 1, boat 2 is moving 30.0° north of east at a speed of 1.60 m/s. A passenger on boat 2 walks due east across the deck at a speed of 1.20 m/s relative to boat 2. What is the speed of the passenger relative to the shore? 78.

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Recit%20%204%2c%20Ch%203-4(1) - PHY2053 RECITATIONS SPRING...

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