Kinematics/Newton's Laws - The Burns Home Page

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9/3/2009 Understanding Understanding Hint: How long would it take a car, starting from rest and accelerating uniformly in a straight line at 5 m/s 2 , to cover a distance of 200m? 1 s v0t at 2 (a) 9.0 s (b) 10.5 s (c) 12.0 s (d) 15.5 s (e) 20.0 s 2 m 1 m 200m 0 t 5 t 2 s 2 s 2 400m t m 5 2 s t 8.9s 2 Hint: A rock is dropped off a cliff and strikes the ground with an impact of 30 m/s. How high was the cliff? We need both the initial and final velocity in the equation (a) 15.0 m (b) 20 m (c) 30 m (d) 46 m (e) 60 m v v 2a s 2 2 f i v v 2a s 2 2 f i v v s 2a 2 2 f i 2 2 m m 30 0 s s m 29.8 2 s 45.9m Understanding Understanding A stone is thrown horizontally with an initial speed of 10 m/s from a bridge. If air resistance could be ignored, how long would it take the stone to strike the water 80 m below the bridge? Hint: We need only think about the vertical components. (a) 1 s (b) 2 s (c) 4 s (d) 6 s (e) 8 s 1 s v0t at 2 m 1 m 80m 0 t 9.8 t 2 s 2 s 2 160m t m 9.8 2 s t 4.0s 2 2 A soccer ball, at rest on the ground, is kicked with an initial velocity of 10 m/s at a launch angle of 30 0 . Calculate its total flight time, assuming air resistance is negligible? Hint: We need only think about the vertical time to travel up to its max height, then double it. (a) 0.5 s (b) 1 s (c) 1.7 s (d) 2 s (e) 4 s v v at f i vf vi t a m m 0 10 sin 30 s s m 9.8 2 s 0.51s 2
9/3/2009 Understanding A stone is thrown horizontally with an initial speed of 30 m/s from a bridge. Find the stone’s total speed when it enters the water 4 seconds later? Hint: We need only think about the vertical time to travel to find vertical speed, then combine with horizontal speed (a) 30 m/ s (b) 40 m/s (c) 50 m/s (d) 60 m/s (e) 70 m/s v v at f i m m vyf 0 9.8 2 4s s s m 39.2 s v 2 2 f vx vy m vxf 30 2 2 s m m 39.2 30 2 2 s s m 49.4 2 s Understanding Which of the following statements is true concerning the motion of an ideal projectile launched at an angle of 45 0 to the horizontal? (a) The acceleration vector points opposite to the velocity vector on the way up and in the same direction as the velocity vector on the way down. (b) The speed at the top of the trajectory is zero. (c) The object’s total speed remains constant during the entire flight. (d) The horizontal speed decreases on the way down. (e) The vertical speed decreases on the way up and increases on the way down. Understanding This question concerns the motion of a car on a straight track; the car’s velocity as a function of time is plotted below 20 velocity (m/s) Understanding This question concerns the motion of a car on a straight track; the car’s velocity as a function of time is plotted below 20 velocity (m/s) 10 10 time (s) 1 2 3 4 5 6 7 time (s) 1 2 3 4 5 6 7 −10 −10 −20 (a) Describe what happened to the car at time t=1 s (b) How does the car’s average velocity between time t=0 sand t=1 s compare to its average velocity between times t=1 s and t=5 s? (c) What is the displacement of the car from time t=0 to time t=7s? (d) Plot the car’s acceleration during this interval as a function of time. (e) Plot the object’s position during this interval as a function of time. Assuming that the car begins at s=0. −20 (a) Describe what happened to the car at time t=1 s The car’s velocity remains at 20 m/s, but the acceleration changes from positive to negative at this time (the foot leaves the accelerator) 3
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