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Unformatted text preview: Physics 101 Classical Physics Spring 2010 Homework 6 Solutions DUE IN CLASS ON March 4 1. A 0.4 kg softball is pitched at a speed of 10 m/s. The batter hits it back directly at the pitcher at a speed of 20 m/s. The bat acts on the ball for 0.01 s. What is the magnitude of the average force exerted by the bat on the ball during contact? ( a )1200 N , (b) 400 N, (c) 40 N, (d) 80 N, (e) 120 N F = dp dt F = p t = (0 . 4 kg)(20 m/s- (- 10 m/s)) . 01 s = 1200 N 2. A 2 kg object is acted upon by a single force in the x-direction in a manner described by the graph below. The momentum acquired by the object is: (a) 16 N s , (b) 30 N s, (c) 32 N s, (a) 40 N s (a) 48 N s 6 4 2 2 4 8 Time (s) Force (N) F = dp dt p = Z Fdt This integral is just the area under the force as a function of time which is just the area of the triangle, so p = (1 / 2)(8 N)(4 s) = 16 Ns. 3. A person of mass=75 kg is standing on a sled of mass 150 kg. The sled sits at rest on a very slick, icy pond. The person begins to walk along the sled with a velocity of v = +6 m/s with respect to the sled. What happens to the sled after the person starts walking? (a) the sled moves in the opposite direction with a velocity of -2 m/s (b) the sled moves in the same direction with a velocity of +4 m/s (c) the sled moves in the opposite direction with a velocity of -4 m/s (d) the sled moves in the same direction with a velocity of +3 m/s (e) the sled moves in the opposite direction with a velocity of -3 m/s Using conservation of momentum, 0 = m p v p + m s v s v s =- v p m p m s =- 3 m/s 1 4. A ball hits a hard surface and rebounds with the same speed, as shown below. Taking the + x-direction to the right and the + y-direction up, the changes in the components of the momentum of the ball are: (a) p x > , p y > (b) p x < , p y > (c) p x = 0 , p y > (d) p x = 0 , p y < (e) p x > , p y < There is no change in the momentum in the x-direction. The final momentum in the y- direction is positive, and the initial momentum in the y-direction is negative. Therefore, the change in momentum in the y-direction must be positive. 5. If the angular velocity of a spinning body were pointing downward out of this page, then viewed from above this page the body is spinning: (a) clockwise about an axis that is perpendicular to the page (b) counterclockwise about an axis that is perpendicular to the page (c) clockwise about an axis that is parallel to the page (d) about an axis that is 45 out of the page (e) counterclockwise about an axis that is parallel to the page Using the right-hand rule, if the angular velocity points into the page, then the motion is clockwise about an axis perpendicular to the page....
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