Mastering Physics Solutions

# Mastering Physics Solutions - 8.1 Model The model rocket...

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8.1. Model: The model rocket and the target will be treated as particles. The kinematics equations in two dimensions apply. Visualize: Solve: For the rocket, Newton’s second law along the y -direction is () net R R 22 RR 11 15 N 0.8 kg 9.8 m/s 8.95 m/s 0.8 kg y F F mg ma aF m g m =− = ⎡⎤ ⇒= − = = ⎣⎦ Using the kinematic equation 2 1 1R 0R 0R 1R 0R R 1R 0R 2 , y yy v tt a =+ −+ ( ) 2 2 1 1R 2 30 m 0 m 0 m 8.95 m/s 0 s t =++ 1R 2.589 s t For the target 1T 1R (noting ), = 2 1 1T 0T 0T 1T 0T T 1T 0T 2 x x xvt t a t t ( )( ) 0 m 15 m/s 2.589 s 0 s 0 m 39 m −+= You should launch when the target is 39 m away. Assess: The rocket is to be fired when the target is at 0T . x For a net acceleration of approximately 2 9 m/s in the vertical direction and a time of 2.6 s to cover a vertical distance of 30 m, a horizontal distance of 39 m is reasonable.

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8.2. Model: The model rocket will be treated as a particle. Kinematic equations in two dimensions apply. Air resistance is neglected. Visualize: The horizontal velocity of the rocket is equal to the speed of the car, which is 3.0 m/s. Solve: For the rocket, Newton’s second law along the y -direction is: net R R () y F F mg ma =− = ( ) 2 1 8.0 N 0.5 kg 9.8 m/s 0.5 kg y a ⇒= 2 6.2 m/s = Thus using 2 1 10 01 0 1 0 2 , yy yy v tt a tt =+ −+ ( ) 2 2 1 1R 2 20 m 0 m 0 m 6.2 m/s 0 s t =++ ( ) 22 1 20 m 3.1 m/s t 1 2.54 s t Since 1 t is also the time for the rocket to move horizontally up to the hoop, 2 1 0 1 0 2 xx x xvt t a t t ( )( ) 0 m 3.0 m/s 2.54 s 0 s 0 m 7.6 m −+= Assess: In view of the rocket’s horizontal speed of 3.0 m/s and its vertical thrust of 8.0 N, the above-obtained value for the horizontal distance is reasonable.
8.3. Model: The asteroid and the giant rocket will be treated as particles undergoing motion according to the constant-acceleration equations of kinematics. Visualize: Solve: (a) The time it will take the asteroid to reach the earth is 6 5 displacement 4.0 10 km 2.0 10 s 56 h velocity 20 km/s × == × = (b) The angle of a line that just misses the earth is 11 6 00 R R 6400 km tan tan tan 0.092 yy θθ −− ⎛⎞ =⇒= = = ° ⎜⎟ × ⎝⎠ (c) When the rocket is fired, the horizontal acceleration of the asteroid is 9 2 10 5.0 10 N 0.125 m/s 4.0 10 kg x a × × (Note that the mass of the rocket is much smaller than the mass of the asteroid and can therefore be ignored completely.) The velocity of the asteroid after the rocket has been fired for 300 s is () ( ) 2 0 m/s 0.125 m/s 300 s 0 s 37.5 m/s xx x vv a t t =+ −= + − = After 300 s, the vertical velocity is 4 21 0 m / s y v and the horizontal velocity is 37.5 m/s. x v = The deflection due to this horizontal velocity is 1 4 37.5 m/s tan tan 0.107 0 m / s x y v v = ° × That is, the earth is saved.

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8.4. Model: We are using the particle model for the car in uniform circular motion on a flat circular track. There must be friction between the tires and the road for the car to move in a circle.
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## This note was uploaded on 10/16/2011 for the course PHYS 211/212 taught by Professor Sup during the Fall '11 term at Boise State.

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Mastering Physics Solutions - 8.1 Model The model rocket...

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