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Unformatted text preview: Physics 6A Practice Midterm #2 solutions Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB 1. A locomotive engine of mass M is attached to 5 train cars, each of mass M. The engine produces a constant force that moves the train forward at acceleration a. If 3 of the cars are removed, what will be the acceleration of the shorter train? We just use Newtons 2 nd law here: Net force = (total mass) x (acceleration) Initially, the total mass is 6M (5 cars, plus the engine) After removing 3 cars the total mass is 3M (2 cars, plus the engine) In both cases the Net force is the same (the engine didnt change). Here is the formula in both cases: Initial F = (6M)(a) Final F = (3M)(a final ) Setting these equal gives a final = 2a Another way to think about this one is that since the mass is cut in half, the acceleration must double. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB We can think of this as a single box with total mass 4kg. Then using F = ma we get the acceleration of the whole system. Now we do the same thing, but just for box B: Remember Box B has a mass of only 3kg A B 2 s m system system kg N 2 a a 4 8 N B s m kg B 6 F 2 3 F 2 2. Two boxes are placed next to each other on a smooth flat surface. Box A has mass 1 kg and Box B has mass 3 kg. A constant horizontal force of 8 N is applied to Box A. Find the force exerted on Box B. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB We can think of this as a single box with total mass 4 kg . Then using F = ma we get the acceleration of the whole system. We need to account for friction, so first find the maximum force of static friction: A B N 68 . 15 ) 8 . 9 4 ( ) 4 . ( N 2 max , static max , static s m kg F s F 8N F static This friction force is more than the 8 N force trying to move the boxes, so they are held in place by friction. Thus the acceleration of both boxes is 0. *Note that the actual force of friction holding the boxes in place is only 8 N just enough to keep them from moving. 3. Two boxes are placed next to each other on a flat surface. Box A has mass 1kg and Box B has mass 3kg. The coefficients of friction are 0.3 and 0.4 for kinetic and static friction, respectively. A constant horizontal force of 8N is applied. Find the acceleration of Box B. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB 4. A crate filled with books rests on a horizontal floor. The total weight of the crate and books is 700 N . The coefficients of friction are 0.35 for kinetic and 0.45 for static. A force of 450 N is applied to the crate at an angle of 20 below the horizontal. Find the acceleration of the crate....
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 Spring '07
 STANEK
 Physics, Force, Mass

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