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Unformatted text preview: Physics 8A Homework 4 Solutions Fall 2009 Pauli Kehayias (firstname.lastname@example.org) September 27, 2009 1 A Push or a Pull? Part A The Earth exerts a downward force on the book due to the Earths gravity. The book cant exert a force on itself, and for the purposes of this problem, the table doesnt pull down on the book either. The table just pushes up on the book. Part B Gravity is a long-range force. Two masses dont need to be in contact to exert the force of gravity on each other. An example is projectile motion: the Earths gravity pulls projectiles down without touching them. Part C The force with which Earths gravity pulls on objects is called the weight. Part D The table pushes back up on the book. Since the book is at rest and is not accelerating, there must be no net force on it. This means there must be an equal and opposite force pushing back up on the book, which is provided by the table. Gravity can only pull down, so the Earth cant push up on the book. Again, the book cant exert a force on itself. Part E This is a contact force - it occurs only because the book is on the table. If the book were in the air above the table, the table wouldnt exert any force on it. Part F The table exerts a normal force equal and opposite to the weight. Part G The only horizontal forces on the block are the strings tension and tables friction. The problem tells you that a string pulls the block to the right, so the correct answer is the string. Part H This is a contact force - it only works if the string and block are connected. 1 Part I The force that strings or ropes exert is called the tension. Note that strings and ropes can not push objects, they can only pull them. Part J The other horizontal force in this problem is the tables friction on the block. Kinetic friction opposes an objects motion, and static friction opposes the net force (less friction) to keep an object at rest. If the block is moving, kinetic friction pulls it to the left. If the block is stationary, static friction pulls it to the left to balance the tension. In either case, friction acts to the left. Part K Friction is a contact force - its caused by materials rubbing together. Part L Friction (see above). Part M Once the block is let go, it is no longer being pushed to the right. Kinetic friction acts opposite the blocks motion (to the left), eventually bringing it to a stop. This is the only horizontal force, since the weight and the normal forces are vertical and the block is not being pushed any more. Part N Friction. Mastering Physics suggests that The force of friction does not disappear as long as the block is moving. Once the block stops, friction becomes zero (assuming the table is perfectly horizontal). 2 Newtons 1st and 2nd Laws We can think of a frame of reference as a set of coordinates with which an observer can measure distances and locations. For example, one observer can stand at a train station and use a coordinate system withand locations....
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