Newton's Laws of Motion Review
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Part D: Drawing and Interpreting Free-Body Diagrams
37. Construct free-body diagrams for the following physical situations at the instant in time for which they are described. As is always done in free-body diagrams, label the forces according to type and draw the arrows such that their length reflects the magnitude of the force.
a. A book is at rest on top of a table.
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b. A book is being pushed to the right across a table surface with a constant velocity. (Neglect Fair.)
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c. A book is being pushed to the right across a table surface and accelerating in the direction of the push. (Neglect Fair.)
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d. A student is pushing lightly upon a large box in an attempt to push it to the right across the floor, but the box fails to move.
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e. A rightward-moving box (which was previously set into rightward motion across the floor) gradually slows to a stop.
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f. An air track glider is gliding to the right at constant velocity.
(or replace the Fnorm with Fair)
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g. A ball is dropped from rest from the top of a building. (Neglect Fair.)
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h. Several seconds after being thrown, a football is moving upwards and rightwards towards the peak of its trajectory. (Neglect Fair.)
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i. Several seconds after being thrown, a football reaches the precise peak of its trajectory. (Neglect Fair.)
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j. A falling skydiver is speeding up.
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k. A falling skydiver has reached a terminal velocity.
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l. After reaching a terminal velocity, a falling skydiver has opened her parachute.
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m. A car is moving to the right at a high speed across a level roadway surface; the driver's foot remains on the gas pedal.
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n. A car is skidding to a stop (with wheels locked) while traveling to the right across a level roadway surface.
(Fair could be added to Ffrict)
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o. An elevator (held by a cable) is moving upwards at a constant velocity. (Neglect Fair.)
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p. An upward moving elevator (held by a cable) slows down. (Neglect Fair.)
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q. A downward moving elevator (held by a cable) slows down. (Neglect Fair.)
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r. A picture hangs symmetrically by two wires oriented at angles to the vertical.
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Part E: Force-Mass-Acceleration Relationships
Construct free-body diagrams for the following objects; label the forces according to type. Use the approximation that g=10 m/s2
to determine the magnitude of all forces and the net force and acceleration of the object.
38. A 2-kg box is at rest on a table.
("At rest" would indicate a balance of forces and an acceleration of 0 m/s/s.)
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39. A 2-kg box is free-falling from the table to the ground.
("Free-falling" indicates that the only force that influences the motion is the force of gravity.)
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40. A 2-kg box equipped with a parachute is falling at a terminal velocity after being dropped from a plane.
(A "terminal velocity" indicates a constant velocity and a balance of forces.)
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41. A 2-kg box is sliding to the right across a table. The force of friction upon the box is 5 N.
(Friction is directed opposite the motion and causes a leftward acceleration; no rightward force is spoken of, only a rightward motion.)
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42. An 8-N force is applied to a 2-kg box to move it to the right across the table at a constant velocity of 1.5 m/s.
(A "constant velocity" indicates an acceleration of 0 m/s/s and a balance of forces.)
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43. An 8-N force is applied to a 2-kg box to accelerate it to the right across a table. The box encounters a force of friction of 5 N.
(The horizontal forces can be summed as vectors; divide by the mass to obtain the acceleration value.)
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44. A 500-kg freight elevator is descending down through the shaft at a constant velocity of 0.50 m/s.
(A "constant velocity" indicates an acceleration of 0 m/s/s and a balance of forces.)
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45. A 500-kg freight elevator is moving upwards towards its destination. Near the end of the ascent, the upward moving elevator encounters a downward acceleration of 2.0 m/s/s.
(Begin by multiplying m•a to find the net force - 1000 N, down. The downward gravity force must be 1000 N more than the upward tension force.)
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46. A 150-N rightward force is applied to a 20-kg box to accelerate it to the right across a rough surface at a rate of 2.0 m/s/s.
(Begin by multiplying m•a to determine the net force - 40 N, right. The rightward applied force must be 40 N more than the leftward friction force.)
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