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.

 

 

b. A book is being pushed to the right across a table surface with a constant velocity. (Neglect Fair.)

 

c. A book is being pushed to the right across a table surface and accelerating in the direction of the push. (Neglect Fair.)

 

 

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.

 

 

e. A rightward-moving box (which was previously set into rightward motion across the floor) gradually slows to a stop.

f. An air track glider is gliding to the right at constant velocity. 

(or replace the Fnorm with Fair)

g. A ball is dropped from rest from the top of a building. (Neglect Fair.)

h. Several seconds after being thrown, a football is moving upwards and rightwards towards the peak of its trajectory. (Neglect Fair.)

 

i. Several seconds after being thrown, a football reaches the precise peak of its trajectory. (Neglect Fair.)

j. A falling skydiver is speeding up.

 

 

 

k. A falling skydiver has reached a terminal velocity.

l. After reaching a terminal velocity, a falling skydiver has opened her parachute.

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.

 

 

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)

 

o. An elevator (held by a cable) is moving upwards at a constant velocity. (Neglect Fair.)

p. An upward moving elevator (held by a cable) slows down. (Neglect Fair.)

 

 

q. A downward moving elevator (held by a cable) slows down. (Neglect Fair.)

r. A picture hangs symmetrically by two wires oriented at angles to the vertical.

Useful Web Links

Types of Forces || Drawing Free-Body Diagrams
 

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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.)

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.)

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.)



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.)

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.)


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.) 


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.)  


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.)


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.)


Useful Web Links

Free-Body Diagrams || Finding Acceleration || Finding Individual Forces
 

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