Download 26a Dynamics Review A - stpats-sph3u-sem1-2013

Dynamics Review A
Write full solutions to the following problems on a separate sheet of paper.
Short Answer Questions
1. Give an example which clearly illustrates:
a. Newton’s first law.
b. Newton’s second law.
c. Newton’s third law.
2. Consider an express elevator ride from the ground floor to the top of a building.
Provide a complete description of the sensations one feels as a result of the ride
and an explanation of these sensations using Newton’s laws of motion.
3. Newton’s first law states that objects will remain at rest or in uniform motion
provided no external unbalanced force acts on them. Newton’s third law states
that forces always occur in pairs – the two forces of a given pair acting with equal
strength and in opposite directions. Doesn’t Newton’s third law imply that forces
are balanced? If so, it would never be possible for anything to accelerate. By
citing a specific example, illustrate how this apparent paradox can be resolved.
4. Distinguish between the terms “static” and “kinetic” friction. Provide examples
that illustrate that friction is sometimes desirable and sometimes undesirable. List
two ways that kinetic friction is typically reduced.
5. Clearly distinguish between the terms “mass” and “weight.” Why is it not correct
to refer to orbiting objects as being “weightless”? If orbiting astronauts are in a
constant state of free fall, why do they never reach Earth’s surface?
6. Design an experimental procedure by which the coefficient of kinetic friction
between a pair of surfaces could be determined.
Problems
1. A bowling ball of mass 2.0 kg strikes a stationary pin of mass 5.0 x 102 g. The
collision lasts for 0.60 s after which the pin moves off with a velocity of 12.0 m/s
[W]. Calculate:
a. the acceleration of the pin during the collision. [20 m/s2]
b. the force exerted by the bowling ball on the pin. [10 N [W]]
c. the acceleration of the bowling ball during the collision. [5.0 m/s2 [E]]
Dynamics Review A
2. An elevator and its contents have a combined mass of 6000 kg. It is suspended by
a single cable.
a. Draw a free-body diagram of the elevator.
b. What force must the cable exert on the elevator when it is at rest? [5.9 x
104 N [up]]
c. What force must the cable exert on the elevator when it is moving upward
at 2.0 m/s2? [7.1 x 104 [N]]
d. What force must the cable exert on the elevator when it is moving
downward at 2.0 m/s? [5.9 x 104 N [up]]
e. If the cable snapped, what is the elevator’s motion as it falls. Include a
new free-body diagram. [accelerate at 9.8 m/s2 [down]]
3. A model rocket of mass 4.80 x 102 g accelerates vertically upward at 34.0 m/s2
during launch, overcoming both gravity and air resistance.
a. Draw a free-body diagram of the rocket during launch once it has departed
from the launch pad.
b. Calculate the thrust force applied by the rocket engine during launch if the
air resistance acting on the rocket is 2.40 N. [23.4 N [up]]
4. A force of 1.2 N is applied to an object of mass 1.5 kg. It accelerates at 0.50 m/s2.
Determine the force of friction that is acting and the coefficient of kinetic friction
involved. [0.45 N; 0.031]
5. Two children pull a toy truck of mass 2.4 kg along a rough horizontal surface.
One child pulls with a force of 8.4 N [N] and the other pulls with a force of 3.6 N
[S]. The coefficient of friction involved is 0.18. What is the object’s
acceleration? [4 x 10-1 m/s2 [N]]
6. A hockey puck of mass 200 g slides along the ice with a speed of 1.2 m/s when it
reaches a rough section where the coefficient of kinetic friction is 0.25. How long
will it take the puck to stop sliding? Include a free-body diagram. [0.49 s]
7. An object of mass 40.0 kg rests on the surface of a planet with a mass of 8.2 x
1022 kg and radius 3.6 x 105 m.
a. Calculate the force of gravity acting on the object. [1.7 x 103 N]
b. Determine the gravitational field strength “g” at the planet’s surface. [42
m/s2 [down]]
c. Calculate the force of gravity acting on the object if it is placed at a
position 6.4 x 105 m above the planet’s surface. [2.2 x 102 N]