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Physics 12 Dynamics
Name:
Blk:
1. Newton’s Laws. Write down Newton’s three laws.
a.
b.
F=
c.
2. A force is a vector quantity. That means that it has both ________________ and
___________________ .
3. What is a free body diagram?
4. What does it mean to find the equation for net force?
5. A rope is attached to the top of a 70 kg block and an upward force of 400N is
applied to the rope. If the block remains at rest, find the force of the floor on the
block
a. Sketch the free-body diagram
b. Is this system accelerating? _________
c. Complete the equation for net force
400 N
d. solve
Fnet = Σ F =
70 kg
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6. Friction
a. Friction resists the motion of 2 surfaces that are sliding (sliding friction) or
are trying to
slide (static friction) across each other.
b. Sliding friction: Ff = µ(FN)
 Once the surfaces are sliding, Ff = µ(FN) until the sliding stops.
 Example: skidding tires, ice skates.
c. Static friction: Ff <= µ(FN)
 µ(FN) is the maximum Ff available to resist any force that is trying to
start the surface from sliding (the “lazy” force)
d. For any 2 particular surfaces in contact, µ static > µ sliding, because
surfaces that aren’t yet moving across each other can “grip” better.
e. Ff DOES NOT depend upon the amount of surface area in contact
between the surfaces!!!!!!!!!!
f. Question: If static friction is what keeps a tire from skidding, then why do
we NOT want to “lock up” the wheels on a car when you stop your car in
a hurry?
7. Sliding Bodies. Find the
acceleration of the block at
the right. Assume no
friction.
a. Free-body diagram
b. Resolve forces into
(x,y) components
c. Write equation for
Fnet
d. Solve for acceleration.
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250 N
150
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e. Do question 8 over again with the coefficient of friction µ = 0.4
Remember that
Ff = µFN
8. Hanging Masses. Two masses are hanging over a
pulley as shown in the diagram to the right.
a. Sketch the arrows for the free body diagram for
each box.
b. Write the equation for net force for each of the
masses.
c. By adding the equations or by substitution,
combine the equations into one equation.
d. Solve for the a.
5 kg
10 kg
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M
9. Vertical and Horizontal
Find M for the situation at the right
such that the system will accelerate
at 1.5 m/s2. µ=0.8
5 kg
10. Do # 1 – 10 p. 92
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11. Inclined Planes (Slopes)
a. Find the coefficient of
friction such that
16kg box does not
slide.
16kg
200
b. For the box to accelerate at 0.25 m/s2 down the ramp, what would the
coefficient of friction need to be?
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45N
c.
An 8 kg box is
accelerated up an
incline of 170 by a
45N force. The
coefficient of friction
is 0.3. Find the
acceleration.
30o
8kg
170
12. Do questions 11 - 24
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13. Moving Surfaces
a. A truck with a crate on the bed is stopped at a light. With what maximum
acceleration can the truck carrying the crate accelerate such that the crate
does not slide. The coefficient of friction is 0.75?
b. Suppose the mass of the crate in the previous question was 85kg and that
the bed of the truck is inclined at 70. What minimum acceleration is
needed to keep the box on the truck?
Acceleration
70
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14. Simple Systems
a. Seven boxes cars are pulled by a 65 ton (65,000kg) locomotive. The
coefficient of friction of steel on steel is 0.41. Find the maximum
acceleration of a seven car train assuming the average mass of each of the
cars is 14,000kg. Hint: The locomotive can only pull with the maximum
force of its static friction with the track. All the other cars are rolling and
you may assume no retarding friction from the cars or the locomotive.
b. If each of the cars has an average retarding frictional force of 500N and
the locomotive has a retarding frictional force of 1000N, then what is the
force needed to pull the last 3 cars? Note the acceleration is not the same
as in part “a.”
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c. The coefficient of friction in the following example is the same for both
boxes. A student set this up in class and used an accelerometer to
deterimine the acceleration. He found that the boxes accelerated at
0.85m/s2. What is the coefficient of friction between the boxes and the
surface below them?
16 kg
20kg
150
15. Do # 26, 28, 30, 32 – 40
16. Do # 41 – 48, 52, 58, 59,62
17. Do Dynamics #1
18. Do Dynamics #2
19. Do Dynamics #3
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