Newton’s 2nd Law
Mr. Kuffer
Concepts of Physics
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Complete the Free-Body Diagrams for the following situations
Apply the method described in the paragraph above to construct free-body diagrams
for the situations described below. Answers are shown at the bottom of this page.
1. A book is at rest on a table top. Diagram the forces acting on the book.
2. A girl is suspended motionless from a bar which hangs from the ceiling by two
ropes. Diagram the forces acting on the girl.
3. An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the
forces acting on the egg as it falls.
4. A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant
velocity. Consider air resistance. Diagram the forces acting on the squirrel.
5. A rightward force is applied to a book in order to move it across a desk with a
rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the
forces acting on the book.
6. A rightward force is applied to a book in order to move it across a desk at
constant velocity. Consider frictional forces. Neglect air resistance. Diagram the
forces acting on the book.
7. A college student rests a backpack upon his shoulder. The pack is suspended
motionless by one strap from one shoulder. Diagram the vertical forces acting on the
backpack.
8. A skydiver is descending with a constant velocity. Consider air resistance. Diagram
the forces acting upon the skydiver.
9. A force is applied to the right to drag a sled across loosely-packed snow with a
rightward acceleration. Diagram the forces acting upon the sled.
10. A football is moving upwards towards its peak after having been booted by the
punter. Neglect air resistance. Diagram the forces acting upon the football as it rises
upward towards its peak.
11. A car is coasting to the right and slowing down. Diagram the forces acting upon
the car.
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Newton’s 2nd Law Lab
Concepts of Physics
Mr. Kuffer
Objective: TSW be able to verify the relationship between mass, force and acceleration in
accordance with Newton’s 2nd Law of Motion.
Procedure:
1. Set up the Vernier Logger Pro Bundle, you know the drill.
2. Using Logger Pro and the motion detector, determine
the acceleration of the cart. (slope of v/t graph)
Perform 3 trials. Calculate average acceleration.
Record on data table
3. Calculate “Pulling Force”. Record on data table
4. Calculate “total mass”. Record on data table.
5. A-E Graph accel (Y) vs. FP (X)
6. F-K Graph accel (Y) vs. MTOT (X)
Pulley
M2
M.D.
Track
M1
Tips and Reminders:
1. To attain the total mass: Add column #1 + Column #3 + Column #4
2. See me after trials A – E
3. In trials A – E: Mass is constant. Vary the pulling force (a.k.a. add mass to
hanger).
4. In trials F – J: Pulling force is constant. Vary the mass of the cart system.
5. Convert all distances to meters.
6. Convert all masses to kilograms.
7. Using the mass, calculate the weight (or force) multiply by -10 m/s2
Example: Fg = mg
(.35 kg)(-10 m/s2)
= -3.5 N
8. Do not let the masses crash to the ground! Do not let the carts run off the
table! If this happens ONCE your group will receive a zero for the lab! Don’t
be “that guy”!
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Important Equation:
F = ma
Analysis:
1.
For Trials A – E create a graph of acceleration and pulling force. What is the
significance of this relationship?
2. For Trials F – K create a graph of acceleration and total mass. What is the
significance of this relationship?
3. How does this lab validate Newton’s Second Law? Explain in terms of both
graphs.
Data Table:
M1
Trial
Pulling
mass
(kg)
A
B
C
D
E
F
G
H
I
J
K
.100
.150
.200
.250
.300
.200
.200
.200
.200
.200
.200
M2
Pulling Mass Extra
Force of
mass
(N)
Cart in
(kg)
cart
(kg)
.5
.5
.5
.5
.5
.5
.5
.5
.5
.5
.5
MTOT
Total Accel.
mass (m/s2)
(kg)
Trial
#1
Accel.
(m/s2)
Trial
#2
Accel. Average
(m/s2) Accel.
Trial
(m/s2)
#3
.20
.15
.10
.05
.00
.00
.05
.10
.15
.20
.25
Calculations and Analysis in Lab Notebook!
Graphs on Excel!
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I.
Frictional Forces:
A)
B)
Ff is proportional to the Fg (weight) and the normal force FN,
and depends on the surfaces in contact. Example:
More weightharder to push
On ice  easier to push
On cement  harder to push
A model used to represent this is…
Ff =  FN
Where  is the coefficient of friction between the two surfaces
C)
A table of all coefficients of friction can be found on page 131
of your text.
Frictional Forces
You push a 25-kg wooden box across a wooden floor at a constant
velocity. How much force do you exert on the box? (Hint: b/c the box has a
constant velocity Ff has got to equal FA)
If the force you exerted on the box is doubled, what is the resulting
acceleration of the box?
Draw a Free-Body diagram for the situation above.
Be careful to identify ALL of the forces involved!!
SHOW ALL WORK ON A SEPARATE SHEET OF PAPER!
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Friction Lab
Objective:
Calculate the coefficient of friction between two surfaces.
Theory:
If an applied force pulls horizontally on a mass and moves the mass at
a constant speed, then the free-body diagram is as follows:
FN
Ff
FA
Fg
Applying Newton’s 2nd Law:
FNET = FA – Ff
OR
FNET = FA – ( µFN)  b/c Ff = µFN
OR
FNET = FA – ( µmg)
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Procedure:
Spring Scale
Wood block
Pull this way
1. Using the spring scale, measure the weight of the block (Fg)
2. Using the spring scale, pull the block of wood across the wooden
surface at a constant speed. The reading on the scale is the applied
force (FA)
3. Add 100g of mass to the wood block. Determine the entire weight of
the block and mass. (this is trial #1)
4. Repeat steps 2 and 3 until reaching 1000g
5. Repeat all steps for felt on wood
6. Create a graph of Ff vs. FN.  Hint  Ff = µFN
7. Determine the slope of the graph and its meaning.
8. Determine the Percent error.
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Forces:
Free Body Diagrams and NET Forces
Name:__________________
Description of Motion:
Date:_________
NET Force?
YES or NO
__________
__________
__________
__________
__________
__________
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1. Free-body diagrams for four situations are shown below. For each situation,
determine the net force acting upon the object.
A) __________________
__________________
__________________
__
B) __________________
__________________
__________________
__
C) __________________
__________________
__________________
__
D) __________________
__________________
__________________
__
2. Free-body diagrams for four situations are shown below. In each case, the net
force is known. However, the magnitudes of some of the individual forces are not
known. Analyze each situation individually to determine the magnitude of the
unknown forces.
F = _____
A = _____
B = _____
C = _____
D = _____
E = _____
G = _____
H = _____
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Newton’s Second Law
F= ma
Name:_____________
Date:_________
1. What acceleration will result when a 12-N net force is applied to a 3-kg object? A
6-kg object?
2. A net force of 16 N causes a mass to accelerate at the rate of 5 m/s2. Determine
the mass.
3. An object is accelerating at 2 m/s2. If the net force is tripled and the mass of the
object is doubled, what is the new acceleration?
4. An object is accelerating at 2 m/s2. If the net force is tripled and the mass of the
object is halved, what is the new acceleration?
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Newton’s Second Law
F= ma
Name:_____________
Date:_________
5. Sidney Crosby strikes a 1 kg hockey puck wit his stick. The puck is accelerated at
5 m/s2. With what force (in newtons) did Crosby strike the puck?
6. What acceleration will result when a 12-N net force is applied to a 4-kg object? A
8-kg object?
7. A net force of 20 N causes a mass to accelerate at the rate of 5 m/s2. Determine
the mass.
8. An object is accelerating at 2 m/s2. If the net force is doubled and the mass of the
object remains the same, what is the new acceleration?
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Under Pressure
1. Distinguish between force and pressure.
2. Which produces more pressure on the ground, a person standing up or
the same person lying down? Explain.
3. In attempting to perform the “experiment” shown in the picture
below, would it be wise to start with just a few nails and work your
way upward to more nails?
4. The massiveness of the block plays a critical role in the experiment
(above). Which provides more safety, a less massive or more massive
block?
5. Pressure is the amount of force per unit area. Write the equation
below. Add this equation to your equation sheet.
6. The unit for pressure is a “pascal”. Why a pascal? What is one pascal
equal to?
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Pressure Problems
1. The atmospheric pressure at sea level is about 1.0 x 105 Pa.
What is the force at sea level that air exerts on the top of
a typical office desk, 152 cm long and 76 cm wide?
2. A woman weighs 495 N and wears shoes that touch the
ground over an area of 412 cm2.
a. What is the average pressure in KPa that her shoes
exert on the ground?
b. How does the pressure change when she stands on only
one foot?
c. What is the pressure if she puts all her weight on the
heel of one shoe with the area of the high heel of 2.0
cm2?
3. A car tire makes a contact with the ground on a rectangular
area of 12 cm by 18 cm. The car’s mass is 925 kg. What
pressure does the car exert on the ground? What pressure
does the ground exert on the car? (Reminder… there are
four tires on car!!)
4. A lead brick, 5.0 x 10.0 x 20.0 cm, rests on the ground on
its smallest face. What pressure does it exert on the
ground? (Lead has a density of 11.8 g/cm3)
5. Same situation as above. If the brick rests on its largest
face, what pressure does it exert?
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6. A 0.75 kg physics book with dimensions 24 cm by 20 cm
rests on a table.
a. What force does the book apply to the table?
b. What pressure does the book apply to the table?
7. Why are sharp ice skate important to hockey players?
8. In a Tornado, the pressure can be 15% below normal
atmospheric pressure. Sometimes a tornado can move so
quickly that this pressure drop can occur in one second.
Suppose a tornado suddenly occurred outside your front
door, which is 182 cm high and 91 cm wide. What force
would be exerted on the door? In what direction would the
force be exerted? (Standard atmosphere is 1.01325 x 105
Pascals)
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