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Physics 1710 Chapter 5: Laws of Motion—II
1′ Lecture
• Newton’s Laws of Motion are:
 Acceleration (or deceleration) occurs if and
only if there is a net external force.
 a = F/m [Note this is a vector eqn.]
 The force exerted by a first object on a
second is always equal and opposite the
the force exerted by the second on the
first. F12 = - F21
Physics 1710 Chapter 5: Laws of Motion—II
1′ Lecture –continued (30″ more)
• Weight is the force of gravity equal to
the mass of the object.
g times
• g =9.80 N/kg
• The force of friction is opposed to the motion of a
body and proportional to the normal force.
• Free body diagrams are sketches of all the
forces acting on a body.
Physics 1710 Chapter 5: Laws of Motion—II
Laws of Motion
1st Law:
An acceleration is caused by a net external
force.
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion:
a= F /m
Note the vector nature of the equation:
ax= Fx /m
ay= Fy /m
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
The Newton [N]
is the unit of force in SI units.
[F] = [m] [ a ]
[N] = [kg] [m/s/s]
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
• Consider free fall (in the absence of air
resistance):
Weight = F = m g fact #1
F=ma
fact #2
∴ m ag = m g
∴ ag = g
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
Feather and Penny Demonstration
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
When a sheet of paper and a book are dropped
at the same time, what will happen?
Will
(1) the book accelerate faster that the sheet;
(2) vice versa or
(3) will they each accelerate at the same rate?
Why do you think so?
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
Book and Paper
Book wins!
CDrag v2 Book
- gM
CDrag v2 Paper
- gm
aPaper = (- gm+ CDrag v2 Paper) /m
aBook = (- gM + CDrag v2 Book)/M
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
Book and Paper
Tie!
CDrag v2
- gm
- gM
aBook+Paper = (- gM - gm+ CDrag v2)/(M+m)
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
Atwood Machine
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
Atwood Machine
F = g (m2 – m1)
a = F/(m2 +m1)
a = g(m2 – m1)/(m2 +m1)
a
a
g m1
g m2
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
• How much force is one Newton?
• How much mass weighs 1 N?
F=mg
1.00 N = mN (9.80 N/kg)
mN = 1.00/ 9.80 kg
mN = 0.102 kg
Experience “Newton’s apple”
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
• What acceleration will one Newton produce on
a mass of one kilogram?
a = F /m
a = 1.00 N /1.00 kg
a = 1.00 m/s2
[ m/s2] = [N ]/[kg]
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
Mysterious Spool Puzzle
Spool
F⃗
Pull
Away from F
Toward F
No acceleration
?????????
Physics 1710 Chapter 5: Laws of Motion—II
2nd Law of Motion: Implications
Mysterious Spool Puzzle
Spool
F⃗
Pull
Toward F
a = F/m; a, always in the direction of F
Physics 1710 Chapter 5: Laws of Motion—II
Summary:
• Newton’s Laws of Motion are:
(1) Acceleration (or deceleration) occurs if and only
if there is a net external force.
(2) a = F/m [Note this is a vector eqn.]
(3) The force exerted by a first object on a second is
always equal and opposite the the force exerted by
the second on the first. F12 = - F21
Physics 1710 Chapter 5: Laws of Motion—II
Summary (cont’d.) :
• Weight is the force of gravity equal to g times the
mass of the object.
• g =9.80 N/kg
• The force of friction is opposed to the motion of a
body and proportional to the normal force.
• Free body diagrams are sketches of all the forces
acting on a body.