Physics 106P: Lecture 1 NotesPhysics 101: Lecture 8
Newton's Laws
Today’s lecture will be a review of Newton’s Laws and
the four types of forces discussed in Chapter 4.
Concepts of Mass and Force
Newton’s Three Laws
1
Sir Isaac Newton, English Physicist, 1643-1727
Physics 101: Lecture 8, Pg *
Newton’s First Law
The motion of an object does not change unless it is acted upon by a net force.
If v=0, it remains 0
If v is some value, it stays at that value
Another way to say the same thing:
No net force
velocity is constant
acceleration is zero
Physics 101: Lecture 8, Pg *
Mass or Inertia
to remain at rest or in motion with
constant speed along a straight line.
Mass (m) is the quantitative measure of inertia. Mass is the property of an object that measures how hard it is to change its motion.
Units: [M] = kg
Newton’s Second Law
This law tells us how motion changes when a net force is applied.
acceleration = (net force)/mass
Response is acceleration (effect)
Newton’s Second Law
A vector equation:
Newton’s 1. Law
An airplane is flying from Buffalo airport to O'Hare. Many forces act on the plane, including weight (gravity), drag (air resistance), the trust of the engine, and the lift of the wings. At some point during its trip the velocity of the plane is measured to be constant (which means its altitude is also constant). At this time, the total (or net) force on the plane:
1. is pointing upward
2. is pointing downward
3. is pointing forward
4. is pointing backward
Newton’s 1. Law
Newton's first law states that if no net force acts on an object, then the velocity of the object remains unchanged. Since at some point during the trip, the velocity is constant, then the total force on the plane must be zero, according to Newton's first law.
SF= ma = m0 = 0
M
F1
Find a
Find a
Newton’s Third Law
For every action, there is an equal and opposite reaction.
Finger pushes on box
Box pushes on finger
Third Law:
Newton's Third Law...
FA ,B = - FB ,A. is true for all types of forces
Fw,m
Fm,w
Ff,m
Fm,f
Physics 101: Lecture 8, Pg *
Conceptual Question: Newton’s 3.Law
Since Fm,b = -Fb,m why isn’t Fnet = 0, and a = 0 ?
a ??
Fb,m
Fm,b
ice
Conceptual Question: Answer
abox
Fb,m
Fm,b
ice
Fnet,man = mman aman = Fb,m
Physics 101: Lecture 8, Pg *
Newton’s 2. and 3. Law
Suppose you are an astronaut in outer space giving a brief push to a spacecraft whose mass is bigger than your own (see Figure 4.7 in textbook).
1) Compare the magnitude of the force you exert on the spacecraft, FS, to the magnitude of the force exerted by the spacecraft on you, FA, while you are pushing:
1. FA = FS
2. FA > FS
3. FA < FS
you experience, aA, to the magnitude of the acceleration
of the spacecraft, aS, while you are pushing:
1. aA = aS
2. aA > aS
3. aA < aS
Third Law!
Summary:
The motion of an object does not change unless it
is acted on by a net force
Newton’s Second Law:
Forces: 1. Gravity
G = universal gravitation constant = 6.673 x 10-11 N m2/kg2
Example: two 1 kg masses separated by 1 m
Force = 6.67 x 10-11 N
(very weak, but this holds the universe together!)
r12
m1
m2
F2,1
F1,2
Physics 101: Lecture 8, Pg *
Gravity and Weight
Force on mass:
Fg W = mg
Forces: 2. Normal Force
W
Therefore, weight balanced by another force
FN
FN is always perpendicular to surface and outward
For this example FN = W
Physics 101: Lecture 8, Pg *
Forces: 3. Kinetic Friction
A force, fk, between two surfaces that
opposes relative motion.
Magnitude: fk = kFN
a property of the two surfaces
W
FN
F
fk
Forces: 3. Static Friction
prevents relative motion.
s = coefficient of static friction
a property of the two surfaces
Physics 101: Lecture 8, Pg *
Forces: 4. Tension
Magnitude: same everywhere in rope
Not changed by pulleys
T
Forces: 4. Tension
rope attached to ceiling
So T = W
Examples: Inertia
Seat-belt mechanism (see textbook)
A man dangles his watch from a thin chain as his plane takes off. He observes that the chain makes an angle of 30 degrees with respect to the vertical while the plane accelerates on the runway for takeoff, which takes 16 s.
What is the speed of the aircraft at takeoff ?
Physics 101: Lecture 8, Pg *
Examples: Tension
A lamp of mass 4 kg is stylishly hung from the ceiling
by two wires making angles of 30 and 40 degrees. Find
the tension in the wires.
Physics 101: Lecture 8, Pg *
Examples:
Consider two blocks of mass m1 and m2 respectively
tied by a string (massless). Mass m1 sits on a horizontal
frictionless table, and mass m2 hangs over a pilley. If
the system is let go, compute the aceleration and the
tension in the string.