Download 2/4 Lecture

Newton’s
st
1
and
nd
2
laws
•  Recitation homework due Thursday/Friday
(pages 19-21 and 23-24)
Web page: http://www.colorado.edu/physics/phys1110/
1
Newton’s first law
Although we might all agree that a stationary body has no
net force acting on it, this is also true for a body traveling at
constant velocity (magnitude and direction are constant).
Formulations of Newton’s first law
A body acted on by no net force
maintains the same velocity (can be 0)
A body moving at constant velocity
has no net force acting on it
A body in motion tends to stay in motion;
a body at rest tends to stay at rest
2
Clicker question 1
Set frequency to BA
Q. A weight is being pulled along at a
constant speed. What is the direction
of the net force?
!
v
1
ton
A.  Up
B.  Down
C.  Left
D.  Right
E.  The net force is 0
Newton’s 1st law states that a body moving at constant
velocity must have no net force acting on it.
3
Newton’s first law
!
v
But how can you say there is no net
force operating on the weight when it
is moving at constant velocity?
It is true that the person exerts a force
on the weight through the tension in
the rope. But if the weight is moving at
constant velocity, this force must be
canceled by something (in this case
friction), so there is no net force.
1
ton
!
T
!
n
!
f
!
Fg
If you imagine the weight on ice it would sail along once
you got it started. It would require force to slow it down.
The tendency to keep moving is called inertia.
Sometimes Newton’s 1st law is called the law of inertia.4
Inertial reference frames
The person on the sailboat watching the shore go by and
the person on the shore watching the sailboat go by are in
two different reference frames. Neither one is preferred.
Newton’s 1st law implies there is no real difference
between zero velocity and constant (non-zero) velocity.
Reference frames which move at constant velocity
(including 0) are called inertial reference frames.
Newton’s laws work the same
in all inertial reference frames
5
Inertial reference frames
Is the inside of a car an inertial reference frame?
When traveling at a constant speed in a straight line,
everything at rest will stay at rest so yes in this case.
When the car hits the brakes causing the passenger who is
not wearing a seat belt to hit his head on the dashboard, it
is not an inertial reference frame. The passenger’s head
is staying at rest while the car accelerates.
6
Clicker question 2
Set frequency to BA
Q. Which of the following is an inertial reference frame?
A.  A car traveling at a constant 100 mph down a straight and
level road.
B.  A car in the process of crashing into a concrete barricade.
C.  A car traveling at a constant 20 mph around a curve.
D.  More than one of the above
E.  None of the above
In B and C, the car is accelerating (linear acceleration in B
and centripetal acceleration in C) so they are not inertial
reference frames. Objects inside the car will move relative
to the car with no apparent force applied.
7
What does a force do?
Galileo
The concept that a body will
stay in motion if there is no
external force applied is
attributed to Galileo and stated
more explicitly by Newton.
Newton
If a net force is not required to keep a body moving
at constant velocity, then what does a net force do?
A force is required to change the velocity of a body!
What quantity deals with velocity change?
Acceleration!
8
Newton’s second law
So, a net force causes an object to accelerate.
Does the amount of acceleration depend
on the object or just the amount of force?
Acceleration times time gives final velocity which (along
with angle) determines how far a projectile travels.
If you were to throw a gallon milk and a can of soda
(with the same force), which would travel farther?
Longest shot put (16 lb ball): 23 m
Longest baseball throw (1/3 lb ball): 136 m
Acceleration depends on the force
applied and the mass of the object.
!
Newton’s ! Fnet
a
=
nd
2 law:
m9
Clicker question 3
Set frequency to BA
Situation 1: A constant force is applied for a short time to a
frictionless cart initially at rest on a straight track. The cart
acquires a final velocity vf.
Situation 2: The same constant force is applied for the same short
time interval to the same frictionless cart, initially moving with
velocity v1. The final velocity in this case is v2. The change of
velocity Δv = v2 – v1 compared to the final speed in Situation 1 is…
A.  The same: Δv=vf Masses and forces are the same so
B.  Greater: Δv > vf
acceleration is the same. The !time
! Δv
C.  Less: Δv < vf
interval is the same and a =
Δt
D.  Depends on the
!
!
so
Δ
v
is
the
same:
Δv = v2 − v1 = v f − 0
signs of v1 and v2
10
Analyzing situations involving forces
I slide a book across the table at a constant velocity.
What are the forces acting on the book and what can
you tell me about their direction and magnitude?
Draw a picture:
!
v
Now isolate the object of interest (the
book) and draw a free-body diagram.
!
f
!
n
!
Fhand
!
Fg
!
!
!
Find the net force in each direction Fnet,x = Fhand − f = max = 0
!
!
!
and set it equal to the acceleration Fnet,y = n − Fg = may = 0
!
!
! !
Therefore Fhand = f and n = Fg
11