Transcript Physics_100_chapt_3

Newton’s 3 laws of motion
Isaac Newton 1642 --- 1727
Physics 100 Chapt 3
1st Law: Law of Inertia
A body at rest tends to stay at
rest, a body in motion tends to
keep moving along at a constant
speed and in a straight-line
path unless interfered with by
some external forces.
example
Motorcycle crash dummy
Another example
(watch the ladder)
2nd Law: F=ma
The
acceleration of a body is directly
proportional to the net force acting on
it and inversely proportional to its
mass.The direction of the acceleration
is in the direction of the applied force.
Directly proportional to Force
a
a
Small force
Small acceleration
Large force
Large acceleration
inversely proportional to mass
a
a
Beach
ball
small mass
Large acceleration
Bowling
ball
Large mass
Small acceleration
“Inertial” mass
“Inertial” mass, mi, is the resistance to
changes in the state of motion
Objects with large mi
are hard to get moving
(& once started, hard
to stop),
Objects with small mi
easier to get moving
(& easier to stop),
Units of mass (we cant avoid
them!)
Mass: basic unit = 1kilogram = 1kg
mass of 1 liter of water
10cm
10cm
This much
water!
10cm
Net force
Tip-to-tail method
for adding vector
Net force
is the vector
from the tail of the 1st to
the tip of the 2nd. (0 in
this case).
Slide tail of one to tip
of the other (keep
directions fixed)
Tip-to-tail method
Net force points
down the hill
Slide tail of one to tip
of the other (keep
directions fixed)
Newton’s 2nd law  F=ma
a is proportional to F:
a  F
direction of a
= direction of F:
a  F
a is inversely
proportional to m:
a  1/m
combine:
set proportionality
constant = 1:
a  F/m
a = F/m
multiply
both sides
by
m
Units of Force
F=ma
m
kg 2
s
Unit of force: 1 Newton = 1N = 1 kg m/s2
1 pound =1lb = 4.5 N
What is your mass?
Suppose I
jump off
a tqble
Weight = force of
Earth’s gravity on you
F=ma
a=g
W
W=mg
W
m= g
Two different aspects of mass
Weight: W = m
mggg
Newton’s 2nd law:
a =
F
m
m
i
Force of gravity is
proportional to
“gravitational” mass
Inertia; resistance
to changes in state
is proportional to
“inertial” mass
Experiment
shows: mg = mi
!!!!!
My mass
My weight in pounds: W = 190 lbs
Convert to Newtons:
W = 190 lbs x 4.5 N/lb = 855 N
m = W/g =
855 N
10 m/s2
= 85.5 kg
Newton’3rd Law: action-reaction
Whenever one object exerts a force on a
second object, the second object
exerts an equal in magnitude but
opposite in direction force on the first.
action: I push
on the canoe
reaction: the
canoe pushes
me forward
Action Reaction
I push on the bus
v= 0
F
But I accelerate
v
Newton:
The bus exerted an “equal but opposite” force on me.
Look again
All forces come in pairs!
-F
F
This force causes me
to accelerate backwards
This force tries to accel.
the bus forward
Air-filled balloon
reaction: air
pushes on balloon
action: balloon
pushes on air
recoil
reaction: equal but
opposite force on the gun F1
Produces a recoil
action: gun exerts
force F2 on bullet
making it accelerate
Rocket propulsion
reaction: rocket
gets pushed
in the opposite
direction
action: rocket engine
pushes exhaust
gasses out the rear