Download Newton`s 1st Law:

Mass and Acceleration
Types of Forces
Noncontact Forces
Gravitational
Electromagnetic
Contact Forces
Nuclear
Pushes
Pulls
W
Tension
Electric
Magnetic
Friction
M
f
Normal
N
perpendicular to
surface
T
Newton’s 1st Law:

No net force - no acceleration

No acceleration = constant velocity
Dry Ice on Slate
Motion of the Block
at rest
uniform motion to
the right
moves to the right
and speeds up
without changing
direction
moves to the right,
slows down and
stops without
changing direction
Velocity Acceleration Push or Pull Necessary
to Maintain Motion
0
0
0
0
0
Pretest #3
(“the crate in the snow”)
Fby snow
Fby
?
Pretest #3
(“the crate in the snow”)
Fby snow
Fby
Newton’s 2nd Law:


Net force causes acceleration (not velocity)
The net force is the vector sum of all the individual
forces on an object.
Nr,b
Fnet
a
WE,b
WE,b
Nr,b
Acceleration is in same direction as the net force.
a
Which one is the net force?
Types of Forces
Noncontact Forces
Gravitational
Electromagnetic
Contact Forces
Nuclear
Pushes
Pulls
W
Tension
Electric
Magnetic
Friction
M
f
Normal
N
T
What’s the mass?



A measure of “how much stuff”
A measure of how hard it is to speed up
or slow down
Very different than weight!
Hammer of Death
More net force - more acceleration
(constant mass)
a  Fnet
F
a
F
a
More mass - less acceleration
(constant net force)
1
a  M
F
a
F
a
Putting it all together
Fnet
a  M
Fnet
a  Constant × M
Newton is defined such that constant = 1.
Fnet  M a
Scale reading = ?
10 N
1.0 kg
Scale reading = ?
10 N
20 N
2.0 kg
Free body diagram of 1 kg
crate:
TS,C (Reading on the scale)
WE,C
In General

The force that the Earth exerts on an
object of mass m is given by
W = m (10 N/kg)
Question:

W = m (10 N/kg)
=mg
If Jeff jumps off the physics building in
a vacuum, what will his acceleration be?
(use Newton’s 2nd law)
Fnet = m a
WE,J = m a
WE,J
m (10 N/kg) = m a
10 N/kg is just g
Weight force is given by mg
2nd Law is the translator
a = 4 m/s/s
T = 100 N
M = 5 kg
Nfloor,B = 50 N
ffloor,B= 80 N
Block
Trope,B = 100 N
How big is the
friction force?
a = 4 m/s/s
In the x-direction:
Fnet = m a
T-f =ma
WE,B
= m g = 50 N
100 N - f = (5 kg) (4 m/s/s)
f = 80 N