Transcript Intro Forces and Newton`s 3 Laws

Forces and Newton’s 3 Laws
What is a force?
• Push or pull
• Produce changes in motion or direction
Net force:
• The net force is a combined total force
acting on an object.
• Fnet
• ΣF
• We represent force by using vectorsarrow symbols that represent magnitude
and direction by their length and which
way they point.
Let’s look at forces acting on this box:
Stationary:
FN – The Normal Force- table pushing up on
the box- THIS IS ALWAYS PERPENDICULAR
TO THE SURFACE AND EQUAL TO THE Fg.
F1 = 20 N
F2 = 20 N
10 kg
Fg
What is the Fnet acting on this box:
ΣF = Fnet = F1 – F2 = 20 N - 20 N = 0
FORCES ARE BALANCED
A Fnet of zero means no change in movement.
The box stays stationary.
Let’s look at forces acting on this box:
Stationary:
FN
F1 = 20 N
F2 = 40 N
10 kg
Fg
What is the Fnet acting on this box:
ΣF = Fnet = F1 – F2 = 20 N - 40 N = 20 N
FORCES ARE UNBALANCED
Now the Fnet is not zero which means there is a change
in movement. This box is not going to remain stationary.
Let’s look at forces acting on this box:
Stationary:
FN
F1 = 20 N
F2 = 40 N
10 kg
Fg
What is the Fnet acting on this box:
ΣF = Fnet = F1 + F2 = 20 N + 40 N = 60 N
FORCES ARE UNBALANCED
Again there is a non-zero Fnet which means there is a change
in movement.
But what if the box had an initial motion:
Moving with a constant velocity:
FN
F1 = 20 N
F2 = 20 N
10 kg
Fg
What is the Fnet acting on this box:
ΣF = Fnet = F1 - F2 = 20 N - 20 N = 0
FORCES ARE BALANCED
The Fnet is zero which means there is not a change
in movement or direction. This box will continue to move
with a constant velocity.
What forces are acting on this ball?
So what needs to happen to make this
box move?
10 kg
UNBALANCED FORCES
What about when the box is already moving
with a constant velocity?
What would cause the box to stop?
10 kg
UNBALANCED FORCES
What would happen if there weren’t unbalanced forces? Would the box ever stop?…
…Not according to Newton’s
Law of Motion:
st
1
An object at rest has a natural tendency to
stay at rest, or an object in motion will stay
in motion, unless a force is acting upon it.
This is also known as the law of INERTIA.
INERTIA is an objects resistance to change in motion.
Examples of INERTIA:
1. Not wearing your seatbelt- if you get into an
accident your body wants to keep moving at
the speed you were going.
2. The famous tablecloth trick:
http://www.youtube.com/watch?v=D-a3kwvY0WM
3. Sliding Cars:
http://www.youtube.com/watch?v=KyfjZlOSq2A
The relationship between mass and inertia:
MASS IS A MEASURE OF INERTIA- the
more massive the object, the more that
object tends to resist changes in its
state of motion.
What would be easier to push a small car or a semi?
Adding mass into the mix brings us to
Newton’s 2nd Law of Motion:
If I push both vehicles with the same amount of force
which one would accelerate more? Why?
F
a
m
Which turns into Newton’s 2nd equation:
F  ma
Fnet  ma
kg
m/s2
kg • m/s2 = Newton = N
A medium-sized
apple weighs
about one Newton.
1 N = .22 lbs.
Let’s find the magnitude of the acceleration for this
box if the following forces are applied:
FN
F1 = 20 N
a
F2 = 40 N
10 kg
Fg
a=?
Fnet = ΣF = 20 N - 40 N = 20 N
m = 10 kg
kg  m s2
F
20 N
2

a
=
10
kg
kg
m
This leads us to the Fg:
Fg  ma
g = 9.81 m/s2 ~ 10 m/s2
What’s mass times the acceleration due to gravity?
THIS IS WEIGHT
w  mg
Use the weight equation to find your mass:
w  mg
This needs to be in N. Use 1 N = .22 lbs
w
m
g
Let’s go back and talk about our box again
this time analyzing all the forces:
FN – EQUAL but OPPOSITE to the Fg = 100 N
F1 = 20 N
F2 = 20 N
10 kg
Fg = ma = mg = 100 N
Now we are dealing with forces acting in two directions (x and y).
ΣFy = Fnety = Fg – FN = 100 N - 100N = 0
ΣFx = Fnetx = F1 – F2 = 20 N - 20N = 0
This box is not moving or changing
direction.
Practice with Newton’s 2nd:
1. A tractor pulls a loaded wagon with a
constant force of 400 N. If the total mass
of the wagon is 200 kg, what is the
wagon’s acceleration?
2. A broken down car is being pushed to the
side of the road with a force of 200 N
which is causing it to accelerate at .2 m/s2.
What is the mass of the car?
4. A student weighs 600 N. What is his
mass?
3. The car below was moving with an initial
velocity of 50 m/s until F2 was applied to
slow the car down. What is the
acceleration of the box?
F1 = 300 N
F2 = 500 N
10 kg
How far did the box travel in 10 s?
Remember this guy?
He was pushing that car with a force of 200
N. With how much force is the car pushing
on him?
Let’s go back to our box example…
FN
10 kg
Fg
The force of gravity it pulling the box downward with a
force of 100 N.
But the table is also pushing back on the box with the
same amount of force in the opposite direction.
This brings us to Newton’s 3rd Law:
For every action there is an equal and
opposite reaction (action/reaction law).
http://www.youtube.com/watch?v=mNM5tHou4IQ
http://www.youtube.com/watch?v=7y7KjanpWOk
The air is moving out of the balloon
in this direction causing it to move in
the opposite direction.