Download Newton*s 2nd and 3rd Laws

Newton’s 2nd and 3rd Laws
Objectives
1. Describe the acceleration of an object in terms of its
mass and the net external force acting on it.
2. Predict the direction and magnitude of the
acceleration caused by a known net external force.
3. Identify action-reaction pairs.
4. Explain why action-reaction pairs do not result in
equilibrium.
Net Force
• If there is NO NET FORCE on an object, then it is at
EQUILIBRIUM and either:
MOTIONLESS OR MOVING WITH CONSTANT VELOCITY
• So a “net” or “unbalanced” force will
– CHANGE AN OBJECT’S VELOCITY
• Changing velocity means ACCELERATION
A net force (an unbalanced force) causes an acceleration
Description of Motion
Net Force: Yes or No?
yes
yes
no
no
yes
yes
Newton’s 2nd Law
Force  Acceleration
• How much acceleration?
• Depends on:
– AMOUNT OF FORCE
• MORE FORCE = MORE ACCELERATION
• Acceleration is directly related to force
– MASS OF OBJECT
• MORE MASS = LESS ACCELERATION
• Acceleration is inversely related to mass
Newton’s Second Law
“The acceleration of an object is directly proportional to
the net external force acting on the object and inversely
proportional to the mass of the object.”
F
a
m
Unit of force is the NEWTON (N)
Fnet
a
m
a
a
F
m
Fnet
a
m
• If mass is held constant,
• doubling of the net force results in …
• a doubling of the acceleration,
• halving of the net force results in …
• a halving of the acceleration.
• If force is held constant,
• doubling of the mass results in …
• a halving of the acceleration
• halving of the mass results in …
• a doubling of the acceleration.
Example
• A 2 kilogram box is pushed with a net,
unbalanced force of 10 newtons.
• What is the acceleration experienced by the
box?
a = Fnet / m
a = (10 N) / (2 kg)
a = 5 m/s2
The Big Misconception
• The most common misconception is one
that dates back for ages; it is the idea
that sustaining motion requires a
continued force.
• Newton's laws declare loudly that a net
force (an unbalanced force) causes an
acceleration;
Are You Infected with the Misconception?
• Two students discussing an object that is being acted upon by
two individual forces as shown. During the discussion, Anna
Litical suggests to Noah Formula that the object under
discussion could be moving.
• Noah Formula objects, arguing that the object could not have
any horizontal motion if there are only vertical forces acting
upon it.
• Who do you agree with?
Class work
• Read page 137 Sample Problem 4B
• Page 138 – practice 4B
• Page 138 – conceptual challenge
Newton's Third Law
• For every action, there is an equal and
opposite reaction.
• Forces always come in pairs - equal and
opposite action-reaction force pairs.
• Examples:
– The propulsion of a fish through the water.
– The flying motion of birds.
– The motion of a car on the way to school.
Third Law Examples
• A firefighter directs a stream of water from
a hose to the east. In what direction is the
force on the hose?
There will be a force on the hose to the WEST
• A man getting out of a rowboat jumps
north onto the dock. What happens to the
boat?
The boat will move to the SOUTH
Identifying Action and Reaction Force Pairs
• Identifying and describing action-reaction force pairs is a simple
matter of identifying the two interacting objects and making
two statements describing who is pushing on whom and in what
direction.
Action: air pushing
the balloon out.
Action: bat hit the ball
Action: ball hit the mitt
Reaction: ball hit the bat
Reaction: mitt hit the ball
Reaction: balloon
pushing air in.
Action/reaction forces vs. equilibrium
forces
• Action and reaction
forces don’t cancel out
because they act on
different bodies.
Force on the car
• Equilibrium forces
act on same body.
FN
Fg
Force on the ground
Check Your Understanding
1. While driving down the road, a firefly strikes the
windshield of a bus and makes a quite obvious mess
in front of the face of the driver. This is a clear case of
Newton's third law of motion. The firefly hit the bus
and the bus hits the firefly. Which of the two forces is
greater: the force on the firefly or the force on the
bus?
2. For years, space travel was believed to be impossible
because there was nothing that rockets could push
off of in space in order to provide the propulsion
necessary to accelerate. This inability of a rocket to
provide propulsion is because ...
a. ... space is void of air so the rockets have nothing to
push off of.
b. ... gravity is absent in space.
c. ... space is void of air and so there is no air
resistance in space.
d. ... nonsense! Rockets do accelerate in space and
have been able to do so for a long time.
3. Many people are familiar with the fact that a rifle recoils when
fired. This recoil is the result of action-reaction force pairs. A
gunpowder explosion creates hot gases that expand
outward allowing the rifle to push forward on the bullet.
Consistent with Newton's third law of motion, the bullet
pushes backwards upon the rifle. The acceleration of the
recoiling rifle is ...
a. greater than the acceleration of the bullet.
b. smaller than the acceleration of the bullet.
c. the same size as the acceleration of the bullet.
Class work
• Page 140 – section review
• 4-3 section review