Transcript Document

Force
&
Newton’s Laws
Force
A force is a push or pull upon an object resulting from
the object's interaction with another object
• Forces only exist as a result of an interaction.
• All forces (interactions) between objects can be placed
into two broad categories:
•contact forces
•forces resulting from action-at-a-distance
Contact Forces
Action-at-a-Distance Forces
Frictional Force
Gravitational Force
Normal Force
Electrical Force
Air Resistance Force
Magnetic Force
How do we Measure Force?
Force is a quantity which is measured using a standard metric unit
known as the Newton.
• One Newton is the amount of force required to give a 1-kg mass
an acceleration of 1 m/s2.
•A Newton is abbreviated by an "N." If you say "10.0 N," you
mean 10.0 Newtons of force.
• To fully describe the force acting upon an object, you must
describe both its magnitude (size) and its direction.
Newton’s Laws of Motion
Isaac Newton (a 17th century scientist) put forth
three laws which explain why objects move (or
don't move) as they do and these three laws have
become known as Newton's three laws of motion.
Newtons 1st Law
“The Law of Inertia”
An object at rest tends to stay at rest and an object in motion
tends to stay in motion with the same speed and in the same
direction unless acted upon by an unbalanced force.
Newtons 1st Law - “The Law of Inertia”
What does this mean?
This means that there is a natural tendency of objects to
keep on doing what they're doing. All objects resist changes
in their state of motion. In the absence of an unbalanced
force, an object in motion will maintain this state of motion.
What is the motion in this picture?
What is the unbalanced force in this picture?
What happened to the skater in this picture?
Newton’s 1st Law
Inertia and Mass
•Inertia is the resistance an object has to a change in its
state of motion.
•All objects resist changes in their state of motion, they have
inertia.
•The tendency of an object to resist changes in its state of
motion is dependent upon its mass.
•The more mass an object has, the more inertia it has – the
more it will resist changes in its state of motion.
Slide a book across a table and it will slide to a stop. The book in
motion on the table top does not stop because of the absence of a
force; it is the presence of a force – the force of friction – which
brings the book to a stop. Without the force of friction, the book
would continue in motion with the same speed and in the same
direction – forever! A force is not required to keep a moving book in
motion; it is the force of friction that causes the book to stop.
Newtons 1st Law
“The Law of Inertia”
If the truck were to abruptly stop and the straps were no
longer functioning, then the ladder in motion would continue
in motion. Assuming a negligible amount of friction between
the truck and the ladder, the ladder would slide off the top of
the truck and be hurled into the air.
Check Your Understanding
Read the following question and predict the answer.
1. Imagine a place in the cosmos far from all gravitational and
frictional influences. Suppose an astronaut in that place throws
a rock. The rock will:
a) gradually stop.
b) continue in motion in the same direction at constant speed.
Check Your Understanding
Read the following question and predict the answer.
Mac and Tosh are arguing in the cafeteria. Mac says that if he
throws his jello with a greater speed it will have a greater inertia.
Tosh argues that inertia does not depend upon speed, but rather
upon mass.
With whom do you agree? Why?
Check Your Understanding
Read the following question and predict the answer.
If you were in a weightless environment in space, would it require
a force to set an object in motion?
Check Your Understanding
Read the following question and predict the answer.
Mr. Wegley spends most Sunday afternoons at rest on the
sofa, watching pro football games and consuming large
quantities of food.
What effect (if any) does this practice have upon his inertia?
Explain.
Check Your Understanding
Read the following question and predict the answer.
Ben Tooclose is being chased through the woods by a bull
moose which he was attempting to photograph. The
enormous mass of the bull moose is extremely intimidating.
Yet, if Ben makes a zigzag pattern through the woods, he will
be able to use the large mass of the moose to his own
advantage.
Explain this in terms of inertia and Newton's first law of
motion.
Newton’s 1st Law
“State of Motion”
REVIEW:
Inertia is the resistance an object has to a change in its “state
of motion”.
“The state of motion” of an object is defined by its velocity
• its speed in a given direction.
Inertia is the tendency of an object to resist changes in its
velocity.
Newton’s
nd
2
Law
The Equation Law
The acceleration of an object is dependent upon two
variables – the net force acting upon the object and the mass
of the object.
In terms of an equation, the net force is equal to the product of
the object's mass and its acceleration.
Fnet = M x A
Newton’s 2nd Law… F = MA
Everyone knows that heavier objects require more force to move
the same distance as lighter objects.
F = MA
Check Your Understanding
F = MA
1. What acceleration will result when a 12-N net force is applied to
a 3-kg object? A 6-kg object?
2. A net force of 15 N causes a mass to accelerate at the rate of 5
m/s2. Determine the mass.
F = MA
Newton’s 3rd Law
Action/Reaction Law
"For every action, there is an equal and opposite reaction."
• Forces always come in pairs – equal and opposite
action-reaction force pairs.
Example
A fish uses its fins to push water backwards. But a push on
the water will only serve to accelerate the water. In turn, the
water reacts by pushing the fish forwards, propelling the fish
through the water. The size of the force on the water equals
the size of the force on the fish; the direction of the force on
the water (backwards) is opposite to the direction of the force
on the fish (forwards).
Newton’s 3rd Law
This means that for every force there is a reaction force that is
equal in size, but opposite in direction. That is to say that
whenever an object pushes another object it gets pushed back in
the opposite direction equally hard.
Would it be a good idea to jump from a rowboat to a dock
that seems within jumping distance? Explain.
Check Your Understanding
1. While driving, Anna Litical observed a bug striking the
windshield of her car. Obviously, a case of Newton's third law of
motion. The bug hit the windshield and the windshield hit the
bug. Which of the two forces is greater: the force on the bug or
the force on the windshield?
Check Your Understanding
2. Rockets are unable to accelerate in space because ...
a. there is no air in space for the rockets to push off of.
b. there is no gravity in space.
c. there is no air resistance in space.
d. ... nonsense! Rockets do accelerate in space.
Check Your Understanding
3. In the top picture, a physics student is pulling upon a rope
which is attached to a wall. In the bottom picture, the physics
student is pulling upon a rope which is held by the Strongman.
In each case, the force scale reads 500 Newtons. The physics
student is pulling
a. with more force when the rope is attached to the Strongman.
b. with the same force in each case.
c. with more force when the rope is attached to the wall.
Check Your Understanding
Consider the interaction depicted below between foot A, ball B,
and foot C. The three objects interact simultaneously (at the
same time). Identify the two pairs of action-reaction forces.
Use the notation "foot A", "foot C", and "ball B" in your
statements.
Newton's 1st Law
An object at rest tends to stay at rest and an object in motion
tends to stay in motion with the same speed and in the same
direction unless acted upon by an unbalanced force
Explains the behavior of objects for which all existing forces
are balanced.
Newton's 2nd Law
The acceleration of an object is dependent upon two variables
– the net force acting upon the object and the mass of the
object.
F = MA
Explains the behavior of objects for which all existing forces
are not balanced.
Newtons’ 3rd Law
"For every action, there is an equal and opposite reaction."
Identifying Action and Reaction Force Pairs
F = MA