Download Forces and The Laws of Motion Newton`s Second and Third Laws

Forces and The Laws of Motion
Newton’s Second and Third Laws
Chapter 4: Section 3
Learning Targets
• Describe an object’s acceleration in terms
of its mass and the net force acting on it
• Predict the direction and magnitude of the
acceleration caused by a known net force
• Identify action-reaction pairs
P3.3A, P3.4A, P3.4 B, P3.4C
Newton’s Second Law
• While Newton’s First Law predicts the
behavior of objects with balanced forces,
Newton’s Second Law applies to the
behavior of objects for which all existing
forces are not balanced
• Newton’s Second Law relates force,
mass, and acceleration
• According to this law, the acceleration of an
object is directly proportional to the net force acting
on the object and inversely proportional to the object’s
mass
– As the force acting upon an object is increased,
the acceleration of the object is increased.
– As the mass of an object is increased, the
acceleration of the object is decreased.
Newton’s First Law
Newton’s Second Law
Second Law Calculations
ΣF = ma
Net force = mass x acceleration
• Sigma (Σ) represents the vector sum of all
external forces acting on the object, or the net
force
 In the following situations, which direction
is the net force acting in?
Units of Force
• Remember that the unit used to measure
force is the Newton (N).
– 1 N = 1 kg * m/s2
• This unit conversion can be explained by
Newton’s Second Law
The Big Misconception
 It is often believed that sustaining motion
requires a continued force
 However, according to Newton’s Second
Law…
“Forces do not cause motion;
forces cause acceleration”
Practice Calculations
• The net force on the propeller of a 3.2 kg model
airplane is 7.0 N forward. What is the acceleration
of the airplane?
• The net force of a golf cart is 390 N north. The
cart has a total mass of 270 kg, what are the
magnitude and direction of the cart’s acceleration?
• A soccer ball kicked with a force of 13.5 N
accelerates at 6.5 m/s2 to the right. What is the
mass of the ball?
Reviewing Force
• A force is a push or a pull upon an object
that results from its interaction with
another object.
– Forces result from interactions
Newton’s Third Law
• Forces always exist in pairs
– For example, when a car exerts a force on a
concrete barrier, the barrier also exerts a force on
the car
• If two objects interact, the magnitude of the
force exerted on object 1 by object 2 is equal
to the magnitude of the force simultaneously
exerted on object 2 by object 1
– These two forces are in opposite directions
• Newton’s Third Law states that for every
action, there is an equal and opposite reaction
– These forces are called action-reaction pairs
• Examples:
– The wings of a bird push air downwards
while the air pushes the bird upward
– Fish fins push water backwards while the
water pushes the fish forwards
Action-Reaction Forces
• The action and reaction forces occur at exactly the
same time and because they coexist, either force can
be called the action or the reaction
• Action and reaction forces each act on different
objects
– Although the action-reaction forces are equal and
opposite, either object may still have a net force acting on
it.
• This makes a change in the state of motion possible
Net Force and Motion
• So how can an object with equal and
opposite forces be in motion?
– If the mass of one object is much larger than
the mass of another, the force exerted by the
larger object would cause the smaller object to
accelerate away
• The force of the smaller object wouldn’t cause
much acceleration at all
• Force and acceleration are related but
they aren’t identical
Field Forces
• Remember that field forces include gravity
and attraction/repulsion forces
– These forces do not involve physical contact
between two objects
• Field forces also exist in pairs
– As a falling object accelerates toward Earth
due to the force of gravity, Earth also
accelerates toward the object
Examples
• Consider the interaction between a
baseball bat and a baseball
• Baseball pushes glove leftwards.
• Bowling ball pushes pin leftwards.
• Enclosed air particles push balloon
wall outwards.