AP B Chapter 4 Newtons Laws PDF

Dynamics- Why do objects move as they do?
What
makes an object at rest, begin to move? What makes a
body accelerate or decelerate? What makes an object
move in a circle?
Force
• A Force is simply a push or a pull.
• A Force gives energy to an object.
• What can that energy do? CHANGE the
motion of an object.
• What do we call a CHANGE in the motion
(speed or direction) of an object?
ACCELERATION
• Forces are vectors with magnitude and
direction.
• Not all objects will move when forces are
applied to them. (push a wall!)
Measuring force
 One way to measure small forces is by
use of a spring scale. As a push or pull
is applied to the spring scale attached to
a block (Or your fruit in the
supermarket), the scale reads the
measure of the force applied by
stretching a spring calibrated in
increments of force (Newtons or Dynes).
Newton’s First Law of Motion
• Isaac Newton (1642-1727) built off the ideas
of Aristotle and Galileo regarding the
relationship between force and motion.
• Aristotle claimed a moving object would
come to rest if left alone.
• Galileo idealized the world to say if friction
were removed, all moving objects would
maintain their motion (speed and direction)
and moving objects slow down only when a
force is exerted on them.
Newton’s First Law
 Newton’s First Law of Motion is very close to
Galileo’s conclusions:
 Every body continues in its state of rest or
uniform speed in a straight line unless acted on
by a nonzero net force.
 The tendency of a body to maintain its state
(rest or motion uniformly) is called inertia.
 Newton’s First Law is often called the Law of
Inertia.
Frames of Reference
 Newton’s first law does not hold for all frames of
reference.
 If you are fixed in a moving car and an object is
resting on the dashboard, it may move toward you
if the car accelerates though neither you nor
anything else touched the cup to make it do that.
 Inertial Reference frames are those frames where
Newton’s first law DOES hold true. Most frames
fixed on Earth are inertial.
 Reference frames where this law DOES NOT hold
true are called noninertial reference frames.
Mass
 Newton used mass as synonym for “quantity

of matter”.
 Quantity of matter is difficult to define.
 More precisely we say mass is the measure
of inertia of an object.
 More mass = more inertia = more difficult to
change motion. (Converse is true also)
Weight is NOT mass
 Though the mass of an object is a
relatively fixed quantity, gravity is always
present…
 The force of gravity acting on a mass
gives the object “weight” which has a
magnitude and direction.
 Weight is a force. Mass responding to
the force of gravity.
 Your mass on the moon vs your weight?
Newton’s Second Law
 Newton’s first law describes an object when
NO NET force acts on it. What happens
when there IS a NET FORCE?
 A net force may increase or decrease the
speed of an object OR change its direction of
motion.
 A net force gives rise to ACCELERATION.
 Greater the net force, greater the
acceleration.
F a
What about mass?
 The greater the mass of an object, the LESS
the acceleration for the same net force.
 Mass and acceleration are inversely related.
1
a
m
Newton’s Second Law summed
up
 “The acceleration of an object is directly
proportional to the net force acting on it and
inversely proportional to its mass. The
direction of the acceleration is in the direction
of the net force acting on the object.”
F
a F ma
m
 ΣF is the vector sum of all forces acting on
the body which we call the Net Force.
Components and units of Forces
• Since a force is an action capable of
accelerating an object, we need to look at each
component of any given force separately.
Fx ma x Fy ma y Fz ma z
• In SI units, with mass in Kilograms, the unit of
force is the Newton (N) where 1N = 1kg*m/s/s
• IN the cgs system, with mass in grams, the unit
of force is the dyne where
1 dyne = 1g*cm/s/s
Ex1: Force to accelerate a fast car
 Estimate the net force needed to accelerate
a 1000-kg car at ½ g.
 Solution: The car’s acceleration is a = ½ g =
½(9.80m/s2 ) ≈5.0m/s/s. We use Newton’s
second law to get the net force needed to
achieve this acceleration.
2
F ma (1000 kg)(5m / s ) 5000 N
Ex2: Force to stop a car
 What net force is required to bring a 1500-kg
car to rest from a speed of 100km/h within a
distance of 55 m?
 Solution: We use Newton’s second law,
ΣF=ma, but first we must determine the
acceleration, a, which we assume is
constant. We assume the motion is along
the +x axis. We are given the initial velocity
v0 = 100 km/h = 28 m/s, the final velocity v =
0, and the distance traveled x-x0 = 55 m.
Carry it out…
 From equation 2-10c, we have
2 2
v v 0 2a ( x x0 )
 So we can rearrange and solve for a we get:
2 2 2
v v 0 0 (28m / s) 2
a 7.1m / s
2( x x0 ) 2(55m)
 The net Force required is then
F ma (1500 kg)( 7.1m / s 2 ) 1.1x10 4 N
 Force is exerted in the opposite direction of

velocity which is what the negative sign tells.
Newton’s Third Law of Motion
 In any interaction between objects, a force is
exerted on an object by another object.
 Forces come in pairs.
 Newton’s Third law of motion states “whenever
one object exerts a force on a second object, the
second exerts an equal and opposite force on the
first.” Consider a hammer and a nail: The hammer
exerts a force on the nail and at the same time the
nail exerts an equal and opposite force on the
hammer.
Newton’s Third Law
 Sometimes this law is paraphrased as “to
every action there is an equal and opposite
reaction.”
 It is important to realize the “action” force
and “reaction” force are acting on different
objects.
Which force is which?
 Consider an ice skater. Since there is very little
friction between the skates and the ice, she will
move freely if a force is exerted on her. If she
pushes on the railing, she moves backwards.
Does her push on the railing cause this? Or is it
the reaction force that causes her acceleration?
Remember, an object accelerates in the
direction of the net force acting on it…
Your turn to Practice

AP B Chapter 4 Newtons Laws PDF

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Dynamics- Why do objects move as they do?

 Newton used mass as synonym for “quantity

 Force is exerted in the opposite direction of

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