-Newton’s Laws
AP Physics
Mrs. Coyle
Early History

Greece
◦ Aristotle (4th Century BC)
Natural and violent motion

Galileo Galilei (1564 AD)
◦ Is a force necessary in order to keep an
object moving?
Galileo’s Experiment with Ball

A force is not required to keep the ball moving on a horizontal
plane.

In the absence of friction, the ball would move perpetually.
Newton’s First Law

Isaac Newton -England (1561)
1st Law – Law of Inertia
An object at rest or moving at constant
velocity will continue to be at rest or at
constant velocity, unless acted upon by a
net force.

Inertia---Mass

Inertia: the resistance of an object to
a change in its state of motion

Mass: amount of matter in an object
◦ a measure of inertia
◦ unit: kg
◦ scalar
Seatbelts!!!!!!!
(http://www.glenbrook.k12.il.us/gbssci/phys/m
media/newtlaws/cci.html)
An airplane drops a flare. Which
is the path of the flare?
v
A
v
B
C
Force: a push or a pull

Vector

Unit: Newton N
1N= 1kg m/s2

U.S. Customary unit for force:
1 lb= slug ft/ s2
1N ~ ¼ lb
The 4 Fundamental Forces
Gravitational (attractive force
between masses)
 Electromagnetic (caused by to
electric and magnetic fields)
 Strong Nuclear Force (holds nuclear
particles together)
 Weak Nuclear Force (involved in
radioactive decay of nuclei)

Types of Forces

Contact Forces (Tension, Normal)

Field Forces (Electrical, Gravitational)
Friction:
a force acting when two surfaces rub
against each other.
◦ Friction opposes motion.
◦ Is caused by the irregularities in surfaces.
Normal Force:
a force acting perpendicular to the
interface of contact between two
objects.
Newton’s 2nd Law
∑F=ma
Weight:
the gravitational force with which
the
earth
pulls an object

vector

unit: N


W=mg
Mass

Inertial Mass (from F=ma)

Gravitational Mass (from W=mg)
Free Body Diagram:
Example 1 (#3)
A 3kg object undergoes an acceleration
given by a=(2.00 i + 5.00j) m/s2. Find the
magnitude of the resultant force.

Answer: 16.2N
Example 2
A 2500kg SUV traveling at 30km/h hits a
foam barrier and is stopped by a net force
of 2000N.
Find the distance the car traveled while
being stopped by the barrier.
Newton’s 3rd Law
When an object exerts a force on a
second object (action), the second
objects exerts an equal and opposite
force on the first object (reaction).
 To every action there is an equal and
opposite reaction.
 Forces occur in pairs.

What is the reaction force to:
 Kicking
 The
a soccer ball?
windshield of a bus hitting a
fly?
 The
swimmer pulling the water?
Can you hit a piece of paper
with a large force with your
hand?
What are the reaction forces to
the each of the forces shown?
Example 1
Two blocks, one 8 times as massive as the
other, are connected by a compressed spring
(sitting on a frictionless surface). When the
spring is released the blocks fly apart.
a) Draw the free body diagram showing the
forces acting on each object.
b) What is the acceleration of the lighter block
relative to the heavier one?
Example 2
Two skaters, a girl of mass 45kg and a
65 kg boy push off of each other. The
boy accelerates at 2m/s2. Find the girl’s
acceleration.
Example 3
A student of mass 55kg is standing on
the surface of the earth.
a)What is the force that the earth pulls
the student with?
b)What is the force with which the
student pulls the earth?
c)What is the acceleration of the earth
because of the pull of the student?