Download Section 4.1 Force and Motion

Mr. Borosky
Physics Section 4.1 Notes
Page 1 of 4
Chapter 4 Forces in One Dimension
In this chapter you will:
Use Newton’s laws to solve problems.
Determine the magnitude and direction of the net force that
causes a change in an object’s motion.
Classify forces according to the agents that cause them.
Sections
Section 4.1: Force and Motion
Section 4.2: Using Newton's Laws
Section 4.3: Interaction Forces
Section 4.1 Force and Motion
Objectives
Define force.
Apply Newton’s second law to solve problems.
Explain the meaning of Newton’s first law.
Read intro paragraph p. 87
Additional Info from old book
Kinematics – study of how objects move
Dynamics – study of why objects move as they do
The causes of acceleration were first studied by Sir Isaac
Newton. The connection between acceleration and its causes are
summarized in Newton’s 3 Laws of Motion.
Newton’s Laws of Motion – laws relating force and acceleration.
FORCE AND MOTION
Read section.
Force – a push or a pull. Forces can cause objects to speed up,
slow down, or change direction as they move. They are Vector
quantities.
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 4.1 Notes
Page 2 of 4
A force exerted on an object causes that object’s velocity to
change, that is, a FORCE CAUSES AN ACCELERATION.
The symbol F is a vector and represents the size and direction of a
force, while F represents only the magnitude.
System – the object of interest.
External World – everything around the object that exerts forces on
it.
CONTACT FORCES AND FIELD FORCES
Read Section.
Contact Forces – forces that exist when an object from the external
world touches a system and thereby exerts a force on it.
Field Forces – forces that exist without being in contact.
example, is gravity.
One
Forces result from interactions.
Agent - a specific and identifiable cause of a Force.
A FORCE needs both an 1) Agent and 2) System.
For example, when you push your textbook your hand is the Agent and
the textbook is the system.
Free Body Diagram - A physical model which represents the forces
acting on a system.
Do Practice Problems p. 89 # 1-5
FORCE AND ACCELERATION
Read Section.
Acceleration Force Graph – a graph that compares the Acceleration
(vertical axis) to the Force (horizontal axis). This shows a linear
relationship between Acceleration and Force where the greater Force
is, the greater the resulting Acceleration.
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 4.1 Notes
Page 3 of 4
The slope depends on the mass.
F = ma
(Force = mass * acceleration)
The Force causes the object to Accelerate.
If you double the force you double the acceleration.
If you apply the same force to several different objects, the one
with the most mass will have the smallest acceleration and the one
with the least mass will have the greatest acceleration.
Newton – the unit we use to measure force. It is defined as the
force that causes a mass of one kilogram to accelerate at a rate of
one meter per second squared (m/s2). It is named after Sir Isaac
Newton and it is denoted by the letter capital “N”.
So we have F = ma
=
1 kg (1 m/s2)
=
1 N
COMBINING FORCES
Read Section.
Net Force – the vector sum of all the forces acting on an object.
Go over the examples and Figure 4-5 p. 92
NEWTON’S SECOND LAW
Read Section.
Newton’s Second Law of Motion – also called Law of Acceleration.
The acceleration of an object is equal to the sum of the forces on
it divided by the mass of the object. It states “The acceleration
of an object is directly proportional to the net force on it and
inversely proportional to its mass.” This law can be summarized in
the equation
F = ma
Force is equal to mass times acceleration
The acceleration is in the same direction as the force causing it.
If the force is in the positive direction then the acceleration is
positive. If the force is in the negative direction then the
acceleration is negative.
Physics Principals and Problems © 2005 Started 2006-2007 School Year
Mr. Borosky
Physics Section 4.1 Notes
The larger the mass the smaller the acceleration.
mass the larger the acceleration.
Page 4 of 4
The smaller the
Do Practice Problems p. 93 # 6-8
NEWTON’S FIRST LAW
Read Section.
A stationary object with no net force acting on it will stay at its
position.
Galileo concluded that in the Ideal Case of Zero Resistance,
horizontal motion would never change.
In the absence of a net force the motion (or lack of motion) of both
the moving ball and the stationary object continues as it was.
Newton’s First Law of Motion – also called the Law of Inertia. The
law states the following: “An object at rest will stay at rest, an
object in motion will stay in motion, unless acted on by an outside
force.” Or the old book version is an object with no net force
acting on it moves with constant velocity. Or new book version “an
object that is at rest will remain at rest and an object that is
moving will continue to move in a straight line with constant speed
if and only if the net force acting on the object is zero.”
Inertia – tendency of an object to resist change.
force.
It is not a
A massive body has more inertia than a less massive body.
Equilibrium – the condition in which the net force on an object is
zero. An object is in equilibrium if it is at rest or if it is
moving at a constant velocity.
Newton’s first law identifies a net force as something that disturbs
a state of equilibrium.
Thus if there is no net force acting on the object then the object
does not experience a change in speed or direction and is in
equilibrium.
Table 4-2 p. 94
Do 4.1 Section Review p. 95 # 9-14
Physics Principals and Problems © 2005 Started 2006-2007 School Year