Chapter 4:
Forces and the
Laws of Motion
4.1 Changes in Motion
 Force
is a vector quantity that causes
changes in motion.
 Force can act either through physical
contact of two objects or at a distance


Contact Force
Field Force
Free Body Diagram
A
Free Body Diagram shows only the
forces that act on one object. These are
the only forces that affect the motion of
that object.
 Practice:
The Physics Classroom - Free Body Diagrams
On whiteboards, draw diagrams.
2.2 Newton’s First Law
 Inertia
is the tendency of an object to
maintain its state of motion.


Object at rest stays at rest
Object in motion stays in motion
 Mass
is a measure of inertia.
Net External Force
 The
net external force acting on an
object is the vector sum of all the forces
acting on it.
 An object is in equilibrium when the net
external force acting on it is zero.
 Practice:


The Physics Classroom - Net Force
Solve for Net Force
More Net Force


Often the net force does not lie on the x or y
axis.
Steps in solving Net Force:




Draw a free body diagram and label.
Draw the resultant vector for the “triangle”.
Use the Pythagorean Theorem to solve for the
resultant vector force (d2=x2+y2)
Use the tangent function to find the angle of
the resultant vector and the horizontal or
vertical axis. Θ= inv. tan (opposite
side/adjacent side)
Problem #3 – Example

A gust of wind blows an apple from a tree. As
the apple falls, the force of gravity on the
apple is 9.25 N downward, and the force of
the wind on the apple is 1.05 N to the right.
Find the magnitude and direction of the net
external force on the apple. (Answer: 9.3 N, 6°
E of vertical)




Draw Free Body Diagram with forces labeled.
Draw the triangle
Use the Pythagorean Theorem to find “d”
Use the tan function to find the angle with
respect to the y axis.
You Try It! Problem 4
 The
wind exerts a force of 452 N north on
a sailboat, while the water exerts a force
of 325N west on the sailboat. Find the
magnitude and direction of the net
external force. (Answer: 557 N, 35° W or N)
Newton’s Second Law
 Force
is proportional to mass and
acceleration
 ΣF= ma
 Net external force = mass x acceleration
Example Problem:
 The
net external force on the propeller of
a 3.2 kg model airplane is 7.0 N forward.
What is the acceleration of the
airplane?(Answer: 2.2 m/s2)
 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?
Newton’s Third Law
 For
every action there is an equal and
opposite reaction.
 The action-reaction pair objects mutually
exert force on each other