Download Forces

MYIB / Honors Physics
• A force is by definition a push or a pull
• Unit is the newton (N)
• Can exist during physical contact
(“contact forces”):
• tension, friction, applied force, etc.
• Can exist with no physical contact, called
“field forces” or “non-contact forces”:
• gravitational, electric, etc.
When forces act on an object they are said
to be balanced or unbalanced.
Balanced Forces are equal in magnitude AND
opposite in direction.
5N
10 N
10 N
5N
10 N
5N
10 N
5N
When balanced forces act on an object the
resultant is always zero.
When balanced force act, the object is in a state
of equilibrium.
Q: What is the magnitude of the unknown force
that will cause equilibrium?
25 N
?N
25 N
?N
15 N
25 N
a.
b.
c.
d.
25 N
50 N
35 N
0N
a.
b.
c.
d.
15 N
40 N
10 N
0N
What effect will balanced forces have on the
motion of an object?
NONE!
When balanced forces act on an object,
there is no change in the motion of the
object.
Unbalanced Forces are not equal in magnitude
OR not opposite in direction.
5N
10 N
5N
3N
5N
4N
Q:What is the resultant force?
Q:What is the resultant
a. +15 N
Q:What isforce?
the magnitude of the
a. +5 N forces
b. -5
N
resultant
force?
When
unbalanced
act
on an object
b.
0
N
c. +5
N resultant is always
a. 5nonzero.
N
the
c. -5 N
d. -15 N
b. 7 N
+10 Nis called
This resultant d.force
c. +1 N the net force.
d. -1 N
What effect will unbalanced forces have on the
motion of an object?
Motion will change!
How could the motion change?
 speed up
 slow down
 change direction
What one word describes all three of theses
changes?
Acceleration!
An net force will cause a mass to accelerate.
Suppose you are at a hockey game when you see a
player strike the puck with his stick. As the puck
slides across the ice, consider the following:
What force causes the puck to start moving?
(a)
(b)
(c)
(d)
The force from the stick.
The force from the player.
The force from the ice.
No force is needed.
Contact / Non-contact
Suppose you are at a hockey game when you see a
player strike the puck with his stick. As the puck
slides across the ice, consider the following:
What force keeps the puck moving as it slides
across the ice?
(a)
(b)
(c)
(d)
The force from the stick.
The force from the player.
The force from the ice.
No force is needed.
Newton’s First Law of Motion
An object in motion will remain in motion,
and an object at rest will remain at rest,
unless acted on by an outside force.
unbalanced
Examples:
 A puck sliding across the ice (constant velocity)
 A spaceship traveling through space (constant velocity)
 A box sitting on a table (at rest)
The tendency of an object to resist a change in
its motion is called inertia.
Newton’s First Law (Inertia)
 A truck is harder to set in motion than a baseball
because it has more inertia.
 The more massive object is said to have more inertia.
Newton’s Second Law
When a net force acts on an object, the object will
accelerate with an acceleration directly proportional to
the strength of the force and inversely proportional to
the object’s mass.
F  ma
5000 kg
1 kg
A Special Case (Newton’s
nd
2 )
 The force of gravity has a special name: “weight”.
 An object’s weight is equal to the force of gravity
on the object. It is equal to its mass times the
acceleration due to gravity.
W  mg
5000 kg
1 kg