Download Balanced Forces

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 Taking
ownership or control of the
information learned in class and taking
responsibility for your work

This means:
• Practicing (reviewing, restating, preparing)
• Applying (creating, thinking, using)
• Planning (studying, completing projects and
homework, being prepared-pencils, books)
Forces & Motion
Chapter 2
Force

Force
a push or pull that one body exerts
on another
 What forces are being
exerted on the football?

Fkick
Fgrav
Measuring Forces
 Forces
are measured
in Newtons (kg .
m/s2).
 Forces are measured
using a spring scale
or force meter.
Force

Balanced Forces


forces acting on
an object that are
opposite in
direction and
equal in size
no change in
velocity
Balanced Forces
(Balanced Forces = No motion)
5N
Object
Notice that all the forces are equal and pointed
in the opposite direction. Hence they balance
each other – or cancel each other.
5N
Equal Pushing
Forces may cancel each other and
produce No Motion!
Balanced Forces
(Balanced Forces = No motion)
5N
5N
Equal Pushing
Equal Pulling
5N
5N
Forces may cancel each other and
produce No motion!
Force
 Net
or unbalanced Force
 unbalanced forces that are not opposite
and equal
 velocity changes (object accelerates)
Fnet
Ffriction
Fpull
N
N
W
Unbalanced Forces
Causes Motion
Adding Forces
Subtracting Forces
Unbalanced Forces
Causes motion
Adding Forces
5N
Object
5N
Notice that all the forces are pointed in the
same direction. Hence they add together.
Subtracting Forces
10 N
=5N 5N
• Two forces can subtract to produce a net
force in the direction of the larger force.
– Negative motion occurs (Slow Down)
Unbalanced Forces
Causes Motion
Subtracting Forces
5N
Object
5N
Notice that all the forces are unequal and
pointed in the opposite direction. Hence they
are unbalanced and in opposition to each
other – or one partially cancels the other.
5N
Balanced Forces
10N
10N
10N
Balanced
Push
i.e. Pushing a
Car
10N
Balanced
Pull
No motion
i.e. Tug-of-war
No motion
Un-Balanced Forces
Additive
10N
10N
Subtractive
10N
10N
10N
Un-Balanced
Same
Direction
i.e. sled
Faster
motion
Un-Balanced
Opposite
Direction
i.e. football
tackle
Slower
motion
Friction

Friction
 force that opposes motion between 2 surfaces
 depends on the:
• types of surfaces
• force between the surfaces
Friction

Friction is greater...
 between rough surfaces
 when there’s a greater force
between the surfaces
(e.g. more weight)
Universal Law of Gravitation
 Mass
and gravity have a directly
proportional relationship.
 The
distance between the centers of
masses of the two objects and gravity
have an inversely proportional
relationship.
Gravity

Gravity


force of attraction between any two
objects in the universe
increases as...
• mass increases
• distance decreases
Gravity

Who experiences more gravity - the astronaut or
the politician?

Which exerts more gravity the Earth or the moon?
less
distance
more
mass
Gravity

Would you weigh more on Earth or
Jupiter?

Jupiter because...
greater mass
greater gravity
greater weight
Balanced Forces
(Balanced Forces = No Motion)
Ground pushes up
Gravity pulls down
Gravity pulls down on you…
The ground pushes back up…
THIS KEEPS YOU WHERE YOU ARE!
If these football players push
on each other equally as hard,
will either one move?
Newton’s First Law
 Newton’s

First Law of Motion
An object at rest will remain at
rest and an object in motion
will continue moving at a
constant velocity unless acted
upon by a net or unbalanced
force.
Two Key Points
Newton’s First Law

Newton’s First Law of Motion


“Law of Inertia”
Inertia
 tendency of an object to resist any change
in its motion
 increases as mass increases
Can you explain how this trick
works, using newton’s first law of
motion?
Newton’s Second Law

Newton’s Second Law of Motion

The acceleration of an object is directly proportional
to the net force acting on it and inversely
proportional to its mass.
F = ma
Newton’s Second Law
F
a
m
F = ma
F
m a
F: force (N)
m: mass (kg)
a: accel (m/s2)
1 N = 1 kg ·m/s2
Calculations

What force would be required to accelerate a 40
kg mass by 4 m/s2?
GIVEN:
WORK:
F=?
m = 40 kg
a = 4 m/s2
F = ma
F
m a
F = (40 kg)(4 m/s2)
F = 160 N
Calculations

A 4.0 kg shotput is thrown with 30 N of force.
What is its acceleration?
GIVEN:
WORK:
m = 4.0 kg
F = 30 N
a=?
a=F÷m
F
m a
a = (30 N) ÷ (4.0 kg)
a = 7.5 m/s2
ConcepTest
 Is

the following statement true or false?
An astronaut has less mass on the moon
since the moon exerts a weaker gravitational
force.

False! Mass does not depend on gravity, weight
does. The astronaut has less weight on the
moon.
Newton’s Third Law
 Newton’s

Third Law of Motion
When one object exerts a force on a
second object, the second object exerts
an equal but opposite force on the first.
Newton’s Third Law

Action-Reaction Pairs

The hammer exerts a
force on the nail to the
right.

The nail exerts an equal
but opposite force on
the hammer to the left.
Newton’s Third Law

Action-Reaction Pairs
 The rocket exerts a
downward force on the
exhaust gases.
 The gases exert an equal
but opposite upward force
on the rocket.
FG
FR
Newton’s Third Law

Action-Reaction Pairs

Both objects accelerate.

The amount of acceleration depends on the
mass of the object and the force applied.

Force = mass x acceleration F=ma

Small mass  more acceleration

Large mass  less acceleration