What is a force?
A force is a push or
pull, or an action that
has the ability to
change motion.
Forces can increase or
decrease the speed of a
moving object.
Forces can also change
the direction in which an
object is moving.
5.1 How are forces created?
Forces are created in many ways.
For example, your muscles create force
when you swing a baseball bat.
Two Kinds of Forces
1.Contact-actual
touching
2. Long-Range—
forces acting
at a distance
Frictional Force
Tensional Force
Normal Force
Air Resistance
Force
Applied Force
Spring Force
A Gravitational
B. Electromagnetic
C. Nuclear
5.1 How forces act
One way forces act is the
result of direct contact.
A contact force is
transmitted by matter
directly touching other
matter such as wind
acting to slow a
parachute.
5.1 How forces act
The force of gravity
between Earth and Moon
appears to be what
people once called
“action-at-a-distance”.
Today we know that the
gravitational force is
carried from the Earth to
the Moon by a force field.
Four Elemental
Forces
All forces in the
universe come
from only four
basic forces.
Electromagnetic
forces are
important to
technology.
Gravity is a
universal force.
Classify these forces as contact
forces or the result of force fields.
Effects of Forces
Forces can cause objects to
 Change speed
 Change velocity
 Change shape
Measuring Forces
To measure a force, we can use
 Spring scales
 Force gauges
What are the units for
FORCE?
Newtons-abbreviated (N)
-metric unit for force
Pounds-abbreviated (lb)
-English unit for force
5.1 Units of force
The pound is a unit of force commonly
used in the United States.
For smaller amounts, pounds are
divided into ounces (oz.).
There are 16 ounces in 1 pound.
5.1 Newtons
Although we use pounds all the time in
our everyday life, scientists prefer to
measure forces in newtons.
The newton (N) is a metric unit of force.
5.1 Unit conversions
The newton (N) is a smaller unit of
force than the pound (lb).
If one pound of force equals 4.448
newtons, then a 100 lb person weighs
444.8 newtons.
5.1 Contact forces from ropes
and springs
Ropes and springs are often used to
make and apply forces.
Ropes are used to transfer forces or
change their direction.
The pulling force carried by a rope is
called tension.
Tension always acts along the
direction of the rope.
5.1 Spring forces
The force created by a
spring is proportional
to the ratio of the
extended or
compressed length
divided by the original
(resting) length.
If you stretch a spring
twice as much, it
makes a force that is
twice as strong.
Vectors
(Representing forces)
Vectors or scalars are used to represent
quantities in physics. We will use vectors
to represent forces.
A scalar is a line segment, whose length
represents only magnitude (amount)
A vector is an “arrow” whose length
represents magnitude and the direction it
points represents the direction the force
acts.
5.1 The force vector
The direction of a force makes a big
difference in what the force does.
That means force is a vector, like
velocity or position.
Arrows are often used to show the
direction of forces in diagrams.
Drawing Vectors
Length represents magnitude, or
amount
Direction is direction of force
Draw a vector representing a force
of 25 Newtons acting to the right.
5.1 Drawing a force vector
The arrow points in the direction of
the force.
Drawing Vectors
Length represents magnitude, or
amount
Direction is direction of force
Draw a vector representing a force
of 25 Newtons acting to the right.
Draw a vector representing a force of
50 N acting to the left
Resultant Force
Sometimes, forces act together. When they do,
it is as if they were equal to just one force
A NET FORCE is the force
that results from the
addition of two or more
forces.
VA
VB
VR
Tip to Tail Method
Place the tail of one vector at the
tip of the other.
The vector sum (also called the
resultant) is shown in red. Draw
resultant vectors from beginning
(tail) first vector to end (tip) of last
vector.
The resultant is also called the
“NET FORCE”
21 N
12 N
9N
12 N
9N
20 N
16 N
20 N
16 N
4N
Draw the Vectors…..
 Two workers are moving a
refrigerator. One pushes
forward with a force of
200 newtons, the other
pushes forward with a
force of 300 newtons.. The
net force they (together)
exert on the refrigerator
is:?
Draw the Vectors…..
Two students are moving a
refrigerator. One pushes forward with
a force of 200 newtons, the other
pushes backward with a force of 300
newtons. Force is a vector. The total
force they (together) exert on the
refrigerator is:
Forces in Combination
Forces acting in same direction
are added together to find “Net
Force”
Forces acting in opposite
directions are subtracted to find
“Net Force”
Equilibrium
If the net force acting on an
object is equal to zero, the object
is said to be in equilibrium, and
the forces are “balanced”
Equilibrium
Unbalanced
Forces
When net force
acting on an
object, the forces are
“Unbalanced”
Unbalanced forces cause objects
to change their motion, either by
changing their speed or velocity
What will happen to the load?
Gravitational Force
The attractive
force acting
between all
masses
This is a major
force but it is
Factors effecting the Stength of a
gravitational attraction
Masses of two masses
Larger masses =
greater
attraction
Distance between the
two masses
Larger distances
between masses =
less attraction
5.1 Gravity
 The force of gravity on an object
is called weight.
At Earth’s surface, gravity exerts a
force of 9.8 N on every kilogram of
mass.
5.1 Weight vs. mass
Weight and mass are not the same.
Mass is a fundamental property of
matter measured in kilograms (kg).
Weight is a force measured in
newtons (N).
Weight depends on mass and
gravity.
Weight depends on mass and gravity
A 10-kilogram rock has the same mass no matter where it
is in the universe. On Earth, the10 kg. rock weighs 98 N..
On the moon, the same rock only weighs 16 N.
5.1 Calculating weight
Solving Problems
 Calculate the weight of a 60-kilogram
person (in newtons) on Earth and on
Mars.
1. Looking for:




…weight of person in newtons on both planets
Given:
…mass = 60 kg; g = 3.7 N/kg on Mars;
…implied g = 9.8 N/kg on Earth
2. Relationships:
 W=mxg
3. Solution:
 60 kg x 9.8 N/kg = 588 N
 60 kg x 3.7 N/kg = 222 N
Sig. fig. = 600 N
Sig. fig. = 200 N
Would you weigh more or
less on a mountain
compared to a plain?
Ocean Tides
The Moon is primarily responsible
for ocean tides on Earth.
The Sun contributes to tides also.
What are spring tides and neap
tides?
Spring Tides
Full Moon
Earth
New Moon
Sun
Neap Tides
First Quarter
Earth
Last Quarter
Sun