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Forces & Motion
Forces
“Trust the Force Luke”
Forces
The Cause of Forces
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.
How do we label force???
F = ma
Mass is measured in kg
Acceleration is measured in m/sec2
If we multiply these two units :
1 kg m/sec2 = 1 newton (N)
Unit of Force
Force is measured in
newtons (N).
1 newton is the force
needed to cause a 1 kg
mass to accelerate at a
rate of 1 m/s2 .
1 N = 1 kg m/s2 .
5.1 How are forces created?
Forces are created in many ways.
For example, your muscles create force
when you swing a baseball bat.
Four Elemental
Forces
All forces in the universe
come from only four basic
forces.
Strong nuclear force
Electromagnetic force
Weak force
Gravity
Electromagnetic forces are
important to technology.
Gravity is a universal force.
Newton’s Law of Universal Gravitation
Every object in the universe attracts
every other object.
Depends on two factors:
Distance between the objects
Mass of the objects
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 Drawing a force vector
The arrow points in the direction of the
force.
Representing forces
We often use arrows.
Force is a vector.
You can combine force
vectors by adding them.
Larger the arrow, the
greater the force
Representing forces
Combined balanced
forces equal zero
Normal force = the force
acting perpendicular on
an object in contact with
another object.
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.
5.1 How forces act
One way forces act is the
result of direct contact.
Matter directly touching
other matter, such as wind
acting to slow a parachute,
transmits a contact force.
5.1 How forces act
The force of gravity
between Earth and Moon
appears to be what people
once called “action-at-adistance”.
Today, we know that the
gravitational force is
carried from the Earth to
the Moon by a force field.
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.
The winning person pushes harder against the ground
than the losing person.
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.
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
Chapter 5.3 Learning Goals
Determine the net force acting on an
object.
Define equilibrium.
Draw free-body diagrams to represent all
forces acting on a body.
5.3 Forces and Equilibrium
The sum of all the forces on an object is
called the net force.
The word net means total but also means
the direction of the forces has been taken
into account.
In what direction
will this plane go?
5.3 Adding forces
To figure out if or how an object will
move, we look at ALL of the forces
acting on it.
Four forces act on a plane:
1.
2.
3.
4.
weight
drag (air friction)
the thrust of the engines, and
the lift force caused by the flow of air over
the wings.
5.3 Equilibrium
When several forces
+
act on the same object:
1. The net force is zero,
or
2. The net force is NOT +
zero.
5.3 Normal forces
When the forces are
balanced, the net
force is zero.
When the net force
on an object is zero,
we say the object is
in equilibrium.
5.3 Equilibrium and normal forces
A normal force is
created whenever an
object is in contact with
a surface.
The normal force has
equal strength to the
force pressing the object
into the surface, which is
often the object’s weight.
The normal force is
sometimes called
the support force.
5.3 The free body diagram
How do you keep
track of many forces
with different
directions?
Draw a free-body
diagram that
contains the objects,
like a book on a
table.
5.3 Solving equilibrium problems
For an object to be in equilibrium, all
the forces acting on the object must
add up to zero.
Is this object in
equilibrium?
Solving Problems
Two chains are used to support a small
boat weighing 1,200 newtons.
One chain has a tension of 600
newtons.
What is the force exerted by the other
chain?
Solving Problems
1. Looking for:
…tension on chain 2
2. Given
…weightboat = 1,200N; tension1 = 600 N
Implied: weight and tension are forces
3. Relationships:
Net force on boat = zero
Solving Problems
4. Solution:
Draw free body
diagram
 Upward force of chains = weight of boat
 600 N + tension2 = 1,200 N
 tension2 = 900 N
Balanced Forces
Forces opposite in
direction and equal in
size.
Net result: no change
in motion.
Equilibrium Means "Zero Acceleration"
Pictures from: Physical Science – Hewitt, Suchocki, Hewitt
Forces in balance: Equilibrium
No change in motion!
Balanced forces will keep an object
moving at a constant velocity
Zero acceleration does not mean zero
velocity
FRICTION
ENGINE
Unbalanced forces = Net force
Unbalanced forces = acceleration
Assign 132-135 Concepts 3-7, 11, 20-25
Problems 1, 9, 10, 14, 16, 17, 18
Applying What you Know 2 and 6