Transcript force

Chapter 5
5.1 Forces & 5.3 Forces
& Equilibrium
pp. 108-115 & pp. 124-129
FORCES
► Where
do you see a force happening in the
room right now?
► Which
object is exerting the force? Which
object us receiving the force?
IMPORTANT DEFINITIONS
► Force
– a push or a pull
All forces have both a size (a
number) and a direction
5.1 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.
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.
► Electromagnetic
forces are important
to technology.
► Gravity is a universal
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.
CONTACT FORCES
►A
contact force acts as a result of direct
contact. Contact forces include:
► Tension
► Friction
► Normal Force – the perpendicular force that
a surface exerts on an object that is
pressing on it
► Air Resistance
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 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:




2.

3.


…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
Relationships:
W=mxg
Solution:
60 kg x 9.8 N/kg = 588 N
60 kg x 3.7 N/kg = 222 N
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 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.
BALANCED FORCES
► When
the net force on an object is zero, the
forces are balanced.
► Balanced forces do not cause a nonmoving
object to begin moving.
► Balanced forces will not cause a change in
motion of a moving object.
UNBALANCED FORCES
► When
the net force in an object is not zero,
the forces on the object are unbalanced.
UNBALANCED FORCES
► Unbalanced
forces produce a change in
motion (acceleration)
► Unbalanced forces are needed to cause a
nonmoving object to begin to move
► Unbalanced forces are also needed to
change the motion of moving objects.
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 1,500 newtons.
One chain has a tension of 600 newtons.
What is the force exerted by the other
chain?
Solving Problems
1.

2.
Looking for:
…tension on chain 2
Given

…weightboat = 1,500N; 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 = 1,500 N
 tension2 = 900 N