Transcript force - washburnhoogheem

Chapter Five: Force
 5.1 Forces
 5.2 Friction
 5.3 Forces and Equilibrium
Chapter 5.1 Learning Goals
 Define force as a vector and describe how it is
measured.
 Explain how forces are created.
 Compare and contrast types of forces.
Investigation 5A
What is a Newton?
 Key Question:
What is force and how is it
measured?
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.
5.1 Pounds
 The pound is a unit of force commonly
used in the United States.
 When you measure weight in pounds on a
postal scale, you are measuring the force
of gravity acting on an object.
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 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.
Opposite
direction
sign direction
 Arrows
are often
used=toopposite
show the
of forces in diagrams.
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-adistance”.
 Today we know that the
gravitational force is carried
from the Earth to the Moon
by a force field.
Two Categories of Forces
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
force is the
force
that isthe
transmitted
through a
The
Tension
acts
along
direction
string,
cable or wire when it is pulled tight by forces acting
of rope,
the rope.
from opposite ends.
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
 The weight equation can be rearranged into
three forms to calculate weight, mass, or the
strength of gravity.
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 5.2 Learning Goals
 Define friction.
 Identify causes of friction.
 Distinguish among various types of friction.
5.2 Friction

Friction is a force
that resists the
motion of objects
or surfaces.

Many kinds of
friction exist.
5.2 Friction
5.2 Friction and two surfaces

Friction depends on both of the surfaces in
contact.

When the hockey puck slides on ice, a thin
layer of water between the rubber and the
ice allows the puck to slide easily.
5.2 Identifying friction forces

Friction is a force,
measured in
newtons just like any
other force.

Static friction keeps
an object at rest
from moving.
5.2 Identifying friction forces

Sliding friction is a
force that resists
the motion of an
object moving
across a surface.
5.2 A model for friction


Friction depends on a
material’s properties
such as roughness,
how clean the surfaces
are, and other factors.
The greater the force
squeezing two surfaces
together, the greater the
friction force.
5.2 Reducing the force of friction

Unless a force is
constantly applied,
friction will slow all
motion to a stop
eventually.

It is impossible to
completely get rid of
friction, but it can be
reduced.
5.2 Reducing the force of friction

The friction between a
shaft (the long pole in the
picture) and an outer part
of a machine produces a
lot of heat.

Friction can be reduced
by placing ball bearings
between the shaft and
the outer part.
5.2 Using friction

Friction is also important
to anyone driving a car.

Grooved tire treads allow
space for water to be
channeled away from the
road-tire contact point,
allowing for more friction
in wet conditions.
5.2 Friction and energy

Friction changes energy of
motion into heat energy.
5.2 Friction and energy

Each time two moving
surfaces touch each
other, tiny bits of
material are broken off
by friction.

Breaking off bits of
material uses energy.
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 1,500 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