Transcript forces - Humble ISD

FORCES
and
st
Newton’s 1 Law
What is a force?
A force is a push or pull on an
object which can cause the
motion of the object to change.
Forces cause accelerations! If an
object at rest is subjected to an
unbalanced force, it will start to
move. If an object that is moving
is subjected to an unbalanced
force, its movement will change.
Newton’s 1st Law of Motion
An object at rest stays at rest
and an object in motion stays in
motion with the same speed and
in the same direction unless
acted upon by an unbalanced
force.
Unbalanced force?
Consider a physics book at rest on a tabletop.
There are two forces acting on the book. One
force - the Earth's gravitational pull - exerts a
downward force. The other force - the push of
the table on the book - pushes upward on the
book.
Unbalanced forces:
Now consider a physics book sliding to the right.
Sometime in the prior history of the book, it may
have been given a shove and set in motion from a
rest position. However, our focus is not upon the
history of the book but rather upon the current
situation of a book sliding to the right across a
tabletop. The book is in motion and at the moment
there is no one pushing it to the right.
Unbalanced forces
Unbalanced forces cause
accelerations! What will happen
to the motion of this book as it
slides to the right?
Two categories of forces:
Contact Forces: forces that arise
from direct physical contact of two
objects
(examples: you push on a door, or
kick a ball)
Field Forces: forces that exist
between two objects even in the
absence of physical contact
between the objects
(examples: gravity, electrostatic
attraction between two charges,
magnetism)
Examples of Forces
Applied force (a direct force)
An applied force is a force that is
applied to an object by a person or
another object. If a person is
pushing a desk across the room,
then there is an applied force acting
upon the object. The applied force
is the force exerted on the desk by
the person.
Forces: Gravity (a field force)
• Weight is a force caused (on Earth)
by the gravitational attraction of a
mass to the Earth’s center.
• The weight of a body, of mass m, is
defined to be the force, W, with
which it is attracted to the Earth.
On Earth, W = mg, where g is the
acceleration due to gravity.
(g ≈ 9.81 m/ s2 on Earth).
• Weight depends on what planet
you’re on. It doesn’t stay the same,
like mass does.
Forces: Tension (a contact force)
Many physics problems involve
objects being pulled or suspended
from a string, spring, or something
similar.
The force that the string (or similar)
exerts on the object in these types
of problems is called tension.
Friction (a contact force)
The friction force is the force exerted by a
surface as an object moves across it or makes
an effort to move across it. Friction usually
opposes the motion of an object. For example, if
the boy pushes on the table, it would continue
moving across the room without slowing down
or stopping if it weren’t for friction.
Forces: Air Resistance
Air resistance is a special type of
frictional force that acts upon objects as
they travel through the air. The force of
air resistance is often observed to oppose
the motion of an object. This force will
frequently be neglected due to its
negligible magnitude (and due to the fact
that it is mathematically difficult to predict
its value). It is most noticeable for objects
that travel at high speed.
Normal force
A mass lies on a horizontal surface.
The weight of the mass pulls it
downwards.
The reason it does not fall to the
ground is because the horizontal
surface exerts an equal and
opposite force on the mass called
the normal force.
The normal force always acts
perpendicularly to the surface that
is causing it.
The Normal Force
Remember, the normal force is perpendicular to the surface
that causes it. It’s not always perpendicular to the ground!
Force Diagrams:
A force diagram shows all the forces
acting on an object, including the
force's direction and its magnitude.
Example: If a box were dragged along
the floor, these would be the forces
acting on it:
Force Diagrams:
An egg is free-falling from a nest in a
tree. Neglect air resistance.
Force Diagrams
A tire is suspended motionless from
a tree branch by two ropes.
Now you try some:
• A rightward force is applied to
a book in order to move it
across a desk with a rightward
acceleration.
Now you try some:
• A rightward force is applied to
a book in order to move it
across a desk with a rightward
acceleration.
How would this one be different?
A rightward force is applied to a
book in order to move it across a
desk at constant velocity.
How would this one be different?
A rightward force is applied to a
book in order to move it across a
desk at constant velocity. Consider
frictional forces. Neglect air
resistance.
If the book is moving with a constant velocity, it isn’t
accelerating. Therefore, the forces must be balanced.
How about this one?
A football is moving upwards
towards its peak after having
been kicked by the punter.
(Unless specifically mentioned,
neglect air resistance.)
How about this one?
A football is moving upwards
towards its peak after having
been kicked by the punter.
(Unless specifically mentioned,
neglect air resistance.)
Answer:
A football is moving upwards
towards its peak after having
been booted by the punter.
(Unless specifically mentioned,
neglect air resistance.)
One more…
A car is coasting to the right and
slowing down.
One more…
A car is coasting to the right and
slowing down.
Brakes work by applying friction to the wheels of a car!