Forces
What is a force?
•It is a push or a pull
exerted on some object
An applied force can cause a
change in velocity
• Question – what is the definition of
change in velocity over time?
The SI unit of force is a Newton
• Newton is abbreviated “N”
• The unit of a Newton is:
kg . m
s2
•
So… think about what these units mean…
Newton’s Second Law
• The acceleration of an object is directly
proportional to the force acting on it, and it
is inversely proportional to the object’s
mass.
F=ma
How can you tell a force is being applied?
• If an object (with mass) is
accelerating, there is a FORCE
being applied.
Which of the following is
experiencing an applied force?
(think about the acceleration of the object)
• An object that is slowing down
• An object at rest
• An object that is speeding up in the
negative direction
• An object traveling at a constant velocity
Different types of Forces
• Contact Forces
– These result in the physical contact of two objects
• Friction
• Tension
• Field Forces
– Forces that can exist between objects, even in the
absence of physical contact between the objects.
• Gravitational
• Electromagnetic
Gravity is a Force!
• The force of gravity that is experienced by
objects is known as WEIGHT.
• We will be using the units of Newtons to
express weight.
– Calculate the weight of an object with a mass of 20kg:
Newton’s First Law
• An object at rest remains at rest,
and an object in motion continues
in motion with constant velocity
(in the same direction).
• Also known as the Law of Inertia
Inertia
• The tendency of an object not to
accelerate.
– The tendency of an object in motion to stay in
constant motion
Example: Ketchup in a bottle…
• It doesn’t want to come out (it wants to
remain at rest).
•So then you bang it on the bottom to make it
come out (apply an outside force).
Basically, objects just keep on
doing whatever they are doing
unless they are acted upon by
an unbalanced force.
Net Force
• Net External Force is defined as the sum
of all the forces acting on an object.
– Also known as: Σ Forces
• An object may experience many forces,
but still have a net force of zero.
An object may have a net force of zero, but still
experience motion!
Free-Body Diagrams
Free-body diagrams are used to show the
relative magnitude and direction of all
forces acting on an object.
Free-Body Diagrams
– Draw the free-body force diagram of an object that is
experiencing multiple forces
• 26N to the right
• 32N to the left
• 32N to the left
• 8N to the right
• 19N to the right
• 19N to the left
Net Force
Normal Force
• The Normal Force is the force exerted on
one object by another that is in a direction
perpendicular to the surface of contact.
Free Body-Force Diagrams
• Forces are vectors
• Draw the free-body force diagram of a
book sitting on a table
• The forces can be drawn at any direction –
depending which way the force is coming
from!
•If an object is at rest:
…then the net force is zero
• The force vectors in the “x” direction will equal
zero.
• The force vectors in the “y” direction will equal
zero.
– Draw the free-body force diagram of a book sitting on a desk
A chair sitting on the ground has a mass of
35kg.
Now,
a boy
over andofpushes
• Draw
the walks
force diagram
the chair.
down
on normal
the chair
with a force of 5N.
– What
is the
force?
•Draw a force diagram of this situation.
•What is the normal force now?
An egg is free-falling from a nest in a tree.
Neglect air resistance. Draw a free-body
diagram showing the forces involved.
A leaf is falling from a tree to the ground at
constant velocity. Consider air resistance. A
free body diagram for this situation looks
like…
Remember the
** important concept**
about balanced forces and velocity
Remember the
** important concept**
about balanced forces and velocity…
• A rightward force is applied to a book in
order to move it across a desk. Consider
frictional forces. Construct a free-body
diagram for the following situations:
The object
with constant velocity.
The
objectisismoving
accelerating.
A football is moving upwards toward its peak
after having been booted by the punter. Draw a
free-body diagram.
Tension
• Tension is the magnitude of a pulling
force.
• Objects that are hanging or being pulled
will experience tensional forces.
• If an object is static, the tensional force will
be equal and opposite of the weight
vector.
A 30kg box is sitting on the ground (it is not
moving), but it is being pulled up with a tensional
force of 30N.
• Draw a force diagram of the object.
• What is the normal force of the ground on the object?
Newton’s Third Law
• If 2 objects interact, the magnitude of the
force exerted on object 1 by object 2 is
equal to the magnitude of the force
exerted on object 2 by object 1. These
two forces are equal and opposite.
– Example: book sitting on a table
Newton’s Third Law
• “For every action there is an equal and
opposite reaction.”
– The forces that are mutually exerted on two
objects are called an “action-reaction pair”
– Action and reaction forces do not always
result in equilibrium…
• Hammer and nail
A 6.0 kg object undergoes an acceleration of
2.0m/s2.
• What is the magnitude of the net external force acting on it?
• If this same force is applied to a 4.0kg object, what
acceleration is produced?