Transcript Ch 4 – Forces and the Laws of Motion

Ch 4 –
Forces and the Laws of Motion
What is a force?
• A force is a push or pull
• A force causing a change in velocity
– An object from rest starts moving
– A moving object comes to rest
– A moving object changes direction
• The SI Unit for Force is the Newton
– 1 N = 1 kgm/s2
2 Types of Forces
• Contact Force
– Force that arises from the physical contact of
two objects
• Field Force
– Force that can exist between objects, even in
the absence of physical contact between
objects
Free Body Diagrams
• These drawings help us by isolating an
object and the forces acting on it.
Newton’s First Law
• An object at rest remains at rest, and an object
in motion continues in motion with constant
velocity unless the object experiences a net
external force.
• The tendency of an object not to accelerate is
called inertia.
• If there is a net external force, this will cause an
acceleration
• Net external force is the sum of all forces acting
on an object
Mass is a measurement of Inertia
• The more mass something has, the
greater tendency it will have to keep doing
what it’s been doing
Equilibrium
• Equilibrium – the state in which there is
no change in a body’s motion.
• The net external force acting on a body in
equilibrium must be equal to zero
– An object stays at rest
– An object continues to move at a constant
velocity
• Just because something is moving, DOES
NOT mean a force is acting on it!
Example Problem
• An agriculture student is designing a
support to keep a tree upright. Two wires
have been attached to the tree at right
angles to each other. One exerts a force
of 30 N north on the tree; the other wire
exerts a 40 N force due west. Determine
the placement and force in the wire for a
third wire so that the tree will have zero
net force from the 3 wires.
a2 + b2 = c2
402 + 302+ = c2
30 N
θ
c = 50 N
Tan-1 (40/30) =
53.1° W of N
40 N
θ
To balance the first 2 wires,
we need a 3rd wire that is of
equal magnitude and
opposite direction of the
resultant.
Therefore, the 3rd wire must
be 50 N at 53.1° E of S
Newton’s 2nd Law
• The acceleration of an object is directly
proportional to the net external force
acting on the object and inversely
proportional to the object’s mass
ΣF = ma
Net external force = mass x acceleration
How will the acceleration of the
elephant and feather compare?
This is the situation WITH air
resistance.
NEGLECTING air resistance, how
will the acceleration of the elephant
and feather compare?
Both have the same acceleration –
Gravity!
Although the mass of the elephant
is greater, so is the force. The two
proportions end up equaling the
same magnitude for acceleration.
Newton’s 3rd Law
• For every action there is an equal and
opposite reaction
• If two objects interact, the magnitude
of the force exerted on object 1 by
object 2 is equal to the magnitude of
the force simultaneously exerted on
object 2 by object 1, and these two
forces are opposite in direction.
Action-Reaction Pairs
• A pair of simultaneously equal but
opposite forces resulting from the
interaction of two objects
• The most important thing to remember is
that each force acts on a different object!
• Ex. Hammer on nail,
nail on hammer
Newton’s Cradle
Ch 4.4 – Everyday Forces
• Weight – the magnitude of the force of
gravity on an object
Weight = mass x gravity
Weight, unlike mass, is dependent on the force
of gravity
We usually designate weight on a free body
diagram as Fg or mg
Everyday forces, cont.
• Normal Force – a force exerted by one
object on another in a direction
perpendicular to the surface of contact
• The normal force is always perpendicular
to the surface of contact, but not always
opposite to the force of gravity
Fn
Fg
Fn
Fg
How we abbreviate forces
• Forces are all represented by a capital
letter F
– The subscript of each F tells us where the
force comes from
•
•
•
•
Fg – Force of gravity
Fa – Applied Force
Fn – Normal Force
Ff (sometimes also seen as Fk and Fs) – Force of
friction
Fun with Friction
• Friction is a force that resists the
motion between two objects in contact
with one another.
Causes of Friction
• The electrons of the two surfaces in
contact with one another
• The surfaces themselves
• Deformation of the surface
Why Friction…Why?!
• Is friction bad? Yes!
• If friction good? Yes!
Forms of Friction
• Sliding Friction
• Rolling Friction
• Friction caused by
passing through fluids
Types of Friction
• Static Friction = the resistive force that
opposes the relative motion of two contact
surfaces that are at rest with respect to one
another
• fs  static force of friction
• Kinetic Friction = the resistive force that
opposes the relative motion of two contacting
surfaces that are moving past one another
• fk  kinetic force of friction.
Kinetic friction is always smaller
than static friction.
• It’s harder to get
something moving
than to keep
something moving
Finding the Frictional Force
• The frictional force is proportional to the normal force.
Greater weight = Greater Normal Force = Greater
Friction
• Since there are two types of friction, static and kinetic,
we have two equations:
f k  k n
f s  s n
• s  coefficient of static friction
• k  coefficient of kinetic friction
• The value of the coefficients depends on the two
surfaces in contact with one another. These values are
found by experiment.
• These values have no units and are always less than 1
Example Problem
• A 24 kg crate initially at rest on a
horizontal floor requires a 75 N horizontal
force to set it in motion. Find the
coefficient of static friction between the
F
crate and the floor.
F
F
• Answer: 0.32
n
s
a
Fg
A 25.0 N wood block is pulled across a wooden table at
constant speed. What is the force needed to do this? For
the coefficient of kinetic friction use 0.35.
fk
 k n
n
fk
F
Fg
fk
 0.35  25 N   F 
8.8 N