Download Force - Purdue Physics

Today: (Ch. 2 & 3)

Develop the equations to describe motion

Look at some situations where we can
apply them
Force


A force is a push or a pull on an object
Force is a vector quantity



The magnitude of the force is the strength of
the push or pull
The direction of the force is the direction of
the push or pull
Denoted by F
Fundamental Forces

Gravity

The strong and weak nuclear forces

The electromagnetic Forces
Inertia
• The Principle of Inertia
– An object will maintain its state of
motion unless it is acted upon by a force
– Inertia is also a measure of an object’s
resistance to changes in its motion
Newton’s laws of motion

The laws are statements about how things
move

Newton’s first law is a statement about inertia

Newton’s Second Law gives the link between
motion and forces

Newton’s Third Law explains where forces
come from
Newton’s laws of motion

If no force acts on an object, then its speed
and the direction of motion do not change.

A non zero force action on an object causes
its state of motion to change.


Fnet  ma

In an interaction between two objects, the
forces that each exerts on the other are equal in
magnitude and opposite in direction
Newton’s second law
• The acceleration of an
object is directly
proportional to the total
force that acts on it.
•
F  m a
– Forces are vectors
• The direction of the
acceleration is parallel
to the sum of the forces
Direction
• The direction of the
acceleration is always parallel
to the direction of the total
force
• The velocity and the total force
do not need to be in the same
direction
• Example
– Initial velocity is upward
– The total force is downward
– The acceleration is
downward
Newton’s third law
• Force applied on first
object by second is equal
in magnitude and
opposite in direction
– Often called the actionreaction principle
• Example
– Force on ball
– Force on bat
Measuring Forces
Hook’s law is applied for springs. Springs
have property that is expansion and
compression.
Hook’s law says that expansion or
compression is roughly proportional to the
force exerted on the ends of the spring.
Hooks law for ideal spring
F  kx
Motion – Constant Velocity
• The velocity is zero
– Position is constant (not
necessarily zero)
• The velocity is non zero and
constant
– Position is changing steadily
(v = 0)
(v = 0)
Constant Acceleration
• The acceleration is a constant
– On the graph, a straight
horizontal line
• The velocity is changing
– On the graph, this is an
upward sloping straight line
• The position is changing
– Not the same change each
second
Equations to Describe Motion with
Constant Acceleration
• vf = v o + a t
– vo is the velocity at some initial time t = 0
– It depends on what happened prior to t = 0
• xf -xo= vo t + ½ a t2
– xo is the position at some initial time t = 0
• v2 = vo² + 2 a (xf - xo)
– Eliminates t from the equation
• Which equation to use depends on what information
you are given in the problem and what you are asked
to find
Constant Acceleration Equations,
Summary
Weight
• Weight is the force of gravity exerted by the Earth on
an object : Denoted by F
grav
• If an object has a mass m, then
– The force of gravity is a consequence of Newton’s
Law of Universal Gravitation
• The value of g is approximately the same for all
locations near the surface of the Earth, which is :
g ≈ 9.8 m/s²
• The weight will be different on another planet
– Since it is due to the gravitational attraction of that
planet
Weight, cont.
• The value of g is independent of the mass of the
object
• g is commonly referred to as the “acceleration due to
gravity”
• Weight will be measured in Newtons
– It is a force
• Since weight acts vertically, it will be along the y-axis
• Since the weight acts downward, Fgrav = - m g
– It acts toward the center of the Earth
Weight and Normal Force, Example
 ΣF = -m g + N = m a = 0
 a = 0, since person is
stationary
N=mg
 Here, the normal is
equal in magnitude to
the weight and
opposite in direction to
the person’s weight
Free Body Diagram
• A free body diagram should be used for analysis
using Newton’s Second Law
• It is a simplified diagram showing all the forces
acting on each object involved in the problem
Free body diagram
F2y = N
F1y = W
What is Fnet?
F2y = N
F1y = W
Net Force: Vector addition
F2y = 90 N
F1x = 60 N
F2x = 120 N
F2x = 60 N
F2x = 120 N
F1y = 40 N
F2y = 90 N
Fnetx = ? N
Fnety = ? N
F1y = 40 N
Contact forces

F

W

N


f

F  Applied force W  Weight of the block


f  Friction force N  Normal force
 f s max  s N
Static friction force
f k  k N Kinetic friction force
Tomorrow: (Ch. 3)

Develop the equations to describe motion

Look at some situations where we can
apply them
Contact forces

Tension