Download Newton`s 2nd Law of Motion

Newton’s 2nd Law of
Motion
Forces
• A push or pull
• The cause of an acceleration
• Cause of a change in an object’s state of
motion
• Cause objects to speed up or slow
down
• Cause a change of direction
• Unit of force: Newton (N)
Free Body Diagrams
• One object only
• Forces represented as arrows
placed where they act
• Length of the arrows should
represent the magnitude of the
force
• Fg down, FN perpendicular to
surface, Ff parallel to surface
opposing motion
Finding Fnet
• We define forces pointing up or right as
positive
• We define forces pointing left or down as
negative
• To get Fnet:
• Add all parallel forces and subtract off
antiparallel forces
Example
A mover pushes a box with a 20 Newton force to the right.
His friend sees him struggling so he helps push the box by
applying a 25 Newton force in the same direction.
What is the net force on the box?
Example
• Now a 3rd mover, not being too bright, decides to help
by pushing with a 14 Newton force in the opposite
direction. What is the net force now?
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 mass of the object
or:
Fnet =ma
Types of Forces
• Contact forces:
• Tension - (Ft or T) - The force that a
“string” pulls on an object
• Normal Force - (FN or N) - Force that
a surface applies to an object (the
direction is  to the surface)
• Friction - (Ff) - To be defined later
F, applied force
 A physical push or a pull
FN, Normal Force (N)
• A force that a surface applies to an
object
• “Normal” means perpendicular
• The direction of the normal force is
perpendicular to the surface
surface
Tension - (Ft or T)
 Tension (force) in a string
or rope
 Strings only pull
Types of Forces
• Field Forces (Action at Distance)
• Force of Gravity (Fg) or Weight
• Fg =Weight, - WEIGHT IS THE FORCE
OF GRAVITY
• Fg= mg
• Where g = 9.8 m/s2
• Electricity/Magnetism
Free Body Diagrams
• Used to analyze the forces affecting
the motion of a single object
• Shows only the forces acting on an
object
Free Body Diagrams
• One object only
• Forces represented as arrows
starting at the center of the object
• Length of the arrows should
represent the magnitude of the
force
• Fg down, FN perpendicular to
surface, Ff parallel to surface
opposing motion
Examples
Force of Friction Ff
• Always parallel to the surface
• Acts opposite to the direction of motion
F f = μ FN
Where μ is the coefficient of friction
It is determined between the two surfaces in contact (it will vary
depending upon the surface)
Common μ’s
Materials
Oak on oak, dry
μ
0.30
Steel on steel,
dry 0.41
greasy 0.12
Steel on ice
Rubber on asphalt,
0.01
Dry 1.07
wet 0.95
Rubber on ice
0.005