Download Link to Notes - Coweta County Schools

Forces and
Momentum
Chapters 4, 5 and 9
Force
 A push or pull exerted on an object
 It causes a change in velocity (and therefore
acceleration)
 SI unit is a newton (N)
 It is a vector quantity (it has magnitude and
direction)
 2 types:
 Contact forces
 Ex. A book on a table
 Force fields
 Ex. Gravity pulling on a falling apple
Free Body Diagrams
 A pictorial way to show all
the forces acting on an
object
 Use an arrow for each
force on the object
 Arrowhead points in the
direction the force is exerted
 Length of the arrow
indicates the magnitude of
the force
 Remember to choose your
coordinate system (which
direction is positive and
which is negative)
Relating Force and
Acceleration
 More force gives more acceleration
 More mass means you need more force
to get the same acceleration
 So a = F/m or F = am
 This is newton’s 2nd law
 Acceleration is directly proportional to the
force exerted on an object and inversely
proportional to the mass of an object
nd
2
Using Newton’s
Calculate Weight
 Weight is the force of gravity
acting on your mass
 Weight changes from location to
location, but mass is constant
 Fg = mag
 ag = 9.8 m/s2 on the surface of
the Earth
 The unit for weight is a N
because it is a force exerted on
you by the mass of the Earth
(or whatever planet is pulling
on you)
Law to
Net Force
 However, when we talk
about force with Newton’s
2nd law, we mean NET
force
 If forces are in the same
plane (or dimension) then
they can just be added
 Remember though, if in
opposite directions then one
must be negative according
to the coordinate system that
you’ve established
 If the net force on an object
is 0, then the acceleration
with also be 0
 It is at equilibrium
Newton’s 1st Law
 When there is no net force acting on an object,
it will continue to behave in the same manner
 An object at rest stays at rest, an object in motion
remains in motion, unless an outside force acts on it
 Inertia
 The resistance of a body to change
 Measured in mass (more mass means more inertia)
 A scale measures your weight because the net
force on you must be zero (a = 0)
 The scale actually measures how hard it has to
push back up on you, not how hard you are pushing
down
 Scale reading are inaccurate when you are
accelerating
Apparent Weight
Friction
 The force that opposes motion
 2 types:
 Static friction
 When an object isn’t moving (v = 0)
 Starts at 0 and increase as you push harder
until the maximum is exceeded
 Kinetic friction
 When an object is moving
 As long as push equals kinetic friction, the
object continues to move at a constant
velocity
 If an object is moving at a constant
velocity (equilibrium), then friction must
equal the force of the push (net force = 0)
 Not moving is just a special type of
equilibrium when v = 0
Calculating Friction
 Is determined by the material the surface
is made of (measured by the coefficient
of friction, μs)
 Also affected by how hard the materials
push against each other (measured by
the normal force, FN)
 This is always equal to the weight (mg) of
the object, but in a direction perpendicular to
the surface the object rests on
 So, Ff = μs FN
Air Resistance (or Drag)
 The frictional force the air exerts on a falling
object (opposes motion)
 Can be altered by the objects mass and
surface area
 More mass, the more drag that can build up
 The more surface area, the quicker the drag builds
up
 So, heavy, compact objects fall more quickly than
light, spread out ones
 When air resistance equals an object’s weight,
the net force = 0 and the acceleration = 0 (but
velocity doesn’t)
 This is the terminal velocity of the object
Creating Forces
 When you push on an object, the object
actually pushes back on you in an equal
and opposite direction (Newton’s 3rd law)
 Forces always occur in pairs of equal
magnitude and opposite direction and on
2 different objects that are exerting forces
on each other
 Ex. A bat hits a baseball, then the baseball
must also hit the bat with the same force
The Same Force Paradox
 If the force on each object is the same, then why don’t
they experience the same effect in the collision
 Their masses differ, and therefore they undergo different
accelerations
 If the forces are equal and opposite, why don’t they
cancel out to a net force of 0
 Because the forces are on 2 different objects, forces only
cancel if they act on the same object
Finding Net Force if Vectors
Aren’t in the Same
Dimension
 This can be done graphically
using the tip to tail method
 As long as the direction and
magnitude of a vector remain
unchanged, you can move it
anywhere
 Move the tip of one vector so that
it touches the tail of another
 Draw an arrow connecting the
exposed tail to the exposed tip
 The magnitude and direction of
this line is the combined effect of
the 2 vectors (we call this the
resultant)
Momentum
 The combined effect of an
object’s mass and it’s
velocity
 Unit is kgm/s
 A change in momentum is
caused by an impulse
 A force acting over a time
 The longer the time, the less
force required to cause the
same change in momentum
 More impulse results from a
bounce than from a solid hit
Conservation of
Momentum  Can be passed between objects,
but cannot be lost
 One object can cause another to
move after a collision, but it will have
to slow down
 It’s the momentum that’s conserved,
not the velocity
 Is a vector since velocity is a vector
(the sign matters)
 2 collision types:
 Inelastic – the KE for the system
changes
 Elastic – the KE for the system
remains the same pre and post
collision