Transcript Newton’s Laws of Force

Gravity
Chapter 3 Section 2
Effects of Gravity
 Why
does the apple fall down
from the tree and not float
around in space?
 All objects have forces of
attraction between them
 this force of attraction is called
gravity
 Gravity
is the force of attraction
between objects and the Earth
 the amount of gravitational
force between objects depends
on their masses and the distance
between them
Weight
 The
force that is caused by
gravity. The mass and the
distance determine the weight.
Good test question
 Do
you weigh more, less or the
same on the moon compared to
Earth?
The Third Law of Motion
Chapter 3 Section 3
Newton’s Third Law
Shooting
a gun
rowing a boat
rocket propulsion
ice skater pushing off
 Newton’s
third law of motion
–when one object exerts a force on
a second object, the second one
exerts a force on the first that is
equal in size and opposite in
direction
–to every action force there is an
opposite and equal reaction force
 How
is this law different from the
other two?
 Forces don’t cancel each other out
even if they are equal and opposite.
 There are always two objects
involved
 If forces are equal then how can a
swimmer move through the water?
 Newton’s second law determines
the acceleration of an object
The
forces are equal, but
the masses may not be
the objects with less mass
will accelerate more
Momentum
 Why
is it harder to stop a
loaded train car verses an
unloaded train car?
 Why is it harder to stop a train
car moving at 10 m/s verses a
train car moving at 5 m/s?
 Momentum
is a property an
object has due to its mass and
velocity
 can be calculated by
multiplying mass times velocity
 momentum = mass x velocity
r = m x v
 units are kgm/s
Force & Changing Momentum
How do you stop the baseball from stinging
your hand as you catch it?
 Why to air bags work?
 Newton’s 2nd law says F=ma
v f  vi
 And
a
t
 So F  m( v f  vi )

t
Rearrange to have the equation
 Ft=m(vf-vi) or Ft=mΔv
 Where: Ft is called impulse
 So the impulse equals the change in
momemtum

 Momentum
has direction
 momentum will change only if
its mass or velocity changes
 momentum can be transferred
from one object to another
–pool balls
–car crashes
–hockey players
Law of Conservation of
Momentum
 The
total amount of momentum
of a system does not change
unless an outside force acts on
the system
–pool balls