Transcript Slide 1

Newton’s 2nd and 3rd Laws
Newton’s Second Law
Force is
proportional
to mass and
acceleration
ΣF=m a
If the same force
is applied to two
different objects
with two different
masses, the
object with the
greater mass will
have a smaller
acceleration and
vice versa
F=ma
By this it means that if
F increases, a increases;
but if m increases, a
decreases
Acceleration is inversely
proportional to mass
The greater the mass, the
greater the force must be
for a given acceleration
Acceleration
directly
proportional
to force
Acceleration is in the
direction of the force
Increase speed : If force is in
the direction of the object
Decrease speed : If force is in
the opposite direction
Turn the object : If force is
applied at a right angle
Any other direction will result in
a change in speed and deflection
Newton’s 3rd Law
Forces always exist in pairs
If two bodies 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
For Every Action There is an
Equal and Opposite Reaction
When two objects interact
with one another, the forces
they exert on each other are
called action-reaction pairs
Reaction forces occur at
exactly the same time as the
action force
Examples of Newton’s 3rd Law
When you walk
across the
floor:
you push
against the
floor and the
floor pushes
against you
Swimming
You push the water backward
and the water pushes you
forward
Person Pushing Against a Wall
Rocket at Lift Off
Action:
rocket
pushes on gas
Reaction: gas
pushes on
rocket
These forces depend on friction
A person or
car on ice may
not be able to
exert the
action force to
produce the
needed
reaction force
Action-reaction forces act on
different objects so they don’t
cancel each other out
Action-reaction pairs do not
result in equilibrium because
they act on different objects.
To determine if there is a
change in motion, draw a free
body diagram on the object
you are observing and not
both objects
Mass is also a factor in
Newton’s 3rd Law
When a gun is fired,
the force exerted on
the bullet is as great
as the reaction force
exerted on the gun.
Why is the recoil not
at the same speed as
the bullet?
Hammer & Nail
Nail exerts a force
on the hammer
equal and opposite
the force the
hammer exerts on
the nail
Net force acting on
the nail, drives it
into the wood