Transcript Newton`s 2nd and 3rd Laws

Newton’s 2nd and 3rd
Laws of Motion
Physics: Chapter 4 Section 3
Newton’s 2nd Law
• Force is proportional to mass and
acceleration
• The unbalanced force acting on an
object equals the object’s mass times
the object’s acceleration
• If the same force is applied to 2
objects of different mass, the less
massive object will accelerate more
quickly
Newton’s 2nd Law
It obviously takes less force to make the ball
accelerate because the ball has less mass.
Newton’s 2nd Law
F = m•a
• SI units for force = Newton
***(1 N = 1 kg·m/s2)***
 F = Vector sum of all external forces
acting on a body
 m = mass of the object (in kilograms)
 a = acceleration of the object (in m/s2)
Newton’s 2nd Law
• Example: Think of the force required
to push an empty shopping cart, and
the force required to push a full
shopping cart. What is the
difference?
• More force applied = greater acceleration
Newton’s 2nd Law
•Weight ≠ Mass
 Weight – the
measure of gravity
pulling on an
object
 Mass – the
measure of the
amount of matter
in an object
Newton’s 2nd Law
Weight influences shape
On land a supporting skeleton is
required
 In water less body support required
because the water helps lift the mass
*Animals can be larger if they live in the
water
Newton’s 2nd Law
•Free fall – motion of a
body when the only force
acting on it is gravity
•Terminal velocity –
maximum velocity
reached when air
resistance pushes up as
much as gravity pulls
down
 Force of gravity
pulling down = force of
air resistance pushing up
Newton’s 3rd Law
•For every action there is
an equal but opposite
reaction
***Action and reaction
forces are applied to
different objects, but act in
pairs (both contact and
field forces)
 These forces occur in
pairs at the same time, but
do not cancel out!
Example: rocketry
Newton’s 3rd Law
• Hammer and Nail example
Newton’s 3rd Law
Actions and Reactions
Newton’s 2nd and 3rd Laws Homework
• Page 138 #1-5