Transcript Newton’s Laws

Newton’s Laws
Motion
• Velocity describes motion.
– Speed and direction
– At rest : no velocity
• Planets have a velocity at
each point in their orbits.
– Circle or ellipse
• Acceleration is the change
in velocity.
– Change in speed
– Change in direction
• Orbits require
acceleration.
– Changing direction : circle
– Changing speed: ellipse
Elliptical Motion
• Velocity points along the
motion.
– Red arrows
• Acceleration points in the
direction of change.
– Blue arrows
y
x
Force
Force is
• A push or pull on an
object.
• A property with
magnitude and direction.
Some forces
 Fundamental forces
– Electricity and magnetism
– Nuclear forces
– Gravity
 Contact forces
– Friction
– Tension
– Normal forces
Dynamics
• Ancient scientists looked
to the natural properties of
objects to describe motion.
• Newton defined motion
based on forces acting
from outside an object.
– Motion from external
forces
– Three laws for motion
First Law: Law of Inertia
1
An object continues at rest, or in uniform motion in a
straight line, unless a force is imposed on it.
• This describes constant velocity, including zero.
• No change means no net force, and vice versa.
no force
rocket
constant
velocity
Mass
• Matter has substance.
– Solids, liquids or gases
– Subatomic particles
– Planets and stars
• Mass measures the amount of
matter.
– Standard kilogram
– 1 kg = 1000 g
– 1 hydrogen atom 1.66 x 10-27 kg
Second Law: Law of Acceleration
2
The change in motion is proportional to the net force and
the change is made in the same direction as the net force.
• Net force gives rise to acceleration.
• Force = mass x acceleration.
– 1 newton (N) = 1 kg m/s2
Initial
velocity
Rocket:
has mass
Force:
becomes
acceleration
Final velocity:
changed by the
force
Third Law: Law of Reaction
3
For every action there is an equal and opposite reaction.
• Forces between two objects act in pairs.
• F12 = -F21
reaction force acts
on the rocket
rocket
hot gas is
forced out
rocket and gas
are both affected