Transcript Force, Momentum and Energy

Force, Momentum and Energy
Newton’s Laws of Motion
Our understanding of how an object reacts to force, or how the motion of an
object is affected by force, is summarized by Newton’s Laws of Motion:
First Law of Motion
In the absence of a net force, an object moves with constant velocity.
Second Law of Motion
Force = mass  acceleration,
or,
F=ma
Third Law of Motion
For any force, there is always an equal and opposite reaction force.
First Law of Motion
In the absence of a net force, an object moves with constant
velocity
• An object at rest will remains at rest unless a net force is applied to it.
• An object in motion will continue to move with the same speed in the same
direction unless a net force is applied to it.
• Examples:
– Spacecraft does not need to have its rocket engine on all the time to keep
moving in space.
– Air hockey!
Second Law of Motion
F = m  a,
or,
a
F
m
Eamples:
• Heavy trucks (large m) accelerate very slowly…
• Sport cars accelerate real fast (0-60 mph in 5 seconds?).
• Throwing base balls.
Third Law of Motion
For every force, there is always an equal and opposite
reaction force.
Eamples:
• The rocket launch.
• When you hit somebody with your hand, your hand hurts also.
Newton’s Law of Gravity
• Every mass attracts every other mass through the force of gravity.
• The strength of the gravitational force is directly proportional to the
product of their masses.
• The strength of the gravitational force between two objects decreases with
the square of the distance between them. This is the inverse square law.
• The Gravitational Constant G = 6.67 × 10-11 m3/(kg × s2)
Does rock falls faster than feather?
On the surface of the Earth, the
gravitational acceleration
experienced by all the objects is
a
Gmearth
2

9
.
8
m
/
s
2
rearth
It does not depends on the mass
of the object!
Does heavier object falls faster
than lighter object?
 All the objects falling off the
roof would reach the ground
at the same time, independent
of their masses (click to start
movie)
Click image to start movie