Transcript 8.3 Newton`s law of motion - Hope Christian College Parent and

8.3 Newton’s laws of motion
• Loose change experiment , p.269
First law of motion: a body remains at rest or in
motion with a constant velocity unless acted upon
by an external force.
First law of motion examples
The net force is the total force. It could be the sum of two forces or more
than two forces.
Inertia
Inertia is the resistance of any physical object to a
change in its state of motion or rest,
Second law of motion
When several forces act on an object, they may
cancel one another out to leave no net force. Only a
net (or unbalanced) force can accelerate an object.
Second law of motion
Third law of motion
For every action there is an equal and opposite reaction.
Principle of a Launch Vehicle
Thrust to propel a rocket is based on Isaac
Newton's third law of motion, which states that
for every action, there is an equal and opposite
reaction. The principle of a rocket motor may be
understood by considering the example of a
closed container filled with a compressed gas.
Within this container the gas exerts equal
pressure on every point of its walls. If a hole is
punched in the bottom of the container,
however, the gas at the bottom escapes and
the pressure against the top of the container is
no longer equalized. The internal gas pressure
then pushes the container upwards in reaction
to the jet of air escaping downwards. The
amount of thrust developed by a rocket motor
depends mainly on two factors, the velocity with
which the burning gases leave the combustion
chamber, and the mass of the burning gases.
http://www.grc.nasa.gov/WWW/k-12/airplane/newton3.html
ACTIVITIES
Action: the tires on a car push on the road…
Reaction: the road pushes on the tires.
Action: while swimming, you push the water backwards...
Reaction: the water pushes you forward.
Action: a rocket pushes out exhaust…
Reaction: the exhaust pushes the rocket forward.
One of the original arguments that flight in the vacuum of space was
impossible was that there would be nothing to push against. This actionreaction explains how a rocket can fly in space where there is no air to push
against.
Action: the earth pulls down on a ball…
Reaction: ball pulls up on the earth.
•If I push on a lawn mower, it pushes back on me with an equal, but opposite force.
Explain why we don’t both just stay still.
•The answer is that these forces are acting on different bodies (and there are other
forces to consider).
•It doesn’t matter to the lawn mower that there is a force on me… all that matters to the
lawn mower is that there is a force on it, so it starts to move!
•Another action-reaction pair you need to consider is that I am pushing backwards on
the ground, and it pushes forwards on me.