Download 8.3

8.3
8.3 Newton’s Laws of Motion
First Law of Motion – Newton’s first law of motion states that an object
at rest will remain at rest unless acted upon by a nonzero net force. An
object moving at a constant velocity will continue moving at a constant
velocity unless acted upon by a nonzero net force.
Inertia – all objects, moving or not, resist changes in motion. Resistance
to change in motion is called inertia. Newton’s first law of motion is also
called the law of inertia.
Inertia explains many common events, including why you move forward
in your seat when the car you are in stops suddenly. You keep moving
forward because of inertia. A force, such as the pull of a seatbelt, is
needed to pull you back.
Inertia Depends on Mass – some objects have more inertia than others.
The greater the mass of an object, the greater its inertia, and the greater
the force required to change its motion.
Newton’s Second Law of Motion: An object’s acceleration depends on its
mass and on the net force acting on it.
Acceleration = Net Force
Mass
Or written another way: Net Force = Mass x Acceleration
Changes in Force and Mass – if you increase the force on a shopping cart
without changing its mass, the acceleration of the cart will also increase.
Your cart will accelerate faster if something falls out, because this
reduces the mass of the cart.
Acceleration is measured in meters per second per second (m/s²). Mass
is measured in kilograms (kg). Newton’s second law shows that force is
measured in kilograms times meters per second (kg x m/s²). This unit is
also called the newton (N) which is the SI unit of force. One newton is
the force required to give a 1-kg mass an acceleration of 1 m/s².
Newton’s Third Law of Motion: If one object exerts a force on another
object, then the second object exerts a force of equal strength in the
opposite direction on the first object.
Another way to state this is: For every action there is an equal and
opposite reaction.
Action-Reaction Pairs: pairs of action/reaction forces are all around you.
When you walk, you push backward on the ground with your feet; the
ground pushes forward on your feet with an equal and opposite force.
When swimming, you move forward by exerting an action on the water
with your hands. The water pushes on your hands with an equal reaction
force that propels your body forward.
Newton’s 3rd Law of motion explains why rockets accelerate in space,
even though there is no water or air to push off of. Inside rockets, gas is
produced. When the rockets push that gas backward out of the rocket, a
reaction force occurs that pushes the rocket forward.
Bugs Go Splat!
Splat! A bug has just flown into the windshield of a car. Use Newton’s
law of motion to answer these questions:
1. In order for a bug to fly through the air, a force has to push the bug
forward. What is this force and how does the bug produce it?
2. The driver of the car hates killing bugs. When she saw one coming
towards the windshield, she braked suddenly and hoped it would get out
of the way (sadly, it didn’t). When she hit the brakes, she felt that she
was thrown forward. Use one of Newton’s laws to explain why.
3. The unfortunate bug hits the windshield with a force of 1N. If this is
the action force, what is the reaction force? Does the car hit the bug any
harder than the bug hits the car? Use one of Newton’s laws to explain
why or why not.
Comparing the forces on the bug and car again, use another of Newton’s
laws to explain why the bug goes splat, but the car keeps going, without
noticeably slowing down.