Laws of motion
Name ______________________________
We have described motion (objects moving at a constant velocity, objects
accelerating) but in this section we will explain why objects move.
Newton’s law of inertia – Newton’s 1st law
An object at rest stays at rest, and an object in motion keeps moving unless an
unbalanced force acts on it.
* an unbalanced force is needed to change an objects state of motion.
*when forces are unbalanced, acceleration occurs because there is a change
in velocity (either speed or direction).
*forces are balanced if they are of equal magnitudes (numbers) and opposite
direction
*inertia is the property of matter that resists a change in motion
(inertia depends on mass).
In class:
Example 1: Describe what happens to the ball on the chair after the chair is stopped
from moving forward.
(the ball keeps moving forward)
http://www.physicsclassroom.com/class/newtlaws/Lesson-1/Newton-s-First-Law
Example 2: (From physics classroom) Describe what happens to the person who is
not wearing a seatbelt and his car runs into a wall.
Example 3: Describe what happens to the ladder on the truck when the truck stops.
So the greater an objects inertia, the greater the force needed to change motion.
Increase mass………increase inertia………increase force (direct relationship)
How much force is needed to push an empty rolling chair from a stand still vs. the
rolling chair with a football player in it from a stand still?
All objects have mass, so if the objects are moving, then they have momentum. The
amount of momentum depends on how much matter is moving and how fast; so
momentum depends on mass and velocity.
(units: kg m / s)
P=mxv
Momentum is a vector quantity so it needs to show magnitude and direction.