Introduction to Newton's Laws in a 9th grade physics class. Inertia/balanced vs. unbalanced forces, force diagrams and system schemas
Inertia is a force which keeps moving objects in motion at a constant velocity.
In a gravity-free environment, it would not require any force to set an object in motion.
The amount of inertia an object has depends on where it is in the universe.
Spaceships can travel at enormous speeds without firing their engines after being launched.
A balance of forces is demonstrated by an object which is slowing to a stop.
It would take an unbalanced force to keep an object in motion.
If an object is constantly accelerating then the forces are balanced.
A free-falling object experiences a balance of forces (before reaching terminal or constant velocity).
Unbalanced forces cause moving objects to come to rest.
If a football is moving upwards and rightwards towards the peak of its trajectory, then there are both rightwards and upwards forces acting upon it.
If an object is at rest then there are no forces acting upon the object.
A pendulum bob is set into its usual back-and-forth periodic motion. You and your good friend Pat, are watching it swing and, after some time (perhaps 10 minutes), the pendulum bob comes to a rest position.
Pat says, “This is explained by the idea of inertia - all objects eventually come to rest.”
Do you agree with Pat’s explanation? If so, say why. If not, say why not. Use the language and vocabulary of Newton’s 1st Law.
Which of the free body diagrams could depict an object moving to the right with a constant speed?
Select all that apply.

Construct free-body diagrams below each description for the instant in time for which they are described. As is always done in free-body diagrams, label the forces according to type and draw the arrows such that their length reflects the magnitude of the force.
Ignore Fair .
1) A hockey puck is gliding to the right at constant velocity. Ignore friction.
Construct free-body diagrams below each description for the instant in time for which they are described. As is always done in free-body diagrams, label the forces according to type and draw the arrows such that their length reflects the magnitude of the force.
Ignore Fair .
2) An object is being pushed rightward across the floor at a constant velocity. Consider friction.
Construct free-body diagrams below each description for the instant in time for which they are described. As is always done in free-body diagrams, label the forces according to type and draw the arrows such that their length reflects the magnitude of the force.
Ignore Fair .
3) A bucket is being lowered by a rope from a frictionless pulley into a well at a constant speed.
Construct free-body diagrams below each description for the instant in time for which they are described. As is always done in free-body diagrams, label the forces according to type and draw the arrows such that their length reflects the magnitude of the force.
Ignore Fair .
4) A football is flying and upwards and rightwards through the air.