1. Aim: How do we define external and internal forces in a SYSTEM!?
Do Now: Quiz

2. People are Amazing (Sport Edition)
For fun, lets take a look at some extreme activities. What are some external and internal forces you notice occurring throughout the video?

3. Internal Forces: Do not change the Mechanical Energy of a system
FG pulls the block down
Gravity, Electrical, Magnetic
And Spring
A tool used to move objects from one height to another.
Allows for the movement of an object without lifting it directly against gravity.

4. External Forces: Do change the Mechanical Energy when NET work is done by it.
Applied Force
Tension Force
Friction Force
Air Resistance

5. Pick out the External Forces
A box is pushed across a rough floor at a constant speed. Treat the box as a single object system.
Force of Friction
Normal Force
Weight
Force Applied
ALL would change the ME if there was a NET force, but it is zero.
Net work done is also zero.

6. Pick out Internal Forces
2. Two birds collide with each other and immediately begin to fall. Treat the system as consisting of the 2 birds.
Force of bird 1 on bird 2
Force of bird 2 on bird 1
The weight is the force of the earth on the birds. This is an external force because the earth is external to the system. It causes an increase in ME.

7. Soooo positive and negative work?
Work is done on a system by external forces. W=FdCosθ
Positive work, means ME gained.
Negative work, means ME lost.
THIS DEPENDS ON HOW THE SYSTEM IS DEFINED!!!
Bold means: “IMPORTANT” ^^^

8. Sounds simple…, but what’s the deal with SYSTEMS?
A system consists of any objects defined for that system.
Systems help define specifically if a force is external or internal.
Internal forces occur within a system and external act on it from outside.

9. Systems Defined
Treat the ball as a single object system. How is weight defined?
100 % A
Treat as the ball-Earth system. How is weight defined? B
ME changes for the single object system, but is unchanged for the ball earth system.
100 % C

10. Summary
How do we determine if a force is external or internal for a situation?
A jet is refueled mid air by another jet as it travels horizontally at constant speed.
What are the external forces for the Jet-fuel system?
Is momentum conserved horizontally if the plane slows down? Explain.