1. Warm-up
What forces were acting on your rocket when it was moving up into the air? What forces were acting on your rocket when it was coming back down to earth?

2. Newton’s Third Law of Motion
“Every Action has an equal and opposite Reaction.”
Forces always come in pairs!
Action-Reaction Forces always occur between two objects.

3. Free-body diagrams
Free-body diagrams are pictures that show the size and direction of all forces acting on an object.

4. Steps to drawing a free body diagram
Pick one object to analyze
Draw a box to represent the object
Draw an arrow to represent each force acting on the object
Make sure the arrow shows the direction and relative size of the force

5. Force Symbol Definition Direction
Friction Ff
The contact force that acts to oppose sliding motion between surfaces
Parallel to surface & opposite direction of sliding
Normal FN
The contact force exerted by a surface on an object
Perpendicular to & away from the surface
Spring
Fsp
A restoring force, that is, the push or pull a spring exerts on an object
Opposite the displacement of the object at end of spring
Tension FT
The pull exerted by a string, rope, or cable when attached to a body & pulled taut
Away from object & parallel to spring, rope or cable at point of attachment
Thrust
Fthrust
A general term for the forces that move objects such as rockets, planes, cars & people
In same direction as acceleration of object
Weight Fg
A long-range force due to gravitational attraction between two objects, generally Earth & an object
Straight down toward center of Earth

6. Example 1
A book is at rest on a table top. Diagram the forces acting on the book.

7. Example 1
In this diagram, there are normal and gravitational forces on the book.

8. Example 1
The forces are balanced (they cancel each other out)

9. Example 2
An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.

10. Example 2
Gravity is the only force acting on the egg as it falls.

11. Example 2
The forces are unbalanced, so the egg will accelerate downward.

12. Example 3
A flying squirrel is gliding from a tree to the ground at constant velocity. Consider air resistance. Draw a free body diagram for the squirrel.

13. Example 3
Gravity pulls down on the squirrel while air resistance keeps the squirrel in the air for a while.

14. Example 4
A rightward force is applied to a book at rest, in order to move it across a desk. Consider frictional forces. Neglect air resistance. Construct a free-body diagram for the book.

15. Note the applied force arrow pointing to the right
Note the applied force arrow pointing to the right. Notice how friction force points in the opposite direction. Finally, there are still gravity and normal forces involved.

16. Example 5
A skydiver is falling with a constant velocity. Consider air resistance. Draw a free-body diagram for the skydiver.

17. Gravity pulls down on the skydiver, while air resistance pushes up as she falls.

18. Example 6
A man drags a sled across loosely packed snow with a rightward acceleration. Draw a free-body diagram of the forces acting on the sled.

19. The rightward force arrow points to the right
The rightward force arrow points to the right. Friction slows his progress and pulls in the opposite direction. Since there is not information that we are in a blizzard, normal forces still apply as does gravitational force since we are on planet Earth.

20. Example 7
A football is moving upwards toward its peak after having been booted by the punter. Neglect air resistance. Draw a free-body diagram of the football in mid-air.

21. The force of gravity is the only force described
The force of gravity is the only force described. It is not a windy day (no air resistance).

22. Example 8
A car runs out of gas and coasts to a stop on flat ground. Draw a free body diagram of the forces acting on the car.

23. Even though the car is coasting down the hill, there is still the dragging friction of the road (left pointing arrow) as well as gravity and normal forces.

24. **TURN IN YOUR ROCKET PACKETS! MODEL ROCKETS SHOULD BE COMPLETE
Warm-up
**TURN IN YOUR ROCKET PACKETS!
MODEL ROCKETS SHOULD BE COMPLETE
Draw a free-body diagram for the scenarios below, then draw the direction of the Net Force.
A rock is dropped from the top of a building and is in free-fall (neglect air resistance).
A rock is dropped from the top of a building and is in free-fall (consider air resistance).
A feather is dropped from the top of a building and floats to the ground at a constant velocity.

25. Get out your Homework (#1-11)
Warm-up
Get out your Homework (#1-11)
Draw a free-body diagram for the scenarios below, then draw the direction of the Net Force.
A rocket’s parachute deploys as it is falling back to the ground at a constant velocity.
A man is pushing a car and it is moving at a constant velocity.
Draw a free-body diagram for the car.
Draw a free-body diagram for the man.

26. Get out your Homework What is the Net Force on the object below?
Warm-up
Get out your Homework What is the Net Force on the object below?

27. Vector Addition for Forces

28. Masses on an Incline Plane

29. Masses on an Incline Plane

31. Quantitative Free Body Diagrams

32. Example 1

33. Example 2 0.5 kg chandelier hangs for the ceiling.
Calculate the forces at work.

34. Quantitative Free Body Diagrams on an Incline Plane
Set up the Free Body Diagram (tilted axis)
Draw component forces for forces at an angle
Use the vector triangle to solve.

35. Example 3 A 2 kg box sits on a 30° inclined plane at rest.
Draw a Free Body Diagram & calculate the forces at work.