Practicing our FBDs

Links and useful resources

• START HERE: Physics 2024 class outline
• Physics classroom online interactive tools
• OpenSTAX high school physics
• NotebookLM physics notebook
• Physics projects
• AP Physics 1 Dan Fullerton videos

Lightning Round Questions

• gr7: [lightning::2]
• gr10: [lightning:: 1]
• Compare and contrast a theory and a law.
• What are the named SI prefixes for ${10}^{-9}$ up to ${10}^9$?
• Which subatomic particle has a positive charge?
• What is a hypothesis?
• What is a kilogram of falling figs? A fig newton.
• There's a new theory of inertia, but it doesn’t seem to be gaining momentum.

Demonstration

Concept summary and connections

• solving free body diagrams
• sine and cosine
• vector addition

Lesson content with examples

Conceptual diagramming

1. A book is at rest on a tabletop. Diagram the forces acting on the book.
2. A gymnast holding onto a bar, is suspended motionless in mid-air. The bar is supported by two ropes that attach to the ceiling. Diagram the forces acting on the combination of gymnast and bar.
3. A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. Diagram the forces acting on the squirrel.
4. A football is moving upwards towards its peak after having been booted by the punter. Neglect air resistance. Diagram the forces acting upon the football as it rises upward towards its peak.
5. A car is coasting to the right and slowing down. Neglect air resistance. Diagram the forces acting upon the car.

Solving FBDs

When you "solve" an FBD, you're really just adding up all of the vectors to see what the resultant force is. The resultant force is a force that has the exact same effect on the object as all of the other forces working together. Since forces are vector quantities, all you have to do to calculate it is add them up!

1. Do problems 5.8.3 and 5.8.1 on this page
2. The Rocinante is maneuvering in space. Its main engine is putting out ${10}^8$N of force, and it has a maneuvering thruster at 90$\mathrm{°}$ to the right that is putting out ${10}^6$ N. Draw a free body diagram to show the situation.
   1. If the ship has a mass of ${10}^7$ kg, what will its acceleration be as a result?
   2. From your perspective the ship starts out moving to the right at ${10}^4\frac{m}{\sec }$, and the main thrust is pointed to the right while the maneuvering thruster is pointed down. Draw a motion diagram that represents the ship's path from your viewpoint.
3. The flying squirrel from above weighs 1N, and is following a flight path that is 30$\mathrm{°}$ from horizontal. Use the diagram you created to figure out what the resultant force must be.
   1. How much wind resistance is the squirrel facing in its direction of motion?
   2. What is the lift force (the force perpendicular to the "wings" that keeps it gliding instead of falling)?

Media resources

• Youtube search for "solving free body diagrams"
• Youtube search for "sine and cosine"
• Youtube search for "vector addition"

Guided practice

In class, we will model and solve FBDs for these situations:

• Pendulum, mid-swing forces
• Ball rolling on flat surface

Other questions to solve:

• Driving at 20m/s when a deer jumps in front of car 35m in front of you. Your reaction time is 0.5s, and the maximum deceleration of the car is 10m/s^2. How much distance is left between you and the deer when you stop?
• A basketball player can jump to a height of 55 cm. How far above the floor can he jump in an elevator that is moving downward at a constant 1 m/s?

Homework

• #hw (physics) Balancing Forces Interactive Try to reach level 10! 2025-01-27