Understanding Forces in Equilibrium

1. Unit 3: ForcesNotes #4 - Forces in Equilibrium Students will be able to describe forces in equilibrium by… • Drawing vectors to scale. • Describing and distinguishing different types of forces. • Creating and analyzing free body diagrams

3. The floor exerts an equal and opposite force upward on the box called the normal force. • The box is in equilibrium at 0N.

4. Vector quantities • A vector is a quantity that includes both magnitude and direction. • Magnitude: shows the quantity or strength (Number & Unit). • Force vectors have the same units as force – Newtons • Vectors can combined.

5. The force vector • We define positive direction as up or right. • We define negative direction as down or left. • A vector must be drawn to scale to show magnitude. • EX: 1 cm = 1 N or 1 graph square = 1N.

6. Free-body diagrams (FBD) • A free-body diagramis a diagram that uses vectors to show all of the external forces acting on an object. • A free-body diagram of a book shows only the forces acting onthe book.

7. Free body diagrams (FBD) • A free-body diagram for a monkey hanging from two ropes includes all forces acting on the monkey. • The tension forces of the ropes (F1 and F2) acts on the monkey, as well as gravity (Fw).

8. The net force & Free body • Consider the forces… • Book weighs 30 N • Table weighs 200N

9. Types of Forces

10. Gravitational force • Symbol: Fg • This is the object’s weight– it always points straight down. Fg

11. FN • Normal force • Symbol: FN • This is the upward force that is perpendicular to the surface an object sits on. Fg

12. FN Fapp • Applied force • Symbol: Fapp • This is the push or pull put on an object. • In the example, you’re pushing the object to the left. Fg

13. FT FT • Tension • FT • This is for hanging objects – in the direction the string is pulling the object. Fg A chandelier hanging from the ceiling by two cables

14. FN Fapp • Friction • Ff • This goes in the opposite direction of motion. • Air resistance = friction Ff Fg

15. Air resistance • When something falls through air, the air exerts an additional force. • This force, called air resistance, acts opposite to the direction of the object’s motion.

16. Terminal velocity • Objects only accelerate until the force of air resistance equals the force of gravity. • The net force then becomes zero and the object reaches a constant velocity called the terminal velocity.

17. Forces and equilibrium • When the net force on an object is zero, we say the object is in equilibrium. • Newton’s first law says an object’s motion does not change unless a net force acts on it. • If net force is 0N, then the object does not accelerate (move).

18. Forces and equilibrium • Gravity is a force that always present. • If is not moving, normal force is present. • Gravity always points down to Earth. • In mathematics, the normal force is means perpendicular (90 degrees).

19. Example of Drawing Free body Diagrams • A skydiver is descending with a constant velocity. Consider air resistance. A free-body diagram for this situation looks like

20. Example of Drawing Free body Diagrams • A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. • Consider frictional forces. Neglect air resistance. A free-body diagram for this situation looks like this:

21. Review: Draw vectors to scale 1. Draw a vector that is 5N to the right. 2. Draw a FBD with a gravitational force of 1N and a normal force of 2N. Use a scale!

22. Review: Describe and distinguish between different types of forces. • What force pulls objects toward the ground? • What force is a support force? • Air Resistance is an example of this type of force. • Weight is measured using this force.

23. Review: Create & analyze free body diagrams • Draw a free body diagram of this cheerleader in the air.