Understanding Forces: Equilibrium and Free Body Diagrams in Physics

1. New UnitForces • What are forces? • How do we represent them in a picture? • How do we calculate each force individually? • How do we deal with multiple forces acting at once? DO NOW: Complete the following chart (add as many rows as you can)

2. A AIM: What does it mean to be in physical equilibrium C B At rest Free Falling with small air resistance Constant velocity while pedaling Do Now: Look at the following situations and Decide if the object is in equilibrium or not Give a reason for your answer D F Moon in orbit around Earth E Terminal velocity Constant speed around a curve

3. A cup At rest

4. B F Constant speed while pedaling Terminal velocity bike man

5. Moon in orbit around Earth E D Constant speed around a curve moon car

6. C Free Falling with small air resistance santa

7. Equilibrium: • An object is in equilibrium when: • The total net force (vector sum of all the forces) acting on the object is equal to zero • If the net force is equal to zero, the object is not accelerating. • This means the object is either • At rest • Moving at a constant VELOCITY • Constant velocity means your speed is constant AND your direction is constant

8. Not in Equilibrium • On object is not in equilibrium (accelerating) when: • The total net force (vector sum of all the forces) acting on the object is NOT equal to zero • The acceleration of an object will ALWAYS be in the exact same direction as the net force. • REMEMBER: acceleration can either be a change in the magnitude of your velocity OR a change in the direction of your velocity

9. Representing the Forces acting on an object using a Free Body Diagram • A free body diagram is a graphical way to show all the forces acting on an object. • To draw a free body diagram; • Represent the object in question with a dot • Represent each force acting on that object with an arrow • The arrow should start at the dot and point in the direction of the force • The length of the arrow should represent the relative strength of the force. • The arrow should be labeled with the force it represents

10. Do Now: 1. Draw a free body diagram for the following2. State whether the body is in equilibrium or not • A car moving at a constant velocity • A person sitting at a desk • A planet in orbit around the sun • A motorcycle accelerating to the right (include friction) • An elevator accelerating upwards • An elevator moving upwards at a constant speed

11. Finding NET FORCEVector Addition of Forces • To find the Net Force (resultant force) • Draw all forces head to tail • Use trig and SohCahToa to either break down angled forces or construct resultants • Graphically draw each force head to tail using a scale at the given angle • Construct the resultant from start to finish. • Measure its length and angle

12. Equilibrant Force • The equilibrant force is the force that would produce equilibrium for an object • It is equal in magnitude to the resultant force, but in the exact opposite direction!

13. Maximum and Minimum Resultant • For any two concurrent forces (forces that act on the same object at the same time), • The maximum resultant • is the sum of the two forces • They will be 0o relative to each other • The minimum resultant • Is the difference of the two forces • They will be 180o relative to each other

14. Find the Resultant and Equilibrant for the following combinations of forces. Complete work on a separate piece of paper SHOW ALL WORK! A • B+E • B-E • A+B • A+B+E • A + D (advanced) • B+C (advanced) D 10N 7N 12N B 25o E 2N 50o 5N C

15. HOME WORK • Pick 5 objects/situations (that are not the same) • Describe what is happening in 1 sentence • Draw a free body diagram of the object • Label the forces • Make sure the length of each arrow represents its relative strength