Chapter 2 Mechanical Equilibrium

Transcription

1 Chapter 2 Mechanical Equilibrium

2 I. Force (2.1) A. force is a push or pull 1. A force is needed to change an object s state of motion 2. State of motion may be one of two things a. At rest b. Moving uniformly along a straight-line path.

3 B. Net force 1. Usually more than one force is acting on an object 2. combination of all forces acting on object is called net force. 3. The net force on an object changes its motion

4 4. Can add or subtract to get resultant net force 5. If forces acting on object equal zero then we say the net force acting on the object = 0 6. Scientific units for force are Newtons (N)

5 C. Tension and Weight 1. Tension is a stretching force

6 2. When you hang an object from a spring scale the there are two forces acting on object. a. Force of gravity pulling down (also called weight) b. Tension force pulling upward c. Two forces are equal and opposite in direction and add to zero (net force = 0)

7 D. Force Vectors 1. Forces can be represented by arrows a. length of arrow represents amount (magnitude) of force b. Direction of arrow represents direction of force c. Refer to arrow as a vector (represents both magnitude and direction of force

8 2. Vector quantity- needs both magnitude and direction to complete description (i.e. force, velocity, momentum)

9 3. Scalar quantity- can be described by magnitude only and has no direction (i.e. temperature, speed, distance)

10 Scaler or Vector quantity? Time interval? scaler Mass? scaler Speed? scaler Velocity? vector Temperature? scaler Displacement? vector Acceleration? vector

11 Arrow-tipped line segment Length represents magnitude Arrow points in specified direction of vector

12 Vectors are equal if: magnitude and directions are the same Vectors are not equal if: have different magnitude or direction or

13 Are these vectors equal or not? NO YES NO YES NO

14 II. Mechanical Equilibrium (2.2) A. Mechanical equilibrium- a state wherein no physical changes occur (state of steadiness) 1. When net force equals zero, object is said to be in mechanical equilibrium a. Known as equilibrium rule

15 b. Can express rule mathematically as F 0 1). symbol stands for the sum of 2). F stands for forces 2 N 2 N F ( 2 2) 0-2 N -2 N

16 III. Support Force (2.3) A. support force- the upward force that balances the weight of an object on a surface 1. The upward force balances the weight of an object 2. Support force often called normal force Support force weight

17 B. For an object at rest on a horizontal surface, the support force must equal the objects weight. 1. Upward force is positive (+) and the downward force is negative (-). 2. Two forces add mathematically to zero F 0 2 N 2 N F ( 2 2) 0-2 N -2 N

18 IV. Equilibrium of Moving Objects (2.4) A. Equilibrium can exist in both objects at rest and objects moving at constant speed in a straight-line path. 1. Equilibrium means state of no change 2. Sum of forces equal zero F 0

19 B. Objects at rest are said to be in static equilibrium C. Objects moving at constant speed in a straight-line path are said to be in dynamic equilibrium Static equilibrium dynamic equilibrium

20 V. Vectors (2.5) A. Parallel vectors 1. Add vectors if in same direction 2. Subtract vectors if in opposite direction 3. The sum of two or more vectors is called the resultant vector. + = +

21 B. Parallel vectors simple to add or subtract add subtract

22 Let s say you are taking a trip to Hawaii. The distance to Hawaii is 4100km and you travel at 900km/hr. How long should it take you to reach Hawaii? Let s do the math. (4100km ) (900km/h) (4100h) = = 4.56 hours (900) It should take you the same amount of time to return.. Right? Does it? Why not?

23 Remember, we can use vectors to describe things such as velocity. Vectors tell us direction and magnitude Let s look at the velocity vectors that might describe the airplane s velocity and the wind s velocity Airplane vector to Hawaii = Wind vector = subtract vectors = Airplane vector from Hawaii = What is the difference in speed? What about the direction? Wind vector = Add together =

24

25 B. Non-parallel vectors 1. Construct a parallelogram to determine resultant vector 2. The diagonal of the parallelogram shows the resultant a. Perpendicular vectors R

26 At one point the lunar lander was traveling 15 ft/sec forward while traveling down at 4 ft/sec. What was the direction they were actually moving?

27 Combining vectors that are NOT parallel Result of adding two vectors called the resultant Resultant of two perpendicular vectors is the diagonal of the rectangle with the two vectors as sides Resultant vector

28 Use simple three step technique to find resultant of a pair of vectors that are at right angles to each other. First draw two vectors with their tails touching.

29 Second - draw a parallel projection of each vector with dashed lines to form a rectangle

30 Third - draw the diagonal from the point where the two tails are touching resultant

31 b. Perpendicular vectors with equal sides (special case) 1). For a square the length of diagonal is or ). Resultant = x one of 2 sides 2 1 Resultant = 2 1

32 c. Parallelogram (not perpendicular) Construct parallelogram Construct with two vectors as sides Resultant is the diagonal resultant

33 C. Applying the Parallelogram Rule - as angle increases, tension increases.

34

35 Pythagorean Theorum- can be used if vectors added are at right angles R 2 A 2 B 2 or R A 2 B 2 A R 90 B

36 SOHCAHTOA sin, cos, and tan are functions of the angle of a triangle compared to the lengths of the sides of a triangle If you know the distances of the triangle sides, you can determine the inside angles. If you know the angle and one side, you can calculate the length of the other side of the triangle. Remember the following acronym: SOHCAHTOA These will give you the formula depending on which side or angle you need to calculate sin cos tan H O H A A O

37 Components calculated using following formulas A A x y Acos ; therefore, cos Asin; therefore, sin adjacent hypotenuse opposite hypotenuse When angles larger than 90, sign of one or more components may be negative Ax A A y A

38 Remember what the sides of a triangle are called Hypotenuse Opposite θ Adjacent

39 Example: sin 25 = sin 25 = Length of Opposite side Length of Hypotenuse Length of Opposite side 100 meters length of Opposite side = 38.3 meters 100 meters Opposite =? θ Adjacent

40

41 Graphical Addition of Vectors Simple method for combining vectors to get resultant vector Use ruler to measure length of vector Use protractor to measure angle 55