An Introduction to Forces Forces-part 1. Forces are Interactions

Transcription

1 An Introduction to Forces Forces-part 1 PHYS& 114: Eyres Forces are Interactions A force is an interaction between 2 objects Touching At a distance See the Fundamental Particle Chart ( Identifying Interactions Choose a system: circle the object Name the force and both interacting objects 1

2 What Is a Force? A force is a vector. The general symbol for a force is the vector symbol F r. The size or strength of such a force is its magnitude F. Contact forces are forces that act on an object by touching it at a point of contact. Long-range forces (Non-Contact forces) are forces that act on an object without physical contact. Identifying Forces 2

3 Five Types weight: Symbol W Pulling Between 2 objects with mass Tension: Symbol T Pulling Must have a rope or string or spring attached Normal: Symbol N or sometimes n Pushing Due to contact with a surface Spring Force: Symbol F sp Pushing or Pulling Due to contact with a spring friction: Symbol f Opposite direction to motion or the direction of potential motion Rubbing (kinetic friction) or Stickiness associated with trying to move it (static friction) Newton s 1 st Law Newton s Laws If there is no net Force, there is a constant velocity. What might the x vs t graph look like? What might the v vs t graph look like? What might the a vs t graph look like? 3

4 Newton s 1 st Law Newton s Laws If there is no net Force, there is a constant velocity. What might the x vs t graph look like? What might the v vs t graph look like? What might the a vs t graph look like? Newton s 2 nd Law Newton s Laws ΣF = ma What might the x vs t graph look like? What might the v vs t graph look like? What might the a vs t graph look like? 4

5 Newton s Laws Newton s 2 nd Law F = ma What might the x vs t graph look like? What might the v vs t graph look like? What might the a vs t graph look like? Newton s Second Law 5

6 Units of Force The basic unit of force is called a newton. One newton is the force that causes a 1 kg mass to accelerate at 1 m/s 2. 1 pound = 1 lb = 4.45 N Question 1: Select the statement that is NOT correct. A. Force has both magnitude and direction. B. Force is an interaction between two objects. C. The SI unit of a force is the kg. D. Pushes and pulls are examples of forces Pearson 6

7 Question 2: Which type of motion is possible for the system object when the sum of forces is zero? (Select all that are possible.) Be prepared to give an example for your choice(s). A. Constant velocity motion B. Increasing velocity motion C. Decreasing velocity motion D. Zero velocity motion (not moving) 2014 Pearson 1 st Law or 2 nd Law? Graph Sketches? X=0 7

8 1 st Law or 2 nd Law? Graph Sketches? X = 0 Constant Speed therefore v=0 and a=0 therefore 1 st Law x vs. t v vs. t a vs. t 1 st Law or 2 nd Law? Graph Sketches? X=0 8

9 1 st Law or 2 nd Law? Graph Sketches? X=0 Speeding up therefore v not 0 and a not 0 therefore 2 nd Law x vs. t v vs. t a vs. t 1 st Law or 2 nd Law? Graph Sketches? X=0 9

10 1 st Law or 2 nd Law? Graph Sketches? X=0 Slowing down therefore v not 0 and a not 0 therefore 2 nd Law x vs. t v vs. t a vs. t Force Notation I am pushing down on a book that is lying on the table. Always identify the TWO OBJECTS involved in the interaction. Force Type By On Symbol Weight Earth Book WE on B Normal Me Book NM on B Normal Table Book NT on B 10

11 Identifying Forces Drawing force diagrams 1. Sketch the situation. 2. Circle the system. 3. Does the object have mass? Identify the weight (long range) force. 4. What is touching it? Turn these into a list of forces. 5. Place a dot at the side in the center of a coordinate system to represent the object. 6. Draw force arrows to represent the forces (interactions). 7. Label the forces with a subscript containing the two interacting objects Pearson 11

12 Identifying Forces Conceptual Example 4.1: Identifying forces on a bungee jumper A bungee jumper has leapt off a bridge and is nearing the bottom of her fall. What forces are being exerted on the bungee jumper? 12

13 Example 4.7 Forces on an elevator An elevator, suspended by a cable, speeds up as it moves upward from the ground floor. Draw a free-body diagram of the elevator. Example 4.7 Forces on an elevator An elevator, suspended by a cable, speeds up as it moves upward from the ground floor. Draw a free-body diagram of the elevator. 13

14 Conceptual Ex. 4.2: Identifying forces on a skier A skier is being towed up a snow-covered hill by a tow rope. What forces are being exerted on the skier? Example 4.9 Forces on a towed skier A tow rope pulls a skier up a snow-covered hill at a constant speed. Draw a full visual overview of the skier. 14

15 Example Problem Consider pushing a block across the table at a steady speed. Since you re exerting a force on it, why isn t it accelerating? Identify all the forces and draw a free-body diagram. Compare the size of the pushing force and the size of the friction force. Newton s Laws Newton s 3 rd Law If Object 1 pushes on Object 2 then Object 2 pushes on Object 1 On 2 By 1 N 21 On 1 By 2 N 12 This can help to identify forces 15