Chapter 4 : Laws of Motion

1. Chapter 4 : Laws of Motion Weerachai Siripunvaraporn Department of Physics, Faculty of Science Mahidol University email&FB : wsiripun2004@hotmail.com

2. What is in this chapter? Force is the causes of motion.

3. Force In previous chapter, we described motion in terms of position, velocity, and acceleration. But we have not considered the causes of motion. Here, we begin our investigation of the causes of motion. Definition: 1. a force is a push or a pull that causes an object to move. 2. a force is something that causes an object to accelerate. Forces have both magnitude and direction, so forces are vector quantities.

4. Contact and Field Forces • No physical contact is required CH5

5. Fundamental Forces • Gravitational force • Between objects • Electromagnetic forces • Between electric charges • Nuclear force • Between subatomic particles • Weak forces • Arise in certain radioactive decay processes • Note: These are all field forces. CH5 Section 5.1

6. ∑F = F1 + F2

7. F F1 ∑F = F1+ F2 F2 Net Force

8. Net Force Each force and net force can be divided into components.

10. Sir Isaac Newton • 1642 – 1727 • Formulated basic laws of mechanics • Discovered Law of Universal Gravitation • Invented form of calculus • Many observations dealing with light and optics CH5 Section 5.1

11. Inertial frames are frames of reference that are not accelerating (i.e. not moving or moving at constant velocity) A reference frame that moves with constant velocityrelative to the distant stars isthe best approximation of an inertial frame, and for our purposes we can consider theEarth as being such a frame. The Earth is not really an inertial frame because of its orbitalmotion around the Sun and its rotational motion about its own axis, both ofwhich result in centripetal accelerations. However, these accelerations are small comparedwith g and can often be neglected. For this reason, we assume that the Earth isan inertial frame, as is any other frame attached to it.

12. If there is no force acting on it, it remains the same.

13. Newton’s First Law – Alternative Statement • In the absence of external forces, when viewed from an inertial reference frame, an object at rest remains at rest and an object in motion continues in motion with a constant velocity. • Newton’s First Law describes what happens in the absence of a force. • Does not describe zero net force • Also tells us that when no force acts on an object, the acceleration of the object is zero • Can conclude that any isolated object is either at rest or moving at a constant velocity CH5 Section 5.2

14. Applications of Newton’s first law

15. Applications of Newton’s first law

16. Which is easier to pull, a shredder or a fire truck? A fire truck is more resistant to changes in its velocity than the shredder. How can we quantify this concept? • Mass is that property of an object that specifies how much resistance an object exhibitsto changes in its velocity. • The SI unit of mass isthe kilogram (kg). • The greater the mass of an object, the less that object accelerates underthe action of a given applied force.

17. More About Mass • Mass is an inherent property of an object. • Mass is independent of the object’s surroundings. • Mass is independent of the method used to measure it. • Mass is a scalar quantity. • Obeys the rules of ordinary arithmetic • The SI unit of mass is kg. • Mass and weight are two different quantities. CH5 Section 5.3

18. Force is the cause of changes in motion, as measured by the acceleration. • Remember, an object can have motion in the absence of forces. • Do not interpret force as the cause of motion. Notice that the acceleration is in the same direction as the resultant force.

19. is the net “external” force This is the vector sum of all the forces acting on the object. May also be called the total force, resultant force, or the unbalanced force. Newton’s Second Law can be expressed in terms of components: Remember that ma is not a force. The sum of the forces is equated to this product of the mass of the object and its acceleration. The SI unit of force is the newton (N). 1 N = 1 kg·m / s2 Newton’s Second Law CH5 Section 5.4

22. action force reaction force action force reaction force

24. action force reaction force

27. Free Body Diagrams and the Particle Model • The particle model is used by representing the object as a dot in the free body diagram. • The forces that act on the object are shown as being applied to the dot. • The free body helps isolate only those forces acting on the object and eliminate the other forces from the analysis. CH5 Section 5.6

28. External and Internal Forces and System We only care about the external forces. To tell which forces are external forces, we must define system of interest first. If dog is system, External force …

29. External and Internal Forces F1’ F2’ m1 m2 F Force F acting on m1 and m2, there is internal force between m1 and m2. If m1 and m2 are our system of interest, F is external force and F1’ and F2’ are internal forces. If m1 is our system of interest, F and F2’ are external force. If m2 is our system of interest, F1’ is external force.

32. 130 N

34. Forces in every day • Gravitational force and weight • Normal force • Tension force • Friction

35. The gravitational force, , is the force that the earth exerts on an object. This force is directed toward the center of the earth. From Newton’s Second Law: Its magnitude is called the weight of the object. Weight = Fg= mg Gravitational Force & Weight • Because it is dependent on g, the weight varies with location. • g, and therefore the weight, is less at higher altitudes. • This can be extended to other planets, but the value of g varies from planet to planet, so the object’s weight will vary from planet to planet. • Weight has a unit of Newton. CH5

36. But g is not constant, decrease with increasing distance from the surface. Therefore, weight is not constant. g  1/r2 g near the surface is about 9.8 m/s2 and vary from point to point.

37. Mass vs. Weight • Mass and weight are two different quantities. • Weight is equal to the magnitude of the gravitational force exerted on the object. • Weight will vary with location. CH5 Section 5.3

38. What is your mass on Earth and on Moon? What is your weight on Earth and on Moon? g near Earth’s surface is about 10 m/s2 g near Moon’s surface is about 10/6 m/s2 Mass = 60 kg Mass = 60 kg Weight = 60 kg x 10 m/s2 Weight = 60 kg x 10/6 m/s2 = 600 N = 100 N

40. Normal Force N N N

42. A light smooth pulley Tension Force String tension is an electromagnetic force. The molecules in the string are pulling one another. Each portion of the string transmits the force undiminished from end to end.

43. If the acceleration of an object that can be modeled as a particle is zero, the particle is in equilibrium.

44. Fx = max Fy = may

45. Problem-Solving Hints – Applying Newton’s Laws • Conceptualize • Draw a diagram • Choose a convenient coordinate system for each object • Categorize • Is the model a particle in equilibrium? • If so, SF = 0 • Is the model a particle under a net force? • If so, SF = m a CH5 Section 5.7

46. Problem-Solving Hints – Applying Newton’s Laws, cont. • Analyze • Draw free-body diagrams for each object • Include only forces acting on the object • Find components along the coordinate axes • Be sure units are consistent • Apply the appropriate equation(s) in component form • Solve for the unknown(s) • Finalize • Check your results for consistency with your free-body diagram • Check extreme values CH5 Section 5.7

50. If we try to drag a box with an increasing force F, what would happen?