Chapters 4,5, & 6

1. Chapters 4,5, & 6 Newton’s Laws of Motion Conceptual Physics

2. Chapter 4 Newton’s First Law Conceptual Physics

3. Newton’s First LawThe Law of Inertia • An object at rest tends to stay at rest. • An object in motion tends to stay in motion. Conceptual Physics

4. Inertia:A resistance to change, a property of all matter. Conceptual Physics

5. Mass: A Measure of Inertia • Mass is measured in kilograms • Mass is not Weight • Mass is a “built in” property of matter • Just because you leave earth, you don’t change your mass, but you do change your weight • Weight is an force caused by the acceleration due to gravity on the mass Conceptual Physics

6. Weight • Mathematically Weight = mass x gravity Or Fw = mg • So the force of weight on one kilogram of mass on planet earth is given by: Fw = (1 kg)(9.8 m/s2) = 9.8 Newtons Conceptual Physics

7. Sample Problem What is the weight of a 10 kg object a) on earth b) on the moon (g = 1/6 that of earth) Conceptual Physics

8. Solution a) Fw (on earth) = mg = (10 kg)(9.8 m/s2) = 98 Newtons b) Fw (on moon) = mg = (10 kg)(1.6 m/s2) = 16 Newtons Conceptual Physics

9. Net Force and Equilibrium • Net force is the sum of all the forces acting on an object • When the net force on an object equal zero – the object is said to be in equilibrium. Conceptual Physics

10. Fw FN Net Force and Equilibrium When a statue sits on the ground, the weight of the statue pushes downward and a support force* from the ground pushes upward, so the statue is in equilibrium * The support force is also called the Normal Force Conceptual Physics

11. 20 N 20 N Sample Problem If a box with a weight of 20 Newton sits on a table, with what normal force does the table push back? A second, 10 Newton force is added to the top of the first box. Now what is the normal force exerted by the table on the 20 Newton box? What normal force is exerted by the 20 Newton box on the 10 Newton box? 10 N Conceptual Physics

12. Chapter 5 Newton’s Second Law of Motion Conceptual Physics

13. Newton’s Second LawThe Law of Acceleration • Forces cause acceleration • Net force must be greater than zero • Masses resist acceleration due to inertia • This is, in part, why it is harder to start something moving than to keep it moving • Hence we say that acceleration is directly proportional to net force and inversely proportional to mass Conceptual Physics

14. Newton’s Second LawThe Law of Acceleration • The acceleration of an object is in the direction of the force applied. • Acceleration is directly proportional to the force applied. • The harder you push an object the faster it goes • Acceleration is inversely proportional to the mass of the object. • The heavier the object, the less affect a push has. Conceptual Physics

15. Sample Problem What force is required to accelerate this object horizontally at 6 m/s2? Conceptual Physics

16. Solution F = ma = (10 kg)(6 m/s2) = 60 Newtons Conceptual Physics

17. Friction • Forces always come in pairs, hence the Normal force, which is perpendicular to the contact surface, has a companion force that is parallel to the contact surface, this force is friction • Friction always opposes motion • Friction depends upon two things: • The nature of the contact between two objects • How strong the force of contact is (The Normal Force) Conceptual Physics

18. Friction • Friction also occurs in gases and liquids both of which are referred to in physics as fluids. • In fluids we call friction drag and in air we refer to it specifically in air as air resistance. Conceptual Physics

19. Sample Problem If a 200 Newton force is applied to a box that undergoes a 100 Newton resistive force (friction). What is the net force on the box? If it is a 30 kg box, what is its acceleration? Conceptual Physics

20. Solution Fnet = 200 N – 100 N = 100 N Fnet = ma so a = Fnet/m = 100 N/30 kg = 3.3 m/s2 Conceptual Physics

21. Pressure • Pressure is defined as the force exerted divided by the amount of area over which the force is spread • Mathematically this is expressed as: P = F/A • P – Pressure • F = Force – in lbs or newtons • A – area – usually in meters2 or centimeters2 Conceptual Physics

22. Chapter 6 Newton’s Third Law Conceptual Physics

23. Newton’s Third LawAction - Reaction • For every action there is an equal and opposite Reaction. Conceptual Physics

24. Conceptual Physics

25. Sample Problem If a 0.4 kg shotgun shell undergoes a 100 Newton force when it is fired, what is its acceleration? If it was fired from a 2 kg shotgun what is the recoil acceleration of the shotgun? Conceptual Physics

26. Solution Since F = ma; a = F/m So: a = 100 N/0.4 kg = 250 m/s2 Since the recoil force is equivalent to the firing force according to Newton’s Third Law the same equation applied however now you use the mass of the shotgun: a = 100 N/2 kg = 50 m/s2 Demonstrating that it is the mass of the shotgun that keeps it from doing the same damage as the bullet Conceptual Physics