Chapter 4 Forces and the Laws of Motion

1. Chapter 4 Forces and the Laws of Motion

2. 4-1 Changes in Motion Force • Forces describe the interactions between an object and its environment. Force

3. Forces can cause accelerations • A force on an object can change the object’s velocity with respect to time. • A change in velocity with respect to time – an acceleration.

4. The SI unit for force is the newton • The SI unit for force is the newton, named after Isaac Newton (1642-1727) • The newton (N) is defined as the amount of force that, when acting on a 1 kg mass, produces an acceleration of 1 m/s2. • The weight of an object is a measure of the gravitational force exerted on the object. 2 liter bottle of water = about 20 N of force

5. Forces can act through contact or at a distance • Pushes and pulls are examples of contact forces. • Another type of force is a field force, these include gravity and electromagnetism. Forces can act through contact or at a distance.

6. Force Diagrams • If you push a toy, it accelerates. • The acceleration depends on the force’s magnitude. • The direction the toy moves depends on the direction of the force.

7. Force is a vector • Force depends on magnitude and direction, therefore it is a vector quantity. • Diagrams that show force vectors as arrows, are called force diagrams.

8. A free-body diagram helps analyze a situation • When engineers analyze a test crash, they isolate the car from the other objects in its environment. • The forces the car exerts on other objects is not included in the diagram because they do not affect the motion of the car. • A free-body diagram is used to analyze only the forces affecting the motion of a single object as shown below. A free-body diagram helps analyze a situation

9. Practice A:Drawing Free Body Diagrams

10. 4.1 Questions1. T / F Forces describe the interactions between an object and its environment.2. A change in velocity with respect to time is called an _____________3. The SI unit for force is the __________4. Give an example of a field force __________5. A vector quantity depends on magnitude and __________ True acceleration newton gravity direction

11. 4-2 Newton’s First Law Inertia • Can an object with no force acting on it have to be at rest? • The answer is no. If the object is moving at a constant speed in deep space, it just continues at the same speed and direction.

12. Imagine a block sitting on thick carpet. Trying to push it would be difficult. • Now imagine the same block on smooth ice. One push may allow it to slide for a very long distance. • To summarize, an object can move at constant velocity with no force acting on it. This is known as, • The tendency for an object not to accelerate is called inertia. • The more inertia something has, the less likely it is to move. Inertia

13. The sum of the forces acting on an object is the net force • All the forces acting on an object equal the net force. • An external force is a single force that acts on an object as a result of the interaction between the object and its environment. • The net force then is the vector sum of all the forces acting on the object. • If the object is traveling at constant velocity, the vector sum of all of the forces are equal to zero.

14. Mass is a measure of inertia • Newton’s first law states that if two objects have different mass, applying the same force to each will cause the lighter one to move away faster. • The greater the mass of something, the greater its inertia or resistance to change.

15. Practice B:Determining Net Force • Derek leaves his physics book on top of a drafting table that is inclined at a 35o angle. This free-body diagram shows the net forces acting on the book.

17. Equilibrium • Objects at rest or moving at constant velocity are said to be in equilibrium. • To be in equilibrium: the net force acting on a body must be equal to zero. • Another way to look at this is: when the vector sum of the forces acting on the object is equal to zero.

18. 4.2 Questions1. Can an object with no force acting on it have to be at rest?2. T / F The tendency for an object not to accelerate is called inertia.3. All the forces acting on an object equal the ____ force.4. Objects at rest or moving at constant velocity are said to be in __________ No True Net equilibrium

19. 4-3 Newton’s second and third Laws Force is proportional to mass and acceleration • Imagine pushing a stalled car. • If there is a force on the car it will accelerate. • If you get a bunch of friends to help, the force will increase and the car will accelerate faster. • A lighter car will accelerate faster than a heavy truck if the forces were the same.

20. Newton’s second law relates force, mass, and acceleration • The Greek symbol Σ is called sigma. In front of F it represents the vector sum of all the external forces acting on the object. Newton’s Second Law

21. Practice C:Newton’s Second Law 0.73 m/s2

22. Newton’s Third Law • A force is exerted on an object when that object interacts with another object in its environment. • A car can exert a force on a barrier when it collides with it. At the same time however, the barrier exerts a force on the car. • If you close a door, you apply a force on the door and the door applies a force on your hand.

23. Forces always exist in pairs • Newton recognized that a single isolated force cannot exist. • Instead, forces always exist in pairs. Newton’s Third Law

24. An alternate statement of this law is that for every action, there is an equal and opposite reaction. • This gives us an action-reaction pair. • One object exerts the action force and the other object exerts a reaction force that is equal and opposite. • The action and reaction force occur exactly at the same time.

25. Action and reaction forces each act on different objects • A hammer exerts a force on a nail, the nail exerts a force on the hammer as well. • If the force that the nail exerts on the hammer is equal and opposite, why does the nail not stay at rest? • The answer is - the net force is equal. • Note in this free-body diagram. • In this example the nail will be driven into the wood because the net force is zero. • Basically, the nail accelerates and the hammer slows down.

26. Field forces also exist in pairs • The force that the earth exerts on a crash dummy’s head is Fg. • Because this is a force, it will cause the head to accelerate downward. • When the head strikes the earth, it actually effects the earth to a small degree. The earth is so massive compared to the head that we are not able to measure the net force. • If you compare the gravitational pull of the moon with the earth, you can calculate the effects they have on each other.

27. 4.3 Questions1. The acceleration of an object is proportional to the net force acting on the object and inversely proportional to the object’s ______2. Forces always exist in ______3. For every action, there is an equal and opposite __4. If the force that the nail exerts on the hammer is equal and opposite, why does the nail not stay at rest? mass pairs reaction It reacts to slowing the hammer down

28. 4-4 Everyday Forces Weight • How do you know a bowling ball weighs more than a tennis ball? • Could you tell in space if you had never seen the objects before? • Here on earth, gravity exerts a force Fg. The magnitude of this force is called weight. • Weight depends on location. • Here on earth, a 180 lb. person exerts a force of 800 N downward, on the moon only 130 N. Weight

29. The Normal Force • The television below is in equilibrium. • We know another force is acting opposite gravity or Fg. • The force pushing up by the table is called the normal force or Fn. • We use the term normal because it is perpendicular to the table.

30. Force of Friction Friction opposes the applied force • The jug below (a) is in equilibrium. The normal force is in balance with the gravitational force. • If another force is applied from the side of the jug, the jug resists this force due to friction created between it and the table. This is known as static friction. • Until a force is applied that is great enough to start moving the jug, it will remain motionless until the maximum static friction is overcome or Fs, max. Friction

31. Kinetic friction is less than static friction • When enough force is applied to an object that it finally over comes static friction, it begins to move. • Kinetics is the movement of an object. • Moving objects experience slightly less friction than they would at Fs, max. • This third type of friction is called kinetic friction or Fk.

32. The force of friction is proportional to the normal force • The heavier an object is, the more force of gravity acts on it. • If two objects with the same surface area, but different masses, are placed on the same surface, they will experience different frictional forces. • Therefore, the greater the mass, the greater the normal force. This causes friction to be greater as well.

33. Friction can be calculated approximately • Friction is really an effect caused by a complex combination of forces. • We can approximately calculate the force of friction with certain assumptions. • The relationship between the normal force and the force of friction provides a good approximation for the friction.

34. The force of friction also depends on the composition and qualities of the surfaces in contact. • Moving a desk across a tile floor requires less force that a floor of carpet. • Our quantity that expresses frictional force is called the coefficient of friction. • The coefficient of friction is expressed with the symbol μ. It is the Greek letter mu.

35. The coefficient of friction is a ratio of forces • The coefficient of kinetic friction is the ratio of the force of kinetic friction to the normal force. • Remember, kinetic friction is always less than static friction.

36. Practice D:Coefficients of Friction μ= 0.32

37. Practice E:Overcoming Friction 2 m/s2

38. Air resistance is a form of friction • Another type of friction is air resistance. • Both water and air are considered fluid mediums. • They can provide resistance to an object’s motion. • Air resistance FR on a moving object is proportional to the square of the object’s speed. • If you double the speed of a car from 50 to 100 mph, the resistance increases 4 times! • That is why a skydiver does not keep falling faster and faster. They reach a terminal speed of about 120 mph.

40. There are four fundamental forces • The strongest of the fundamental field forces is the strong force. It holds the atomic nucleus together but has extremely short range. • The next strongest is the weak force. It is responsible for nuclear decay and also has a limited range. • Next weakest is the electromagnetic force. Magnets have greater ranges and are strong enough to lift objects to oppose gravity. • The weakest force is gravity. As far as we know, it has unlimited range!

41. 4.4 Questions1. Here on earth, gravity exerts a force Fg. The magnitude of this force is called _________2. We use the term normal because it is ________ to a table or floor.3. When enough force is applied to an object that it finally over comes _____ friction, it begins to move.4.T / F kinetic friction is always less than static friction. weight perpendicular static True

42. End