FORCES AND THE LAWS OF MOTION

Transcription

1 FORCES AND THE LAWS OF MOTION

2 FORCE A force is the cause of an acceleration, or the change in an object s velocity (speed or direction). Forces are usually thought of as a push or a pull. The SI unit of force is the Newton (N). The Newton is defined as the amount of force that, when acting on a 1 kg mass, produces an acceleration of 1 m/s². Therefore, 1 N = 1 kg m/s².

3 FORCES CAN ACT THROUGH CONTACT OR AS A FIELD Contact Forces are forces that arise from the physical contact of two objects. (Push or Pull) Field Forces are forces that can exist between objects, even in the absence of physical contact between the objects. Examples: gravity, electromagnetic, weak/strong nuclear

4 Physics 151: Lecture 5, Pg 10

5 WEIGHT The weight of an object is measure of the gravitational force on the object. It is the result of the interaction of an object s mass with the gravitational field of another object, such as Earth. 1 lb = N 1 N = lb

6

7 NEWTON S FIRST LAW Newton s First Law: An object at rest remains at rest, and an object in motion continues in motion with constant velocity (that is, constant speed in a straight line) unless the object experiences a net external force. The tendency of an object to maintain its state of motion is called inertia. Newton s first law is often referred to as the Law of Inertia.

8 Newton's First Law Newton's first law states: An object at rest will remain at rest, an object in uniform motion will stay in motion - UNLESS acted upon by an outside force Outside Force This is why you should always wear a seat belt!

9 NEWTON S FIRST LAW

10

11 MASS IS A MEASUREMENT OF INERTIA Imagine a basketball and a golf ball at rest side by side on the ground. Newton s first law states that both remain at rest as long as no net external force acts on them. Now imagine supplying a net force by striking each ball with a golf club. If the two are struck with equal force, the golf ball will accelerate much more than the basketball. The basketball experiences a smaller acceleration because it has more inertia than the golf ball.

12 ACCELERATION IS DETERMINED BY NET EXTERNAL FORCE The net external force is the vector sum of all the forces acting on an object. It is sometimes referred to as the resultant force. When there is no net force the object is at rest or constant motion. When there is a net force there is acceleration in the direction of the the resultant force. The Σ symbol means the sum of. The net external force must be found on the x and the y axis.

13 EQUILIBRIUM Objects that are either at rest or moving with constant velocity are said to be in equilibrium. Newton s first law states one condition that must be true for equilibrium:the net external force acting on a body in equilibrium must be equal to zero. Σ Fx = 0 and Σ Fy = 0

14 EQUILIBRIUM

15 EQUILIBRIUM

16 NON-EQUILIBRIUM Force of Push

17 Types of Forces Contact Forces Action-at-a-Distance Tensional Force Forces Electrical Force Frictional Force Normal Force Magnetic Force Air Applied Resistance Force Force Gravitational Force Spring Force Field Forces

18 TYPES OF FORCES Weight: the magnitude of the force of gravity acting on an object (always a downward force) Fw = mag Normal Force: a force exerted by one object on another in a direction perpendicular to the surface of contact.you must have a surface to have a normal force. Tension: The pull exerted by a string, rope, or cable when attached to a body and pulled taut.

19 TYPES OF FORCES Spring: A restoring force, that is, the push or pull a spring exerts on an object Thrust: A general term for the forces that move objects such as rockets, planes, cars, and people Push/Pull forces: Physical contact force Friction: the resistive force that opposes the relative motion of two contacting surfaces that are at rest with respect to one another.

20 FORCE DIAGRAMS A force is a vector. It has magnitude and direction. A force diagram is a diagram of the objects involved in a situation and the forces exerted on the objects. A free-body diagram helps analyze a situation. In a force diagram, vector arrows represent all the forces acting in a situation. A freebody diagram shows only the forces acting on the object of interest.

21 FREE-BODY DIAGRAMS

22 UNBALANCED FORCES

23 Physics, Friday, November3 Turn in your illustrations in the basket! Grab a whiteboard and expo!

24

25 FORCE PROBLEMS

26 ANSWERS 0 N 5 N; LEFT

27 FORCE PROBLEMS

28 ANSWERS 0 N 15 N; UP

29 FREE-BODY DIAGRAM PRACTICE A book is at rest on a table top. Diagram the forces acting on the book. A girl is suspended motionless from a bar which hangs from the ceiling by two ropes.

30 ANSWERS

31 FREE-BODY DIAGRAM PRACTICE A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book. A car is coasting to the right and slowing down. Diagram the forces acting upon the car.

32 ANSWERS

33 QUESTIONS 1 AND 2 Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose an astronaut in that place throws a rock. The rock will: a) gradually stop. b) continue in motion in the same direction at constant speed. An 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving with the same speed and in the same direction?

34 ANSWERS 1 AND 2 B 0 N

35 QUESTIONS 3 AND 4 Mac and Tosh are arguing in the cafeteria. Mac says that if he throws his jello with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. With whom do you agree? Why? If you were in a weightless environment in space, would it require a force to set an object in motion?

36 ANSWERS TO 3 AND 4 Tosh YES!!! An object has mass even in space Newton s laws still apply

37 CONCEPTUAL QUESTION 5 Ben is being chased through the woods by a bull moose which he was attempting to photograph. The enormous mass of the bull moose is extremely intimidating. Yet, if Ben makes a zigzag pattern through the woods, he will be able to use the large mass of the moose to his own advantage. Explain this in terms of inertia and Newton's first law of motion.

38 Exit Ticket 1. Explain the relationship between Weight and Mass 2. What is the net force on this object: 3. What is the net force on this object:

39 CONCEPTUAL QUESTION 6 Several physics teachers are taking some time off to play a little putt-putt golf. The 15th hole at the Hole-In-One Putt-Putt Golf Course has a large metal rim which putters must use to guide their ball towards the hole. Mrs. Davis guides her golf ball around the metal rim. When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow?

40 NEWTON S SECOND LAW Newton s Second Law: The acceleration of an object is directly proportional to the net external force acting on the object and inversely proportional to the object s mass. Σ F = ma (The sum of forces on a particular axis equals mass times acceleration) A small force on an object causes a small acceleration, but a larger force causes a larger acceleration.

41

42

43 PRACTICE PROBLEM An applied force of 50 N is used to accelerate an object to the right across a frictional surface. The object encounters 10 N of friction. Use the diagram to determine the normal force, the net force, the mass, and the acceleration of the object. (Neglect air resistance.)

44 ANSWER FN: 80 N (not moving on y-axis so FN = Fw) MASS: 8.15 kg (Fw/ag=mass) Fnet: 40 N; right a: 4.91 m/s² ; right (Fnet/m)

45 NEWTON S THIRD LAW Newton s Third Law: If two objects interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction. For every action there is an equal, but opposite reaction.

46 NEWTON S THIRD LAW Forces always exist in pairs. Action-reaction pair: a pair of simultaneous equal, but opposite forces resulting from the interaction of two objects. Field forces also exist in pairs.

47

48 INTRO TO NEWTON S THIRD LAW

49 NEWTON S 3 RD LAW

50 NEWTON S 3 RD LAW

51 ACTION-REACTION PAIRS Newton s Third Law Pairs

52 What are the Newton Third Law Pairs you identified yesterday? Station 1: standing from the ground Station 2: two spring scales Station 3: two lab chairs Station 4: Wheels of the car Station 5: tennis ball

53 QUESTION 7 While driving, Anna observed a bug striking the windshield of her car. Obviously, a case of Newton's third law of motion. The bug hit the windshield and the windshield hit the bug. Which of the two forces is greater: the force on the bug or the force on the windshield?

54 The same force ANSWER 7

55 QUESTION 8 A gun recoils when it is fired. The recoil is the result of action-reaction force pairs. As the gases from the gunpowder explosion expand, the gun pushes the bullet forwards and the bullet pushes the gun backwards. The acceleration of the recoiling gun is... a.greater than the acceleration of the bullet. b.smaller than the acceleration of the bullet. c.the same size as the acceleration of the bullet.

56 ANSWER 8 B; The forces are equal, but the mass of the bullet is smaller than the gun and the acceleration is inversely proportional to the mass

57 Friction occurs because the surface of any object is rough. Even surfaces that appear smooth are really covered with microscopic hills and valleys. FRICTION %20motion.asp

58 FRICTION Friction opposes the applied force Kinetic friction is the resistive force that opposes the relative motion of two contacting surfaces that are moving past one another. Static friction is the resistive force that keeps a stationary object at rest Kinetic friction is less than static friction.

59 FRICTION The force of friction is proportional to the normal force. The coefficient of friction (µk or s)is the ratio of the force of friction to the normal force acting between two objects. FF = µ FN µ = F F / FN Friction depends on the surfaces in contact.

60 FRICTION

61 TYPES OF KINETIC FRICTION Sliding Friction: type of friction that occurs when objects slide past each other Rolling Friction: type of friction when a rounded object rolls over a flat surface. Rolling < Sliding

62 REDUCING UNWANTED FRICTION Use low-friction materials Lubricants: substances that are applied to surfaces to lower the friction between them. Ex) oil, wax, grease

63 BALL BEARINGS A bearing can reduce friction by virtue of its shape, by its material, or by introducing and containing a fluid between surfaces. cal)

64 INCREASING HELPFUL FRICTION Helpful friction is increased by making surfaces rougher. Friction is also greater if the force pushing the surfaces together is increased. Paper weights increase force and therefore increase friction. Shoe treads increase friction by increasing roughness. Putting sand on icy roads.

65

66 EXIT TICKET Explain why the force normal and the force of gravity on a book sitting on a table are NOT a Newton s third law force pair (action-reaction pair).