Big Questions. Physics 201, Lecture 7. Newton s First Law. What Did the Big Guys Say. Today s Topics. What does it take to maintain a motion?

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Norman Atkins
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Big Questions Physics 201, Lecture 7 oday s opics n n What does it take to maintain a motion? ewton s Laws of Motion (Chap.

) It is very important that you have previewed Chapter 5. What Did the Big Guys Say A force is a must for any and all motions!

ewton s 2nd Law is mostly right, except when the object is at very high speed.

1 Big Questions Physics 201, Lecture 7 oday s opics n n What does it take to maintain a motion? ewton s Laws of Motion (Chap. 5) n First Law, Force and Inertia Forces n Second Law, F=ma Mass n hird Law, Action and Reaction What does it take to change a motion? (and quantitatively how?) It is very important that you have previewed Chapter 5. What Did the Big Guys Say A force is a must for any and all motions! v Force is not necessary for a motion in constant velocity v But to change a motion, a net force is a must! --ewton: 3 laws say it all. ewton s 2nd Law is mostly right, except when the object is at very high speed. + Aristotle ( BC) Galileo Galilei ( ) Isaac ewton ( ) Albert Einstein ( ) ewton s First Law q he Law: An object will maintain a constant motion* (with constant velocity) until it is acted upon by a net force. Ø Message #1: Motion (with constant velocity) is possible even without a force! Ø Message #2: We do need a force to change a motion. Ø Some simple examples: Without a force (or net force =0) If stationary (v=0), the object will remain stationary. If moving at a velocity, it will keep moving at the same velocity. q Sometimes, ewton s First Law is also called Law of Inertia All matter has an intrinsic tendency to resist change of motion. * Unless otherwise specified, the motions we are talking about in this course are measured in something called a inertia reference frame. 1

Force q Force: A measure of action one object exerting on another q Force has a unit of ewton () 1 = 1 kg m / s 2 q

(Described by both magnitude and direction) q otal Force: If an object is subject to more than one forces, the total

(pushing) force on the ball electric force on q friction force supporting force (normal force) F 1 =1.0 F 2 =2.

2 gravitational force ( weight ) Gravitational Force (Weight) q All objects on earth are experiencing a

he direction of the gravitational force is always downwards (to the Earth) and the magnitude of F G is always

2 Force q Force: A measure of action one object exerting on another q Force has a unit of ewton () 1 = 1 kg m / s 2 q Force is a vector! (Described by both magnitude and direction) q otal Force: If an object is subject to more than one forces, the total (net) force on it is the vector sum of all forces F net = F pulling force on box Many Forms of Forces kicking (pushing) force on the ball electric force on q friction force supporting force (normal force) F 1 =1.0 F 2 =2.0 F 1 +F 2 =3.0 F 1 +F 2 =2.2 gravitational force ( weight ) Gravitational Force (Weight) q All objects on earth are experiencing a gravitational force F G (exerted by the Earth). he direction of the gravitational force is always downwards (to the Earth) and the magnitude of F G is always proportional to the mass (M) of the object by a factor of g=9.81 m/s 2. (i.e. F G = ) ormal Force q ormal force () is a supporting force exerted on an object by a supporting surface. ormal Force is always normal to the surface ormal force is always pointing towards the object (away from the supporting surface) F G = he magnitude of the normal force is an adaptive quantity that depends on other factors such as the weight, inclination, and even the acceleration of the object. q It is important to note that for an object with a certain ( fixed) mass, its weight on earth is fixed. (i.e. both direction and magnitude are fixed) 2

Friction q Friction is a kind of dragging force on the contacting surface between two objects.

3 ension q ension () is a pulling force transmitted along a soft string. ension always goes parallel to the string ension is always pulling away from the object he magnitude of the tension is adaptive but it is the same along a massless string. Friction q Friction is a kind of dragging force on the contacting surface between two objects. q here are two kinds of frictions: Kinetic Friction (F k ): Friction when the object is moving on the surface. moving along the surface F k v Static Friction (F s ): Friction when the object is not moving but has a tendency to move on surface. pulling ot moving but has a tendency to move due to pulling F s Kinetic Friction q Kinetic Friction (F k ): Friction when the object is moving on the surface. F k is always parallel to the surface in the opposite direction of the relative motion F k is always proportional to the corresponding ormal force: F k = µ k (µ k : kinetic coefficient of friction) F k v Static Friction q Static Friction (F s ): Friction when the object is not moving but has a tendency to move on surface. F s is always parallel to the surface in the opposite direction of the tendency of motion F s is an adaptive force. Its magnitude is self adjustable to maintain the object to be at rest. For a given normal force, F s can not exceed a maximum of F s_max = µ s (µ s : Static coefficient of friction) F s pull You foot pretends to move backwards he ground provide an F s forward 3

moving along the surface F k = µ k F k v From Static to Kinetic Friction For most materials, µ s > µ k Static Friction (F s ): Friction when the object is not moving but has a tendency to move on

4 Friction summary q Friction is a kind of dragging force on the contacting surface between two objects. q here are two kinds of frictions: Kinetic Friction (F k ): Friction when the object is moving on the surface. moving along the surface F k = µ k F k v From Static to Kinetic Friction For most materials, µ s > µ k Static Friction (F s ): Friction when the object is not moving but has a tendency to move on surface. F s <= µ s pulling ot moving but has a tendency to move due to pulling F s ewton s Second Law q he Law: At any moment, the acceleration of an object is proportional to the total force on it and inversely proportional to its own mass. F a = m OR F = ma Ø otes: he direction of acceleration is always the same as the direction of the net force. Mass: An physical quantity intrinsically associated with all matter. Mass is additive : M 1+2 = M 1 + M 2 unit: kg Mass is conservative Larger mass more inertia (i.e. harder to change velocity) * Modern Physics (not required for this course): ewton s 2 nd law is valid only at size >> m, and speed<< 3x10 8 m/s. ewton s 2 nd Law in Cartesian Coordinates q ewton s Second Law: F = Fi = ma q Calculating total force : F = F F q Apply standard formulas to decompose forces and kinematic parameters in x,y,z coordinate system. Exercises on hursday... x y z F = ma x y z F = F 1x F = F 1y F = F 1z x F = ma y F = ma z 2x 2 y 2z 3x 3 y 3z

Example: Acceleration By Gravitational Force q Recall: all objects on earth are experiencing a gravitational force F G, the magnitude of F G is F G = If the F G is the only force on an object of mass

Conventionally, g is called gravitational acceleration. he motion under sole gravitational force is called Free Fall. F 1 M F 2 M=10 kg F 1 =200 F 2 = 100 What is a?

5 Example: Acceleration By Gravitational Force q Recall: all objects on earth are experiencing a gravitational force F G, the magnitude of F G is F G = If the F G is the only force on an object of mass M, what is the acceleration of the object? F 1 rivial Examples M M=10 kg F 1 =200 What is a? a = F net /M = 200/10kg = 20 m/s 2 Solution: a = F G /M = /M = g = 9.8 m/s 2, and pointing downwards. Conventionally, g is called gravitational acceleration. he motion under sole gravitational force is called Free Fall. F 1 M F 2 M=10 kg F 1 =200 F 2 = 100 What is a? a = F net /M = ( )/10kg = 10 m/s 2 Weight Readings Quiz: Acceleration of an Elevator q Which of the four cases has the direction of acceleration marked wrong? v v v v A floor scale reads A Spring scale reads We feel our weight by the pressure on our feet moving up with increasing speed a moving up with decreasing speed a correct: a down with increasing speed a down with decreasing speed a 5

6 Weight Reading Inside an Elevator (Apparent Weight) ewton s hird Law q he Law: When two objects intact, the force F 12, exerted on object 2 by object 1 is always equal in magnitude and opposite in direction to the force F 21 which is exerted on object 1 by object 2, regardless of size or mass of the objects. note: ewton s 3 rd law is about two objects F 12 =-F 21 F 12 =-F 21 2 F 2 21 F 21 F 12 1 F 12 1 Attractive Repulsive * F 12 and F 21 are often called a pair of action-reaction ot moving, or moving at constant v = accelerating up or decelerating down = + M a > up w/ slower v or down w/ faster v = M a < ewton s hird Law: Example Example of Bad hinking F A,B = - F B,A. is true for all types of forces Since F m,b = -F b,m, so F net = 0, and a = 0! Incorrect! F w,m F m,w a?? m F b,m F m,b F m,f ice F f,m Action-Reaction are two forces on two objects! An action-reaction pair must always refer to two objects rivial Quiz, so what is a? 6

End-of-Lecture Quizzes q What is the net force on the block? 6.0, 0.0, other. 3.0 F 2 3.

7 End-of-Lecture Quizzes q What is the net force on the block? 6.0, 0.0, other. 3.0 F F 1 F net = F 1 +F 2 = 3+ (-3)=0 q If the two forces are not back to back, can they ever balance to produce a zero net force? Yes o One More End-of--Lecture Quiz q We have shown that if the person is standing still, the supporting force F must be equal to his weight F G in magnitude and opposite to F G in direction. Do F and F G form an action-reaction pair? YES, F and F G are action-reaction pair as F =-F G O, F and F G are O action-reaction pair as they are forces on the same object. F F G a third force is necessary to balance 7

Forces and Newton s Laws Reading Notes. Give an example of a force you have experienced continuously all your life.

Forces and Newton s Laws Reading Notes Name: Section 4-1: Force What is force? Give an example of a force you have experienced continuously all your life. Give an example of a situation where an object