The Laws of Motion. Newton s first law Force Mass Newton s second law Newton s third law Examples

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2 The Laws of Motion Newton s first law Force Mass Newton s second law Newton s third law Examples Isaac Newton s work represents one of the greatest contributions to science ever made by an individual.

3 Kinematics and Dynamics Kinematics: Describing object s motion by answering: When? Where? How fast? How far? How long? without asking: Why is object moving in a certain way? Dynamics: Describing object s motion by answering: Why is the object moving in a certain way? What causes the object to change its velocity? Dynamics studies motion on a deeper level than kinematics: it studies the causes of changes in objects motion!

4 Dynamics Describes the relationship between the motion of objects in our everyday world and the forces acting on them Language of Dynamics Force: The measure of interaction between two objects (pull or push). It is a vector quantity it has a magnitude and direction Mass: The measure of how difficult it is to change object s velocity (sluggishness or inertia of the object)

Forces The measure of interaction between two objects (pull or push) Vector quantity: has magnitude and direction May be a contact force or a field

5 Forces The measure of interaction between two objects (pull or push) Vector quantity: has magnitude and direction May be a contact force or a field force Contact forces result from physical contact between two objects Field forces act between disconnected objects Also called action at a distance

6 Forces Gravitational Force Archimedes Force Friction Force Tension Force Spring Force Normal Force

7 Fundamental Forces Gravitational forces between objects Electromagnetic forces between electric charges Strong forces between subatomic particles Weak forces radioactive decay processes

8 Vector Nature of Force Vector force: has magnitude and direction Net Force: a resultant force acting on object F net F F1 F2 F3... You must use the rules of vector addition to obtain the net force on an object F tan F F ( F F cm ) 26.6

9 Balanced (equal) forces, therefore no motion. Equal in magnitude but in opposite directions. 3-9 Section 3.1

10 Unbalanced forces result in motion Net force to the right 3-10 Section 3.1

12 Example 1: What is the net force of the block and in what direction is moving?

Newton s First Law An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force An

13 Newton s First Law An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force An object at rest remains at rest as long as no net force acts on it An object moving with constant velocity continues to move with the same speed and in the same direction (the same velocity) as long as no net force acts on it Keep on doing what it is doing

14 Newton s First Law An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force When forces are balanced, the acceleration of the objection is zero Object at rest: v = 0 and a = 0 Object in motion: v 0 and a = 0 The net force is defined as the vector sum of all the external forces exerted on the object. If the net force is zero, forces are balanced.

15 Q Check1: Is it possible for an object to have motion in the absence of forces on the object? 1. YES 2. NO

16 Q Check2: Is it possible to have forces on an object in the absence of motion? 1. YES 2. NO

Mass and Inertia Every object continues in its state of rest, or uniform motion in a straight line, unless it is compelled to change that state by unbalanced forces impressed upon it Inertia is a

17 Mass and Inertia Every object continues in its state of rest, or uniform motion in a straight line, unless it is compelled to change that state by unbalanced forces impressed upon it Inertia is a property of objects to resist changes in motion! Mass is a measure of the amount of inertia. Mass is a measure of the resistance of an object to changes in its velocity Mass is an inherent property of an object Scalar quantity and SI unit: kg

18 Who has more inertia a ball that has a mass of a) 2 Kg or b) 4 Kg 1. 2 kg 2. 4 kg 3. Both have the same mass

19 Mass and Weight Mass and weight are two different quantities Weight depend on the gravitational pool Mass is a scalar quantity that does not change Example # 2 What is the weight of a 4 kg ball?

20 If gravity in the moon is 1/6 of the gravity on earth. a) A 5kg ball will weigh more b) The 5kg ball s mass on earth will weigh less, more or the same than in the moon 1. a) earth b) more 2. a) earth b) same 3. a) moon b) less 4. a) moon b) same

21 Newton s Second Law The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass a F m F m net F net F ma

22 Units of Force Newton s second law: F net F ma SI unit of force is a Newton (N) 1 kg N 1 s 2 m US Customary unit of force is a pound (lb) 1 N = lb

More about Newton s 2nd Law You must be certain about which body we are applying it to F net must be the vector sum of all the forces that act on that body Only

23 More about Newton s 2nd Law You must be certain about which body we are applying it to F net must be the vector sum of all the forces that act on that body Only forces that act on that body are to be included in the vector sum Net force component along an axis to the acceleration along that same axis F ma F ma net, x x net, y y

24 Example 3: One or two forces act on a puck that moves over frictionless ice along an x axis, in one-dimensional motion. The puck's mass is m = 0.20 kg. Forces F 1 and F 2 and are directed along the x axis and have magnitudes F 1 = 4.0 N and F 2 = 2.0 N. Force F 3 is directed at angle θ = 30 and has magnitude F 3 = 1.0 N. In each situation, what is the acceleration of the puck? F net, x ma x

25 Example 4: Your turn. Moe, Larry and Curly push on a 752-Kg boat that float next to a dock. They each exert an 80.5 N force parallel to the dock (in the positive x direction). ( a) What is the acceleration of the boat if they all push in the same direction? Give both direction and magnitude. 1. a) 0.32 m /s 2 to the right 2. a) 0.52 m /s 2 to the right 3. a) 0.21 m /s 2 to the right 4. a) 0.11 m /s 2 to the right

26 Example 4: Your turn. Moe, Larry and Curly push on a 752-Kg boat that float next to a dock. They each exert an 80.5 N force parallel to the dock. ( b ) What are the magnitude and direction of the boat s acceleration if Larry and Curly push in the opposite direction to Moe s push? 1. b) m /s 2 to the right 2. b) m /s 2 to the left 3. b) 0.11 m /s 2 to the right 4. a) m /s 2 to the left

27 Gravitational Force Gravitational force is a vector Expressed by Newton s Law of Universal Gravitation: F g G mm 2 R G gravitational constant M mass of the Earth m mass of an object R radius of the Earth Direction: pointing downward

28 Weight The magnitude of the gravitational force acting on an object of mass m near the Earth s surface is called the weight w of the object: w = mg g can also be found from the Law of Universal Gravitation Weight has a unit of N F g G g mm 2 R G w F M m/s 2 R g Weight depends upon location mg R = 6,400 km

29 Normal Force Force from a solid surface which keeps object from falling through Direction: always perpendicular to the surface Magnitude: depend on situation w F g mg N F g ma y N mg ma y N mg

30 A woman in an elevator is accelerating upwards The normal force exerted by the elevator on the woman is, 1. (A) greater than 2. (B) the same as 3. (C) less than The normal force exerted by the elevator on the woman is, the force due to gravity acting on the woman

31 Gravity and Normal Forces A woman in an elevator is accelerating upwards F y mg N ma N mg ma m( g a) The normal force exerted by the elevator on the woman is, (A) greater than (B) the same as (C) less than the force due to gravity acting on the woman

Physics 111 Lecture 4 Newton`s Laws

Physics 111 Lecture 4 Newton`s Laws Dr. Ali ÖVGÜN EMU Physics Department www.aovgun.com he Laws of Motion q Newton s first law q Force q Mass q Newton s second law q Newton s third law q Examples Isaac