Presentation on theme: "Forces and Newton’s Laws of Motion"— Presentation transcript:
1 Forces and Newton’s Laws of Motion Chapter 4Forces and Newton’s Laws of Motion
2 4.1 The Concepts of Force and Mass A force is a push or a pull.Contact forces arise from physicalcontact .Action-at-a-distance forces do notrequire contact and include gravity and electrical forces.
3 4.2 Newton’s First Law of Motion An object continues in a state of restor in a state of motion at a constantvelocity along a straight line, unlesscompelled to change that state by anet force.The net force is the vector sum of allthe forces acting on an object.
4 4.2 Newton’s First Law of Motion Inertia is the natural tendency of anobject to remain at rest or in motion ata constant velocity along a straightline.
5 4.3 Newton’s Second Law of Motion Mathematically, the net force iswritten aswhere the Greek letter sigma denotes the vector sum.
6 4.3 Newton’s Second Law of Motion When a net external force acts on an objectof mass m, the acceleration that results isdirectly proportional to the net force and hasa magnitude that is inversely proportional tothe mass. The direction of the acceleration isthe same as the direction of the net force.
7 4.4 The Vector Nature of Newton’s Second Law The direction of force and acceleration vectorscan be taken into account by using x and ycomponents.is equivalent to
8 4.5 Newton’s Third Law of Motion Whenever one body exerts a force on asecond body, the second body exerts anoppositely directed force of equal magnitude on the first body.
9 4.6 Types of Forces: An Overview In nature there are two general types of forces,fundamental and nonfundamental.Fundamental Forces1. Gravitational force2. Strong Nuclear force3. Electromagnetic force4. Weak Nuclear force
10 4.6 Types of Forces: An Overview Examples of nonfundamental forces:frictiontension in a ropenormal or support forces
11 4.7 The Gravitational Force Newton’s Law of Universal GravitationEvery particle in the universe exerts an attractive force on everyother particle.A particle is a piece of matter, small enough in size to beregarded as a mathematical point.The force that each exerts on the other is directed along the linejoining the particles.
12 4.7 The Gravitational Force For two particles that have masses m1 and m2 and areseparated by a distance r, the force has a magnitudegiven by
13 Definition of the Normal Force The normal force is one component of the force that a surfaceexerts on an object with which it is in contact – namely, thecomponent that is perpendicularto the surface.
14 4.9 Static and Kinetic Frictional Forces When an object is in contact with a surface there is a forceacting on that object. The component of this force that isparallel to the surface is called thefrictional force.
15 4.9 Static and Kinetic Frictional Forces When the two surfaces arenot sliding across one anotherthe friction is calledstatic friction.
16 4.9 Static and Kinetic Frictional Forces The magnitude of the static frictional force can have any valuefrom zero up to a maximum value.is called the coefficient of static friction.
17 4.9 Static and Kinetic Frictional Forces Static friction opposes the impending relative motion betweentwo objects.Kinetic friction opposes the relative sliding motion thatactually does occur.is called the coefficient of kinetic friction.
19 4.11 Equilibrium Application of Newton’s Laws of Motion Definition of EquilibriumAn object is in equilibrium when it has zero acceleration.
20 4.12 Nonequilibrium Application of Newton’s Laws of Motion When an object is accelerating, it is not in equilibrium.
21 ExampleA man can exert a force of 700N on a rope attached to a sled. The rope is at an angle of 30⁰ with the horizontal. If the coefficient of kinetic friction between the sled and ground is 0.4, what is the maximum load on the sled that the man can pull at constant speed?
23 From (ii): m = [FN+ Fsin30⁰]/g = [ ×0.5]/9.8= kg
24 Problems to be solved4.34, 4.40, 4.58, 4.61, 4.91, 4.93, 4.107B4.1 A subway train has three cars, each weighing 1.2×105N. The frictional force on each car is 103N, and the first car, acting as an engine, exerts a horizontal force of 4.8×104N on the rails. (a) What is the acceleration of the train? (b) What is the tension in the coupling between the first and the second cars?
25 (c) What is the tension between the second and third cars. Ans: (a) 1 (c) What is the tension between the second and third cars? Ans: (a) 1.3m/s2 (b) 32000N (c) 16000NB4.2 Block 1 of mass m1= 8.0kg is moving on a frictionless 30.0⁰ incline. This block is connected to block 2 of mass m2= 22.0kg by a massless cord that passes over a massless and frictionless pulley. Find the acceleration of each block and the tension in the cord. Ans: 5.88m/s286.24N
26 B4.3 What minimum force is required to drag a carton of books across the floor if the force is applied at an angle of 45⁰ to the horizontal. Take the mass of carton as 40kg and the coefficient of friction as Ans: N