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.Arrows are used to represent forces. The length of the arrowis proportional to the magnitude of the force.15 N5 N
3 4.3 Newton’s Second Law of Motion SI Unit for ForceThis unit of force is called a newton (N).
4 4.1 The Concepts of Force and Mass Mass is a measure of the amountof “stuff” contained in an object.SI Unit of Mass: kilogram (kg)
5 4.2 Newton’s First Law of Motion Newton’s First Law (Law of Inertia)An object continues in a state of restor in a state of motion at a constantSpeed unless changed by a net forceThe net force is the SUM of allof the forces acting on an object.
6 4.2 Newton’s First Law of Motion The net force on an object is the sum of all forces acting on that object.Individual ForcesNet Force4 N10 N6 N
7 4.2 Newton’s First Law of Motion Individual ForcesNet Force5 N3 N4 N
8 4.3 Newton’s Second Law of Motion Mathematically, the net force iswritten aswhere the Greek letter sigma denotes the vector sum.
9 4.3 Newton’s Second Law of Motion When a net 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.
10 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
21 4.7 The Gravitational Force Definition of WeightThe weight of an object on or above the earth is the gravitational force that the earth exerts on the object. The weight always acts downwards, toward the centerof the earth.SI Unit of Weight: newton (N)
22 Definition of the Normal Force The normal force is the force that a surfaceexerts on an object with which it is in contact– It is ALWAYS perpendicular to the surface.
23 4.8 The Normal ForceA hand pushes down on a block with a force of 11 N. The block weighs 15N. What is the magnitude of the normal force?A rope is now used to lift the block with a force of 11 N. What is the magnitude of the normal force?
24 4.8 The Normal ForceApparent WeightThe apparent weight of an object is the reading of the scale.It is equal to the normal force the man exerts on the scale.
26 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.
27 4.9 Static and Kinetic Frictional Forces When the two surfaces arenot sliding across one anotherthe friction is calledstatic friction.
28 4.9 Static and Kinetic Frictional Forces Note that the magnitude of the frictional force doesnot depend on the contact area of the surfaces.What does it depend on???
29 4.9 Static and Kinetic Frictional Forces Static friction opposes the impending relative motion betweentwo objects.Kinetic friction opposes the relative sliding motion motions thatactually does occur.is called the coefficient of kinetic friction.
30 4.9 Static and Kinetic Frictional Forces Page 104
31 4.9 Static and Kinetic Frictional Forces The sled comes to a halt because the kinetic frictional forceopposes its motion and causes the sled to slow down.
32 4.9 Static and Kinetic Frictional Forces Suppose the coefficient of kinetic friction is 0.05 and the totalmass is 40kg. What is the kinetic frictional force?
33 4.3 Newton’s Second Law of Motion A free-body-diagram is a diagram thatrepresents the object and the forces thatact on it.
34 The net force in this case is: 275 N + 395 N – 560 N = +110 N 4.3 Free Body DiagramsThe net force in this case is:275 N N – 560 N = +110 Nand is directed along the + x axis of the coordinate system.
35 4.6 Types of Forces: An Overview Examples of different forces:Friction forceTension forceNormal forceWeight (Force due to gravity)Gravitational ForceMagnetic ForceElectric Force
36 Cables and ropes transmit forces through tension. 4.10 The Tension ForceCables and ropes transmitforces through tension.
37 This diagram shows four forces acting upon an object This diagram shows four forces acting upon an object. There aren’t always four forces, For example, there could be one or more forces.
38 Problem 1A book is at rest on a table top. Diagram the forces acting on the book.
39 Problem 1In this diagram, there are normal and gravitational forces on the book.
40 Problem 2An egg is free-falling from a nest in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.
41 Gravity is the only force acting on the egg as it falls.
42 Problem 3A rightward force is applied to a book in order to move it across a desk. Consider frictional forces. Neglect air resistance. Construct a free-body diagram. Let’s see what this one looks like.
43 Note the applied force arrow pointing to the right Note the applied force arrow pointing to the right. Notice how friction force points in the opposite direction. Finally, there is still gravity and normal forces involved.
44 Problem 4A block is at rest on an incline and stays in place because of static friction. Draw a free-body diagram
45 A skier is standing motionless on a horizontal tow rope, which is about to pull her forward. The skier’s mass is 59kg and the coefficient of static friction between the skis and the snow is Draw a free body diagram. What is the magnitude of the maximum force that the tow rope can apply to the skier without causing her to move.
46 Objects in Equilibrium Objects that are either at rest or moving with constant velocity are said to be in equilibriumThis means that the net force acting on the object is zeroEquivalent to the set of component equations given by
47 4.11 Equilibrium Application of Newton’s Laws of Motion Reasoning StrategyDraw a free-body diagram.Choose a set of x, y axes for each object and resolve all forcesin the free-body diagram into components that point along theseaxes.
48 4.11 Equilibrium Application of Newton’s Laws of Motion What is the Tension in Rope 1 and 2?The engine has a weight of 3150 N
49 4.11 Equilibrium Application of Newton’s Laws of Motion Forcex componenty component
50 4.11 Equilibrium Application of Newton’s Laws of Motion The first equation givesSubstitution into the second gives
51 4.11 Equilibrium Application of Newton’s Laws of Motion
52 4.12 Nonequilibrium Application of Newton’s Laws of Motion When an object is accelerating, it is not in equilibrium.
53 4.12 Nonequilibrium Application of Newton’s Laws of Motion The mass of the supertanker is 1.5 x 108 kg. There is a drive force with magnitude75000 N and an opposing force of N. The tanker moves with an accelerationof 2.0 x 10-3 m/s2. Solve for the Tension in each of the ropes.The acceleration is along the x axis so
54 4.12 Nonequilibrium Application of Newton’s Laws of Motion Forcex componenty component
55 4.12 Nonequilibrium Application of Newton’s Laws of Motion
56 4.12 Nonequilibrium Application of Newton’s Laws of Motion