CHAPTER 3 NEWTON S LAWS OF MOTION


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1 CHAPTER 3 NEWTON S LAWS OF MOTION
2 NEWTON S LAWS OF MOTION FORCE Forces are calssified as contact forces or gravitational forces. The forces that result from the physical contact between the objects are called contact forces and the forces that do not require physical contact to affect are called field forces. The gravitational and the electromagnetic forces are examples of field forces. The force is a vector quantity. The unit of the force in the SI unit system is Newton, abbreviated N. In the cgs unit system its unit is dyne with 1dyne=105 N, and in the British unit system, the force's unit is pound (lb). 3.2 NEWTON'S FIRST LAW The law states that an object continues in its state of rest or uniform motion until it is forced to change that state by an external force. Newton's first law sometime calle the law of inertia.
3 NEWTON S LAWS OF MOTION MASS AND INERTIA Inertia is the property of matter that resists the change of its state and mass is the measure of this inertia. The mass is a scalar quantity with a unit of kilogram (kg), in the SI unit system and gram (g) in the cgs unit system. 3.4 NEWTON'S SECOND LAW This law states that the acceleration of an object is directly proportional to the net F 1 12 F 21 2 Figure 3.1 Newton s third law. force acting on it and inversely proportional to its mass, i.e., F = m a, (3.1) where F represents the net (resultant) force acting on the mass. Eq. 3.1 can be written in components form as F x = ma x, (3.2a)
4 NEWTON S LAWS OF MOTION 47 F y = ma y, (3.2b) FreeBody Diagram: When solving problems applying Newton's second law it is very essential to identify all the forces acting on the body. A diagram showing all the forces acting on a body is called a freebody diagram. This diagram does not include the forces the body exert on other bodies. If your problem involves more than one object, a separate freebody diagram is needed for each object. 3.5 NEWTON'S THIRD LAW The last law of Newton states that for every action there is an equal, but opposite reaction, that is., if two bodies interact, the force exerted by body number 1 on body number 2 (F21 ) is equal and opposite to the force exerted on body number 1 by body number 2 F12 (see Figure 3.1), namely F = (3.3) 21 F 12
5 NEWTON S LAWS OF MOTION 48 Remark: Action and reaction act on different bodies. That is, the action force and its action force can never be in the same freebody diagram. 3.6 WEIGHT AND THE NORMAL FORCE It is wellknown that the earth exerts a force on every object lying within its field. This force is called the force of gravity of the object, or the weight of the object. The weight is always points vertically downward. From Newton's second law we can deduce that the weight (W) of an object is equal to the mass of that object multiplied by the acceleration of gravity (g), that is W = mg (3.4)
6 NEWTON S LAWS OF MOTION 49 When an object is at rest ( does not move) on a table or any surface, as shown in Figure 3.2, the gravitional force on it (its weight) does not disappear. Why, then, the object doesn't fall? From Newton's second law, for an object to remain at rest the net force acting on it must be zero. This means that there must be another force on the object to balance the gravitational force. The surface, at which the object is resting on, exerts this force. The force exerted by a surface on an object, which is in contact with it, and directed perpendicular to the surface itself is called the normal force (normal means perpendicular). Due to its name this force will be labeled N, as shown in Figure 3.2 (b). It should be noted that the normal force is not a reaction force for the weight. It is rather a reaction to the m N (a) mg (b) Figure 3.2 (a) An object of mass m is resting on a table. (b) A freebody diagem of the object.
7 NEWTON S LAWS OF MOTION 50 force exerted by an object on the surface, while the weight is the force exerted by the earth on the object. Remark There is a difference between mass and weight. While the mass of a body is the measure of inertia of that body (the same everywhere), its weight is the force exerted by gravity on it (depends on the body's position). Strategy for solving problems using Newton s laws: (i) Chose a suitable coordinate system with the positive direction is the direction of the acceleration, if it is known. (ii) Draw a freebody diagram of each body of the system separately (iii) Resolve each force into its components according to the chosen coordinates. (iv) Identify the known and the unknown quantities. (v) Now you can apply Newton's second law for one body or more of the system according to the unknown quantities.
8 NEWTON S LAWS OF MOTION 51 Example 3.1 A boy want to drag a box, that has a mass of 10.0 kg, along a horizontal smooth surface. He pulls the box horizontally with a force of 8.0 N. Find (a) the weight of the box, (b) the normal force of the surface, and (c) the acceleration of the box kg Solution The freebody diagram of the system is shown in Figure 3.3. (a) The weight of thje box is W = mg =10.0(9.8) = 98kg (b) Using Newton's second law and noting that ther is no vertical motion, we have N mg 8.0 N 8.0 N Figure 3.3 Example 3.1 with the freebody diagram of the box. Or F y = ma y = 0 N mg = 0, so N = mg = 98kg
9 NEWTON S LAWS OF MOTION 52 (c) Now, applying Newton's second law along the xaxis we have or F x = ma x F 8.0 a = = = 0.8 m/s 2. m 10.0 Example 3.2 Two masses m1 and m2 (m1> m2) are suspended vertically by a light string that passes over a light, frictionless pulley as in Figure 3.4(a) (Atwood's machine). Find the acceleration of the masses and the tension in the string. a m 2 m 1 (a) T T m 2 g (b) m 1 g a Solution The freebody diagram of the system is Figure 3.4 Example 3.2. (a) The Atwood's machine. (b) the freebody diagram of the two masses. shown in Figure 3.4 (b), with the positive sense is taken downward. T represents the tension, the
10 NEWTON S LAWS OF MOTION 53 force applied by the string. Applying Equation (3.1) for m1, yield m1 = 1 g T m a, 1 and for m2, we obtain m2 2 g T = m a. 2 Solving Equation 1 and Equation 2 for a and T, we get and a T = = m m1 m + m 2 g 1 2, 2mm 1 2 m1+ m2 g.
11 NEWTON S LAWS OF MOTION 54 Example 3.3 Two blocks are in contact on a smooth horizontal table. A constant force F is applied to one block as in Figure 3.5. a) Find the acceleration of the system. b) Find the contact force between the two blocks N 2 m 2 g m 2 F c F c m 1 F N 1 m 1 g Figure 3.5 Example 3.3. with the freebody diagram of the two masses. F Solution Applying Newton's second law for mass m1 : F c = 1 For mass m2 : F ma. 1 Fc = m2a. 2 a) Adding Equations (1) and (2), you get F = m m ) a, ( 1 + 2
12 NEWTON S LAWS OF MOTION 55 F or a = m 1 + m 2 You can find the acceleration of the system by applying Newton's second law to the whole system. b) Substituting for a in Equation 2, you obtain F c m2 F = m + m 1 2
13 NEWTON S LAWS OF MOTION 56 Example 3.4 A boy of mass 45.0 kg stands on a platform N scale in an elevator, as in Figure 3.6. Find the scale reading when the elevator a) moves with constant mg velocity, b) ascends with an acceleration of 3.50 m/s2, c) descends with an acceleration of 3.50 m/s2 Solution Noting that the scale reads its reaction force, and applying Newton's second law in the three cases, you get a) F = N mg = 0, or N = mg = 441N. So the scale will read the actual weight of the man. b) F = N mg = ma, Figure 3.6 Example 3.4.
14 NEWTON S LAWS OF MOTION 57 or N = m( g+ a) = 599 N c) F = N mg = ma, or N = m( g a) = 284 N
15 NEWTON S LAWS OF MOTION FRICTIONAL FORCES It is m the force that two surfaces in contact exert on each other to oppose the sliding of one surface over the other. This kind of force results mostly from the interaction between the atoms and the molecules on the surfaces. Consider that you want to push your textbook along a horizontal table (Figure 3.7). Regarding the frictional force, three cases have to be considered: Case 1: Before starting pushing, the book is initially at rest, and this means that its acceleration is zero. As the force of friction is the only force acting horizontally on the book, this force in this case is equal to zero. Case 2: If you push the book gently, the book will not move and remains at rest. Here, there are two forces acting horizontally on the book: the pushing force F and the frictional force. For the acceleration to be zero, these two forces must be equal in magnitude and opposite in
16 NEWTON S LAWS OF MOTION 59 f s N F f k N F mg mg direction. This force is called the force of static friction and will be denoted hereafter by fs. Case 3. Now, slowly increasing the pushing force, the book will begin to move when the pushing force reaches a critical value fs(max.). Once in motion, the frictional force is less than fs(max.), and is called the force of kineticfriction, denoted by fk. It is found, experimentally, that the frictional forces fs and fk, between two surfaces, are proportional to the normal force N pressing the two surfaces together. i.e., and (a) (b) Figure 3.7 (a) Case 1: f s =0. (b) Case 2: fs = F µ s N. (c) Case 3: f = µ N. k k fs µ s N, (3.4) (c)
17 NEWTON S LAWS OF MOTION 60 fk = µ k N. (3.5) Where the dimensionless constants µ s andµ k are, respectively, the coefficient of static friction and the coefficient of kinetic friction. Remarks: 1 The frictional force is always parallel to the surfaces in contact. 2 The force of staticfriction is always opposite to the applied force 3 The force of kinetic friction is always opposite to the direction of motion. 4 The frictional force, together with the normal force constitute the two perpendicular components of the reaction force exerted by one of the contact bodies on the other.
18 NEWTON S LAWS OF MOTION 61 Example 3.5 A block of mass m slides down a rough, inclined plane with the angle of inclination is θ as shown in Figure 3.8. The coefficient of kinetic friction between the block and the plane is µ. a) Find the normal force of the plane. b) Find the acceleration of the block y x mgcosθ m f k mg θ N mgsinθ Figure 3.8 Example 3.5 with the freebody diagram of the block. Solution: The freebody diagram of the block is shown in Figure 3.8. Note that the xaxis is chosen along the plane. a) It is clear the the motion is entirely along the xaxis and there is no motion along the yaxis. Now applying Newton s second law in the y axis, we get F = N mg cosθ 0, y = From which we obtain
19 NEWTON S LAWS OF MOTION 62 N = mg cosθ (b) Applying Newton's law in the xaxis, we obtain Fx = mg sinθ fk = ma. Substituting for have f = µ N = µ mg cosθ, we k k k a = g(sinθ µ cosθ )
20 NEWTON S LAWS OF MOTION 63 Example 3.6 A worker drags a crate along a rough, horizontal surface by pulling on a rope tied to the crate. The worker exerts a force of 300. N on the rope that is inclined 37.0 o to the horizontal as shown in Figure 3.9. If the mass of the crate is 60.0 kg, and the coefficient of kinetic friction between the crate and the surface is 0.30, find the acceleration of the crate. Solution First we construct the freebody diagram of the system as shown in Figure 3.8. After resolving the applied forces into its components according to the chosen axes, we apply Newton's second law to get, in the yaxis f k N 60. kg mg Fsin37. F=300. N 37. o F Fcos37. Figure 3.9 Example 3.6 with the freebody diagram of the system. F y = N + 300sin37. mg = 0 from which we find
21 NEWTON S LAWS OF MOTION 64 N = = 407 N. Now applying Newton s second law in the x axis, we obtain But so Fx = 300 cos37 fk = ma. f = µ N = 0.3(407) 122 N, k k = 118 a = = 1.97 m/s
22 NEWTON S LAWS OF MOTION 65 Example 3.7 A 2.0kg block is placed on top of a 5.0kg block as shown in Figure A horizontal force of F = 44 N is applied to the 5.0kg block. If the coefficient of kinetic friction between the 5.0kg block and the surface is 0.20, and assuming that the 2.0kg block is in the verge of slipping, a) what is the acceleration of the system? 2.0 kg 5.0 kg b) What is the coefficient of staticfriction between the two blocks?. Solution Study carefully the freebody diagram of the two blocks (Figure 3.10). The normal force acting on the 5.0kg block by the surface is denoted by N 1, while N 2 stands for the normal force acting on the upper block by the lower one. f k N 2 N 1 f s 2g 5g f s N 2 F 44 N Figure 3.10 Example 3.7 with the freebody diagram of the two blocks. Note that the static frictional force is the force that accelerate the upper block.
23 NEWTON S LAWS OF MOTION 66 a) Applying Newton's second law to the 2.0kg block, we have in the xaxis f 2a s =, 1 and in the yaxis or N 2 2g= 0 N = 2g 20. N 2 2 = Similarly for the 5.0kg block, we have F = F f f 5. 0a, 3 x k s = and or F y = N1 5.0g N2 = 0 N = 7.0g 69 N, 4 1 =
24 NEWTON S LAWS OF MOTION 67 where the value of N2 is taken from Equation 2. Note that, the two blocks have the same acceleration because the 2kg block does not slip. Now, Adding Equation 1 and Equation 3, we obtain F µ N 7. 0a, k 1 = where we have substituted for f k = µ k N1. Now substituting for N1 from Equation 3, we have a = = 4.3 m / s. 7.0 b) Substituting for a in Equation 1, we get 2a µ s = = = N 20. 2
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