Module in forces and laws of motion


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Laws of Motion

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Module in forces and laws of motion

  1. 1. Forces and Laws of Motion<br />If a big rock at the middle of a field suddenly starts moving across the ground, you will surely try to find for the cause of its motion. You may want to find out if somebody is pushing it or pulling it. Definitely, the movement does not happen without cause. What causes the motion of the rock is surely due to a certain type of force. Also, what makes it to stop from moving is again due to a force of some kind.<br /><ul><li>In this module, you will:define force (balanced and unbalanced);explain how force affects motion;distinguish between contact and field forces; identify some particular types of forces;state and explain the Newton’s Laws of motion; and,identify some applications of the Newton’s Laws of MotionObjectives</li></ul>In the previous chapter, you have learned how to analyze motion in terms of kinematics. In this module, you will learn about dynamics – the study that deals with how force produces motion. It will give you an idea of the importance of recognizing a force that is acting on an object. Here, you will also compute for the effects of force applied on an object. <br />A. FORCE<br />340042510160It would be very easy for you to give examples of forces. Basically, when you think of force, you think of a push or a pull. That is very elementary. <br />443865068580Force can be operationally defined based on the observed effects. A force can be described in terms of what it does. It can set a stationary object in motion. It can also speed up or slow down (or even stop) a moving object or it can even change the direction of its motion. Hence, a force can produce a change in velocity (and consequently, causes an object to accelerate). Evidently, force causes change in motion (but not necessarily correlated with motion).<br />4391025137795The net force is the vector sum of all the forces acting on a system. If two equal forces act on an object in opposite directions, what would happen? Will the object move? In this case, the net force (Fnet) is equal to zero. These forces are called balanced forces. They do not produce motion. Instead they keep an object stationary. An unbalanced force on the other hand, is a non-zero net force. It produces acceleration, thus it can set objects at rest in motion.<br /><ul><li>TRUE/FALSE: Force always causes an object to move.TRUE/FALSE: An unbalanced force produces accelerated motion.Give an example of a force causing an object in motion to stop. Give an example of a force causing an object at rest to move. Give one example each of contact and field forces.Find the net force: Answer first then turn to page 8 to see if your answer is correct.Self-CheckHave you tried pushing a car or pulling a cart using a rope? Obviously, forces are involved in these situations. These forces result to a physical contact between the objects involved. These forces are called contact forces. There are also forces that do not require physical contact between objects involved. These are called field forces (or non-contact forces). Can you give examples of this kind of forces?</li></ul>SOME PARTICULAR FORCES<br />WEIGHT OF A BODY (W) <br />It is a force that pulls the body directly toward a nearby astronomical body: in everyday circumstances that astronomical body is the earth.<br />The force is primarily due to an attraction between two bodies called gravitational attraction.<br />W = mg<br />where: W = force acting on the body<br />m = mass of the body<br />g = free fall acceleration <br />NORMAL FORCE<br />It is a force exerted when a body is pressed against a surface which is perpendicular to the surface. <br />A book on a table experiences a downward force (W) due to gravity, and an upward force (N) due to the table pushing on the book. The two forces cancel each other out exactly; there is no net force, so the book does not accelerate off the table.<br />3. FRICTIONAL FORCE OR SIMPLY FRICTION<br />401002546355Friction is a force that resists the movement of one object against another. This force is directed along the surface, opposite the direction of the intended motion. <br />If the surface is said to be frictionless, friction is assumed to be negligible. <br /><ul><li>TENSION</li></ul>It is a pulling force that acts in one direction.<br />When a cord (rope, cable or other such object) is attached to a body and pulled, the cord is said to be under tension. Under tension, a material usually stretches, returning to its original length if the force does not exceed the material's elastic limit. Under larger tensions, the material does not return completely to its original condition, and under even greater forces the material ruptures.<br />B. NEWTON’S LAWS OF MOTION<br />In the previous section, you learned about what causes motion. Here, you will study the three laws of motion formulated by Isaac Newton. These laws would explain how objects affected by force behave.<br />B.1. Newton’s First Law of Motion<br />377190083820Inertia, as defined by Galileo, is the tendency of an object to maintain its initial state of motion. Although Galileo did not fully explain motion, he had made a great influence to Newton, who established the laws of motion. Newton related the concept of inertia to mass. Originally, Newton defined mass as a quantity of matter. However, he later redefined it as a measure of inertia. From here, it can be said that a more massive object has greater inertia, or has more resistance to change in motion than a less massive one. Newton’s first law of motion, which is also know as the law of inertia states that: <br />A body at rest will remain at rest or a body in uniform motion<br />in a straight line will maintain that motion unless an external force acts on it.<br />An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. <br />When you ride on a vehicle, what happens when the car suddenly moves forward? How would you feel? Do you feel that your body moves backward? What if, it suddenly stops? Your body will continue to be in motion and so it moves forward until something stops it. This explains the role of seatbelts. A passenger of a fast moving vehicle can meet an accident when the car suddenly stops because the passenger’s body has the tendency to continue moving forward toward the windshield. Seatbelts prevent the passenger from moving in the direction of the motion.<br />B.2. Newton’s Second Law of Motion<br />Try pushing a cart with you alone. And then, have someone help you do the same, both of you pushing the cart. In which of the two cases will you be able to move the cart faster? This shows that the greater force you apply on an object, the faster will it accelerate on the direction of the force. The relationship of the quantities force, mass and acceleration is stated in the Newton’s Second Law Motion (also known as Law of Acceleration) which is stated as: <br />The acceleration of a body is directly proportional to the<br /> resultant force acting upon it and is inversely proportional to the mass of the body.<br />In mathematical form, the second law can be stated as:<br />Fnet = mawhere: Fnet = is the net force m = is the mass a = is the acceleration<br />From the equation above, it can be said that the net force is dependent on two elements – mass and acceleration. Consequently, the unit of force can be expressed as a combination of a unit of mass and the unit of acceleration. In the SI system, the unit of force is the Newton (N). A Newton (N) is the net force when a 1kg mass gives an acceleration of 1m/s2. <br />Illustrative Example:<br />A 50kg cart is pushed and produces an acceleration of 2m/s2 to the North. What is the net external force exerted on the cart?<br />Solution:<br />Given: m = 50kg<br />a = 2m/s2to the North<br />Find: Fnet<br />Fnet = ma<br /> = (50kg)(2m/s2)<br /> Fnet = 100 kg m/s2 or 100N to the North<br />Illustrative Example:<br />A constant net force of 200N is applied to accelerate a cart from rest to a velocity of 40m/s in 10s. Find the mass of the cart.<br />Solution:<br />Given: Fnet= 200N<br />v1 = 0m/s<br />v2 = 40m/s<br />t = 10s <br />Find: m<br />First find the acceleration, <br /> <br />then,<br />m = 50kg<br /><ul><li>An object with a mass of 9 kg experiences a force of 27 N. What is the acceleration of the object?How much force is needed to stop a car with a mass of 5000kg and moving at 20m/s in 10s?Answer first then turn to page 8 to see if your answer is correct.Self-Check</li></ul>B.3. Newton’s Third Law of Motion<br />46672503810Try to push against a wall. In this case, you are exerting a force against the wall. But do you also feel that there is also a force that pushes toward you? The wall pushes back on you a force equal to that which you exert on the wall. If the force exerted by the wall towards you does not exist, you will collapse with the wall. When you quarrel with someone and punch him on the face, the amount of force you exert in punching his face is the same amount of force that his face exerts on your hand, which is why you also feel the pain. Situations like these probably led Newton to formulate the third law of motion (also known as the Law of Interaction) which states that: <br />When an object exerts a force on another object, the second object exerts on the first a force of the same magnitude but in the opposite direction.<br />This can be simply stated as “For every force (action) there corresponds an equal and opposite force (interaction).<br />The third law of motion applies to two different bodies. The forces are equal and opposite in directions. The third law may seem to contradict the second law. If there are always equal and opposite forces, how can there be a non-zero net force? It is important to note that the forces in each pair in the third law are opposing forces that do not act on the same object, while that on the second law are acting on a particular object. <br /><ul><li>Identify the law of motion illustrated by the following:A rocket lifts off from a space shuttle system.A car still moves for some time even after the brakes have been applied.Answer first then turn to page 8 to see if your answer is correct.Self-Check
  2. 2. Write everyday applications which demonstrate the Newton’s Laws of Motion. (Do not use examples cited in this module). Write one situation for each.In this module, you’ve learned that mass and weight are two different things. Weight refers to the force with which gravity pulls upon the object, while Mass refers to how much stuff is present in the object. What is the mass of an object which weighs 98N?Taba Choy wants to diet this summer. Is he aiming to lose mass or to lose weight? Defend your answer.The net force is the vector sum of all the forces which act upon an object. That is to say, the net force is the sum of all the forces, taking into account the fact that a force is a vector and two forces of equal magnitude and opposite direction will cancel each other out. Below, the net force is known for each situation. However, the magnitudes of a few of the individual forces are not known. Analyze each situation individually and determine the magnitude of the unknown forces. Suppose that a car is accelerating at a rate of 5 m/s2. If the net force is doubled and the mass is tripled, then what is the new acceleration of the sled?While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus? Explain. A 50g tennis ball approaches a racket at 25m/s. if it is in contact with the racket strings for 0.005s, then rebounds at 25m/s, what is the average contact force between the ball and the racket? Hint: Compute first for the acceleration.Many people are familiar with the fact that a rifle recoils when fired. This recoil is the result of action-reaction force pairs. A gunpowder explosion creates hot gases which expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. Which has a greater acceleration: the recoiling rifle or the bullet? Why? Write your answers on a clean sheet of paper and submit it to your instructor. Task
  3. 3. FalseTrueAnswers may vary. (e.g. braking on a moving car)Answers may vary. (e.g. pushing a book on top of a table)Answers may vary. (e.g. contact force: pushing a door, kicking a ball; field force: attraction or repulsion of magnets)Fnet = 150N to the righta. Third Law or Law of Interactionb. First Law or Law of InertiaAnswers to Self-Check Questions