Chapter 4 - Forces & Motion - Physics 100 Lecture Note PDF

Chapter 4 - Forces & Motion Eng. Nada Salem Physics 100 Lecture Note 1  Inertia ▪ According to the law of inertia, by nature objects have the tendency to be either at rest or, when in motion, to move along a st...

Chapter 4 - Forces & Motion Eng. Nada Salem Physics 100 Lecture Note 1  Inertia ▪ According to the law of inertia, by nature objects have the tendency to be either at rest or, when in motion, to move along a straight line with constant speed. ▪ We call this property of matter inertia ▪ It means that, by nature, object maintain a constant state of motion (either being at rest or move a straight line) ▪ So objects resist any change of their state of motion ▪ Objects around us never obey the law of inertia because there are always such influences as air resistance, force of gravity, or friction ▪ The bigger an object the harder to change its motion Eng. Nada Salem Physics 100 Lecture Note 2  Galileo’s Inclined Planes Noted that a ball that rolls down an inclined plane picks up speed. When the same ball rolls up the inclined plane, it slows down W hen a ball rolls on a flat horizontal surface, it rolls at near constant velocity Stated that with the absence of friction, the ball would roll forever. Eng. Nada Salem Physics 100 Lecture Note 3  Galileo’s Inclined Planes If you have two inclined planes face each other, a ball rolled down one plane will reach nearly the same height as it rolls up the other plane. He noticed that the ball ended up at the same height, even is the plane was elongated or at a different angle. Eng. Nada Salem Physics 100 Lecture Note 4  Law of Inertia Inertia – the property of a body to resist change (the tendency of a moving body to keep moving and every material object resists change to its state of motion). Mass is the amount of matter in an object The more MASS an object has, the more INERTIA the object has. Bigger objects are harder to start & stop. Eng. Nada Salem Physics 100 Lecture Note 5  Newton's Law of Motion First Law- Inertia Second Law-Acceleration Third Law- Action-Reaction Eng. Nada Salem Physics 100 Lecture Note 6  Newton’s First Law An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force. Eng. Nada Salem Physics 100 Lecture Note 7 Eng. Nada Salem Physics 100 Lecture Note 8  Newton’s First Law Objects at rest remain at rest Objects in motion remain in motion UNTILYOU APPLYA FORCE Objects tend to resist a change in motion. This is called: Inertia Eng. Nada Salem Physics 100 Lecture Note 9  Question Is inertia a force? No, inertia is a property of matter. Something has inertia. Inertia does not act on something. Eng. Nada Salem Physics 100 Lecture Note 10  Newton's Law of Motion ▪ As we have just concluded, the motion of an object changes only if there are external influences on the object. ▪ What are external influences? ▪ If you want to slow down an object, what should you do? Push it forward? Or push it backward? ▪ External influences that push or pull objects in one way or another have the general name “Force” ▪ We can then say, external forces cause the motion of objects to change ▪ How? By Slowing them down or Speeding them up or Changing their direction of motion ▪ Gravity is one such influence and it pulls objects toward the earth center ▪ Friction and air resistance slow things down ▪ Conclusion: Forces cause change in motion or accelerations Eng. Nada Salem Physics 100 Lecture Note 11  Balanced force Versus Unbalanced force Balanced forces cause no acceleration. Eng. Nada Salem Physics 100 Lecture Note 12  Balanced force Versus Unbalanced force Unbalanced forces cause acceleration. 15 Eng. Nada Salem Physics 100 Lecture Note 13  One object & multiple forces: ▪ Most of the time, objects experience push/pull from multiple forces ▪ For example, the motion of an object moving on the surface is hindered by friction and air resistance ▪ How do we handle such multiple forces? ▪ Friction and air resistance oppose the motion so they usually subtracted from the applied force ▪ General rule: If the forces are pushing or pulling in the same direction, add them If the push in opposite directions, subtract Eng. Nada Salem Physics 100 Lecture Note 14 Newton’s Second Law Force equals mass times acceleration. F = ma Acceleration: a measurement of how quickly an object is changing speed. Eng. Nada Salem Physics 100 Lecture Note 15  Newton's Second Law Acceleration Acceleration is a change in velocity [speed or direction] The more force placed on an object, the more it will accelerate [change its motion] Mass is the amount of matter in an object. The more mass [or inertia] an object has, the more force it takes to accelerate the object. Eng. Nada Salem Physics 100 Lecture Note 16  Mass and Force What happens if the same force is exerted on two objects with different sizes? The bigger object's motion will change less In other words, the bigger object will have less acceleration The smaller object's motion will change more In other words, the smaller object will have more acceleration In summary, the acceleration caused by a certain force is inversely related to the size of the object. 21 Eng. Nada Salem Physics 100 Lecture Note 17  Mass and acceleration The relationship between mass and acceleration is written Eng. Nada Salem Physics 100 Lecture Note 18  Newton's Second Law The acceleration caused by a net force acting on an object ▪ is directly proportional to the magnitude (or size) of the force. ▪ is inversely proportional to the mass of the object. ▪ has the same direction a the force causing it. 𝐹 𝑎= 𝑚 ▪ a stands for acceleration ▪ m stands for the mass of the object ▪ F stands for the force acting on the object Eng. Nada Salem Physics 100 Lecture Note 19  Newton's Second Law 𝐹 𝑎= 𝑚 Eng. Nada Salem Physics 100 Lecture Note 20  Summary: Motion & force Eng. Nada Salem Physics 100 Lecture Note 21  Units Quantity Definition Unit Distance Length of path Meter (m) or kilometer (km) Displacement Direct distance b/w two location Meter (m) or kilometer (km) Time Time taken by motion Second (s) or hour (h) Speed Distance/ time m/s or km/h Velocity Displacement/time m/s or km/h Acceleration Change of velocity/time m/s/s or m/𝑠 2 Mass Amount if inertia of abject has Kilogram (kg) Force Mass * acceleration kgm/𝑠 2 or Newton (N) Eng. Nada Salem Physics 100 Lecture Note 22  Examples An object whose mass is 2 kg is pushed by a 20 N force. What acceleration would this force produce? Eng. Nada Salem Physics 100 Lecture Note 23  Examples An object whose mass is 2 kg is pushed by a 20 N force. What acceleration would this force produce? 𝐹 20 𝑎= = = 10m/s 2 𝑚 2 Eng. Nada Salem Physics 100 Lecture Note 24  Mass vs weight Definition of weight: Mass is a measure of the amount of matter in an object. It can also be defined as the amount of inertia an object has. It is measured in kilograms. Weight on the other hand is about how heavy an object is. It is ▪ About how hard it is to lift an object ▪ A measure of how hard an object is pulled by the force of gravity The weight of an object is a measure of the force of gravity on the object Weight= mass X acceleration of gravity Eng. Nada Salem Physics 100 Lecture Note 25  Mass and Weight Note that mass is independent of everything but weight is different on the earth, the moon. Eng. Nada Salem Physics 100 Lecture Note 26  Weight Weight is a force. Kilograms and pounds are units of mass. Weight is a force that references local gravity. Mass is the same measure of the amount of matter in an object, and thus the same wherever you go in the Universe. Examples: What is the weight of an object (on earth) that has a mass of 45 kg? What is the mass of a person who weighs 500 N? Eng. Nada Salem Physics 100 Lecture Note 27  Normal force When an object lies on a table or on the ground, the table or ground must exert an upward force on it, otherwise gravity would accelerate it down. This force is called the normal force. N In this particular case, m N = mg. So, Fnet = 0; hence a = 0. mg Eng. Nada Salem Physics 100 Lecture Note 28  Newton’s Third Law For every action there is an equal and opposite reaction. Eng. Nada Salem Physics 100 Lecture Note 29  Newton’s Third Law “For every action, there is an equal and opposite reaction.” For every action there is an equal and opposite reaction. Eng. Nada Salem Physics 100 Lecture Note 30  Newton’s Third Law Objects exchanging force Eng. Nada Salem Physics 100 Lecture Note 31  Newton’s Third Law 36 Eng. Nada Salem Physics 100 Lecture Note 32  Newton’s Third Law Newton’s third law describes the relationship between two forces in an interaction. One force is called the action force. The other force is called the reaction force. Neither force exists without the other. They are equal in strength and opposite in direction. They occur at the same time (simultaneously). Eng. Nada Salem Physics 100 Lecture Note 33  Forces and Interactions When you push on the wall, the wall pushes on you. Eng. Nada Salem Physics 100 Lecture Note 34  Two objects & one force ▪ When you push an object, you influence its motion ▪ The object also influences your motion ▪ Pushing influences both the motion of the pusher and the motion of the pushed ▪ This is most obvious in slippery surfaces, such as ice rinks Law of force pairs To every action there is always an equal and opposite reaction: or the forces of two bodies on each other are always equal and are directed in opposite directions. Eng. Nada Salem Physics 100 Lecture Note 35  Summary Newton’s First Law: Objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted upon by an unbalanced force. Newton’s Second Law: Force equals mass times acceleration (F = ma). Newton’s Third Law: For every action there is an equal and opposite reaction. Eng. Nada Salem Physics 100 Lecture Note 36