Newton’s Laws of Motion In this lab you will analyze real-world examples to determine if the net force is zero or nonzero. You will also identify the Newton’s law of motion that is demonstrated by each example and explain you selection. Sir Isaac Newton Newton’s three laws of motion are fundamental to how the world functions. These laws influence everyday activities such as walking, driving, and even sitting. Sir Isaac Newton was born in 1643 in Woolsthorpe, England. He studied at Cambridge during the Scientific Revolution in the 17th Century. The new ideas and discoveries of this time period set the stage for his contribution: a groundbreaking paper titled “Philosophiae Naturalis Principia Mathematica” (The Mathematical Principles of Natural Philosophy), which Newton published in 1687. This work contained the basics of his theories, which would become known as Newton’s laws of motion. It was revolutionary in its time and is considered the foundation of modern, non-relativistic physics. Newton developed his ideas about motion through an intense period of scientific experimentation and reflection. The laws have been supported throughout history by repeated experimentation. Newton’s paper continues to be widely regarded as one of the most influential works of all time in physics. It is remarkable that these laws are just as relevant today as they were hundreds of years ago. Image copyright Georgios Kollidas, 2016. Used under license from Shutterstock.com. Figure 1. Portrait of Isaac Newton published in The Gallery of Portraits: with Memoirs encyclopedia. 1|Page Newton’s First Law of Motion Newton’s first law of motion is known as the law of inertia. Inertia is the measure of a body’s resistance to change its state of movement. Inertia has a direct relationship with mass, in that as mass increases so does inertia. For example, it would be harder to change the motion of a large boulder than a small boulder with the same applied force because the large boulder has a larger mass than the small boulder. Inertia is also directly related to force since the force required to change an object’s motion increases with inertia. Combining these relationships, Newton’s first law is stated as: A body at rest tends to stay at rest, and a body in motion tends to stay in motion in a straight line, unless acted upon by a net force. There are several noteworthy points in this first law. First, inertia applies to both non-moving and moving objects. It is often mistakenly believed that Newton’s first law only applies to objects at rest. In fact, the law of inertia refers to a change in movement, whether that change refers to a stationary object beginning to move or a moving object increasing or decreasing its speed. Second, the definition states “motion in a straight line.” Objects in motion require a force to change direction. Finally, the end of the law states that a change in motion requires a net force. Every object in the universe is experiencing a force of some kind at all times, such as gravitational force. The law of inertia indicates that change occurs only when the forces acting on an object are out of balance. If the net, or combined, force in any direction is nonzero, then the object will experience a change in motion. Newton’s first law of motion can be explained using the example of a rocket on a launch pad. Prior to igniting the engines, the velocity, or speed of an object in a particular direction, is zero and the downward force of gravity and the upward normal force of the platform are balanced. This results in a net force of zero, so the rocket remains at rest on the platform. See Figure 2. When the engines are ignited, a net force is applied that increases until it is greater than the weight of the rocket, changing the rocket’s movement, as described by the next law of motion. 2|Page Image copyright Alan Freed, 2016. Used under license from Shutterstock.com. Figure 2. Rocket remaining at rest on the launch pad due to the downward force of gravity and upward force of the launch pad canceling each other out. In summary, the main point of Newton’s first law is that if there is no net force acting on an object, then the object maintains a constant velocity. If the velocity is zero, then the object remains at rest. If the velocity is nonzero, then the object maintains the velocity and travels in a straight line. Newton’s Second Law of Motion Newton’s second law of motion focuses on the concepts of force and inertia. The first law of motion establishes the conditions for an object in motion to change velocity, and the second law of motion provides a measure of the force that is necessary to cause that change in velocity. The law of force and acceleration states that: Acceleration is directly proportional to the net force acting on an object and inversely proportional to the mass of the object. 3|Page Mathematically, this law is written as: Recall that velocity is the speed of an object in a particular direction. Acceleration is the rate of change of an object’s velocity. A large acceleration implies a quick rate of change; whereas, a small acceleration indicates a slow rate of change. Because velocity has both speed and directional components, acceleration can reflect either a change in the speed of an object (speeding up or slowing down) or a change in the direction an object is moving, even if the speed remains constant. An example of the latter is a car traveling around a curve at a constant 25 mph. Even though the speed remains the same, the car is accelerating because it is changing direction, and direction is part of velocity. Notice that acceleration refers to the rate of change – how fast the velocity is changing, and not the magnitude of speed. A rapid change at a slow speed is the result of a large acceleration, while a slow change at a high speed is the result of a small acceleration. For example, a car accelerating from 0 to 30 mph in one second has a larger acceleration than a car accelerating from 70 to 100 mph in 10 seconds. Even though the second car is traveling at a higher speed, its rate of change is smaller, so its acceleration is smaller. Just like Newton’s first law, the second law also refers to net forces. These forces are counterbalanced by the mass of the object itself, meaning that a more massive object requires a larger force than a smaller object to accelerate by the same amount. For example, a larger force is required to throw a bowling ball the same speed as a tennis ball. Newton’s second law can also be applied to a rocket launch. Inside the rocket’s engine, a controlled explosion occurs creating pressure as a force known as thrust. Propellant onboard the rocket is responsible for the largest portion of the rocket’s mass. The thrust from the engine must be greater than the mass of the rocket in order to lift the rocket off of the launch pad. As the propellant is burned up during continued thrust, the mass of the rocket decreases during flight. To maintain balance in Newton’s second law equation, acceleration of the rocket increases as its mass decreases, as shown in Figure 3. This explains why a rocket initially moves slowly and then speeds up as it climbs into space. 4|Page Image copyright John A. Davis, 2016. Used under license from Shutterstock.com. Figure 3. Acceleration of a rocket increases as it climbs into space due to the decreasing mass from the propellant being expelled. Did you know? A rocket must reach a speed in excess of 28,000 km per hour (17,398 mph) in order to reach the low orbit of Earth. A speed of over 40,250 km per hour, known as the escape velocity, enables a rocket to leave Earth and travel into deep space. Newton’s Third Law of Motion When two objects interact they exert forces upon each other. Newton’s third law of motion predicts what occurs to the forces when one object comes in contact with another object. The law states that forces come in equal pairs. Also known as the law of action and reaction, Newton’s third law is stated as: For every action, there is an equal and opposite reaction. The reaction force acts to counter the initial force. Anytime a force is exerted on an object, the object pushes back with the exact same force but in the opposite direction. Note that the forces are balanced in magnitude and act directly against each other. Without this balance there would be no equilibrium. Newton’s third law leads to the question: if every force is perfectly balanced, 5|Page then how is movement possible at all? Newton’s second law of motion answers the question: equal forces do not produce the same effect if the masses of the two objects are unequal. This concept is illustrated by an elephant and a fly colliding head-on. When they collide, the elephant exerts a force on the fly, and the fly exerts an equal force on the elephant (in the opposite direction). Because the elephant is much heavier than the fly, the force exerted by the fly is essentially unfelt by the elephant. However, the force of the collision is substantial for the fly, and results in a complete halt to its motion. It is also important to note that normal force and gravitational force are not an action and reaction pair because they are two different types of forces. Because they are two different types of forces they do not interact as “equal but opposite” as stated in Newton’s third law. A rocket launching into space can also be used to explain Newton’s third law. Gravity on the rocket pushes on the propellant, and the propellant pushes on the rocket. The thrust accelerates the propellant out of the rocket in one direction and the rocket itself in the opposite direction. The action is the expelling of propellant out of the rocket engine, and the reaction is the movement of the rocket in the opposite direction. See Figure 4. Image copyright Everett Historical, 2016. Used under license from Shutterstock.com. Figure 4. Action of propellant expelling downward from the engine causes the reaction of the rocket to lift off in an upward direction. 6|Page Newtons Laws of Motions Learning Objectives • Define inertia, velocity, and acceleration. • Describe each of Newton’s three laws of motion. • Identify the net force of real-world scenarios • Apply Newton’s laws of motion to real-world scenarios • Sketch examples of Newton’s laws of motion to illustrate the relevant forces Materials Read through the procedures listed in the exercises on the next pages before beginning. Then, gather all of the materials listed below and begin Exercise 1. Note: The packaging and/or materials in your kit may differ slightly in appearance from images in the experimental procedures. Student Supplied Camera, digital or smartphone Pen or pencil Paper There are no kit materials needed for this lab. Exercise 1 – Newton’s Laws of Motion In this exercise, you will analyze scenarios and identify objects being acted on by a net force, and explain why the net force is zero or nonzero. You will also explain how each scenario demonstrates Newton’s laws of motion. You will sketch scenarios of Newton’s laws of motions, including arrows representing the relevant forces. Procedure Part 1: Analyzing Newton’s Laws 1. Review each scenario listed in Data Table 1. 2. Record the net force (0 or nonzero) of each scenario in Data Table 1. 3. Explain why the net force is 0 or nonzero for each scenario and record in Data Table 1. 4. Record the Newton’s law of motion (1st, 2nd, or 3rd) that is best demonstrated by each scenario in Data Table 1. Note: Remember that normal force and gravitational force are not action/reaction pairs because they are two different type of forces. Newton’s first law is typically applied to objects at rest when the forces are balanced; whereas Newton’s second law is commonly applied to scenarios in which the forces are not balanced. 7|Page 5. Explain in detail how each scenario demonstrates Newton’s law of motion in action in Data Table 1. Part 2: Sketching Newton’s Laws 1. Sketch (either by hand or on a computer) the scenario of a book sitting on a table from Data Table 1. Draw arrows representing the relevant forces on the sketch and an additional arrow showing the acceleration if necessary. 2. Take a photo of the sketch and upload the image into Photo 1. 3. Sketch (either by hand or on a computer) the scenario of a man pushing a box across a frictionless floor from Data Table 1. Draw arrows representing the relevant forces on the sketch and an additional arrow showing the acceleration if necessary. 4. Take a photo of the sketch and upload the image into Photo 2. 5. Sketch (either by hand or on a computer) the scenario of air inside and outside of a balloon from Data Table 1. Draw arrows representing the relevant forces on the sketch if necessary. 6. Take a photo of the sketch and upload the image into Photo 3. 7. Complete the Postlab Review Questions. 8|Page Newton’s Laws of Motion Data Table 1. Net Force and Newton’s Laws Scenario Net Force Net Force Explanation Newton’s Law Law Explanation A book sitting on a table A puck sliding across frictionless ice A child sitting on a merry-goround turning at a constant speed A person leaning against a wall A rock falling at constant terminal velocity A car driving around a curve at a constant speed of 40 mph A person pushing a box across a frictionless surface A soccer ball placed on the grass Air inside and outside of an inflated balloon 1|Page Photo 1. Newton’s Law Sketch Photo 2. Newton’s Law Sketch Photo 3. Newton’s Law Sketch 2|Page Postlab Questions: 1. Define velocity and acceleration. Explain how acceleration can be used to interpret the velocity of an object. 2. Assuming a frictionless surface, explain how Newton’s three laws of motion are in action as an air hockey puck slides across the table and then hits the side of the table wall. 3|Page
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