Forces and Newton s First Law

Transcription

1 Lyzinski Physics CRHS-South Forces and Newton s First Law Thus far, we have studied the motion of objects. The study of motion is known as. However, we were not interested, yet, about what caused the motion. This branch of mechanics is known as. Thus far, we have been interested in how something moves. Did it accelerate, in what direction, etc? But now let s probe deeper. Why did the object move? Did someone push the moving object? Maybe something that was holding the object up suddenly let it go. What caused the motion? This question leads us into the 2 nd branch of Mechanics. This branch, which studies the causes of an object s motion, is known as. So what actually causes motion, or a change in motion? The answer is forces. Q: How can we define the term force? Q: What is the SI unit of force? Is this unit a base unit or a derived unit? Q: What is the English unit of force? What is the relationship between the SI unit and the English unit? Q: There are two categories of forces. What are they? Describe each one. People have been interested in the motion of objects for thousands of years. We will begin a brief history of motion at the time of the Ancient Greeks (approx. 350 B.C). Aristotle, the famous Greek philosopher, studied the motion of objects and concluded the following: It requires a continuous pushing or pulling to keep an object such as a rolling stone moving. When the pushing or pulling is no longer applied, the stone comes to rest A is required to produce a constant velocity (Heath, pg. 128) Increased force à object moves Decreased force à object moves No force à object will

2 Aristotle s ideas seem very logical. He believed that heavy objects fall faster than lighter ones. When discussing projectiles, he believed that projectiles were pushed along by an external force which was transmitted through the air. His medieval successors internalized this force in the projectile itself and called it "impetus." This impetus caused the object to move in a straight line until it was expended, at which point the object fell straight to the ground (Rice). Aristotle s views on motion dominated the study of motion for almost 2000 years. Galileo Galilei, the great Italian physicist who lived during the Renaissance, used two different thought experiments about the motion of an object on an inclined plane to explain motion. Experiment #1: Galileo imagined a ball rolling down a sloped plane. He figured that it would speed up. He then imagined a ball rolling up a slope. He figured that it would slow down. He reasoned that a ball rolled across a horizontal surface would neither speed up nor slow down, but rather continue to move with a constant velocity. Experiment #2: Galileo again imagined a ball rolling down a sloped plane. However, this time, he allowed the ball to roll up a plane afterward. He reasoned that no matter what the slope of the two planes, the ball would always attain the same height (the height it was rolled down from equals the height it rolls up to). Therefore, he concluded that if no up plane were present at the bottom of the down plane, the ball would roll on FOREVER, trying to but never reaching the height from which it were dropped. Both of Galileo s thought experiments contradicted his observations of the same events in real-life. However, he attributed the differences to Resistance, or what we today call friction. Both thought experiments occurred on a frictionless surface. Galileo argued that it was just as natural for an object to move with a constant speed as it is to be at rest. This contradicted Aristotle s view. Galileo Published these thoughts about motion in the early 17 th century, and his contemporaries immediately dropped the Aristotelian view and embraced Galileo s views of motion.

3 Isaac Newton, an Englishman who lived later in the 17 th century, began his theories of motion by looking at a concept that he called Inertia. Inertia is a property of an object. It can be thought of as laziness. Objects tend to keep doing what they are doing. It takes force to make an object start moving or change direction. The more massive an object is, the larger the force that is required for a given change. (Holt, teacher s addition) Mass is a measure of inertia. Galileo was the first person to formalize this concept. However, Newton used it to develop his famous Laws of Motion, which he formally published in his book Principia Mathematica. This book is widely considered to be the greatest scientific work ever published. Newton s First Law of motion, otherwise known as Newton s Law of Inertia, informally states that #1 This law can be stated in layman s terms as follows What does the term unbalanced mean? It means that when all the forces upon an object are looked at, the. Therefore, the of the forces will not be zero. Also forces have no effect on an object s motion. Only forces affect motion. But if this is in fact a Law of motion, why don t we see this happen in the real world. When an object is pushed, it slows down, even when no other forces act upon it. Or so it would seem. is the invisible force that acts on objects to slow them down. However, in a friction-free case, Newton s 1 st Law will be observed to hold true. And, it holds true in all cases, we simply have a hard time observing it because of friction. Examples of Newton s 1 st Law include: A person (without a seatbelt) going through the windshield in a car accident. A magician pulling the tablecloth off a table but leaving the place-setting. Getting a car to start moving vs. keeping it rolling once it s started. Punching a light speed bag vs. punching a heavy bag.

4 So, why is Newton s 1 st Law often called the Law of inertia? To answer this, we first need to define the term inertia. Inertia is For example, when a heavy grocery cart is pushed quickly down the super market aisle, it is difficult to stop, or eve slow down. Why? Because is has a lot of inertia. It has a great tendency to resist a change in its current motion. On the other hand, a textbook is very easy to slide accelerate across a desk. It has very little inertia, and thus has very little tendency to resist this change in its motion. The bottom line is this: all objects have a natural tendency to either stay at rest or to keep moving at a constant velocity. Remember, these are the two natural states of motion for an object (according to both Galileo and Newton). Some objects have more tendency, while others have less. An object s MASS is a measure of its inertia. Which only makes sense, since massive objects are much harder to either move from rest or to budge from a state of constant velocity. Therefore, in summary: Forces are capable of causing accelerations and are measured in N or lb. Objects naturally want to stay at rest or in a state of constant velocity. The term constant velocity implies that F net = 0. and if F net = 0, then an object will stay at rest or keep a constant velocity. If F net = 0, then a = 0. Inertia can be quantitatively measured by measuring an object s mass.

5 Newton s First Law Worksheet 1. Fully and clearly state Newton s 1 st law 2. An object is at rest. What does the object have that tends to keep it at rest? 3. What would it take to force the object to move? Be specific, using Newton s first law. 4. An object is moving along at a constant velocity of 100 cm/s. What is the external force acting on the object? Explain using Newton s 1 st Law. 5. An object is sliding with a velocity of 10 m/s [S] along a perfectly frictionless surface. Instantly, a 45 N [N] force acts upon the object. Based on your knowledge of Newon s 1 st Law, what do you think will happen? Will the object remain in motion or will its motion change? If the motion changes, what kind of change occurs? 6. An object is sliding with a velocity of 4 m/s [NE] along a perfectly frictionless surface. Instantly, 3 forces act upon the object. The forces are 5N [W], 15N [E], and 10N [W]. Based on your knowledge of Newon s 1 st Law, what do you think will happen? Will the object remain in motion or will its motion change? If the motion changes, what kind of change occurs? 7. An object is at rest on a perfectly frictionless surface. Instantly, a 10 N [W] force acts upon the object. Based on your knowledge of Newon s 1 st Law, what do you think will happen? Will the object remain in motion or will its motion change? If the motion changes, what kind of change occurs? 8. According to Newton s 1 st Law, there are two natural states of motion that an object tends to be in. What are they? 9. Convert 10 N to lbs. Then, convert 85 lbs to Newtons. (recall that 1 N = lbs) 10. A car is driving along a straight road. A passenger is sitting in the front-passenger seat, which just so happens to be greased with Crisco. There are no doors on the car and no seatbelts (illegal? Maybe.). The car comes to a hard turn in the road and attempts to complete the turn without slowing down. Explain, using Newton s 1 st Law, what will happen to the passenger in the front seat. 11. In real-life, objects in motion don t tend to stay in motion. What is the unbalanced force that keeps most objects that are in motion from remaining in motion indefinitely? Give three different examples of this type of force. 12. The SI unit of Force is the Newton. Since this is not a base SI unit but rather a derived unit, we must always remember what base units make up the Newton. Write 1N in terms of only kg, sec, and m. (look it up on the internet if you need to google it J) 13. Give two different examples of contact forces. Then, give two different examples of field forces. 14. Find the net force in each situation below. Make sure to give both a magnitude and a direction. a) 15 N [right] and 42 N [left] b) 65 N [West] and 50 N [North] c) 32 N [E 30 N] and 16 N [South]

6 Read Buffa pp a, taking notes in the space below. Buffa Book Problems to Understand: pg. 131 # s 1-10.