Newton s First Law. ICP-Physics, Ave

Transcription

1 Newton s First Law ICP-Physics, Ave

2 Newton s First Law of Motion 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. AKA: Law of Inertia

3 Two Clauses and a Condition There are two clauses (parts) to this statement one that predicts the behavior of stationary objects and the other that predicts the behavior of moving objects. See summary below

4 Two Clauses and a Condition The condition is described by the phrase, unless acted upon by an unbalanced force. As long as the forces on an object are balanced, the First Law of Motion applies.

5 Real World Application of Newton s First Law

6 Inertia and Mass Newton s First Law states that 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. Objects tend to keep doing what they are doing. It is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion is described as inertia.

7 Galileo and the Concept of Inertia Galileo developed the concept of inertia in the 17 th century. Galileo reasoned that moving objects eventually stop because of a force call friction. In experiments using a pair of inclined planes facing each other, Galileo observed that a ball would roll down one plane and up the opposite plane to nearly but not quite, the same height. He postulated that if friction could be entirely eliminated, then the ball would reach exactly the same height. Newton built on Galileo s thoughts about motion.

8 Galileo and the Concept of Inertia

9 Mass as a Measure of the Amount of Inertia The tendency of an object to resist changes in its state of motion varies with mass. Mass is that quantity that is SOLELY dependent upon the inertia of an object. The more inertia an object has, the more mass that is has. A more massive object has a greater tendency to resist changes in its state of motion.

10 Mass as a Measure of the Amount of Inertia

11 Check Your Understanding Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose that you visit that place (just suppose) and throw a rock. The rock will a) Gradually stop. b) Continue in motion in the same direction at constant speed.

12 Check Your Understanding Imagine a place in the cosmos far from all gravitational and frictional influences. Suppose that you visit that place (just suppose) and throw a rock. The rock will a) Gradually stop. b) Continue in motion in the same direction at constant speed.

13 Check Your Understanding A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction?

14 Check Your Understanding A 2-kg object is moving horizontally with a speed of 4 m/s. How much net force is required to keep the object moving at this speed and in this direction? 0 N, An object in motion will maintain its state of motion. The presence of an unbalanced force changes the velocity.

15 Check Your Understanding Mac and Tosh are arguing in the cafeteria. Mac says that if he flings the Jell-O with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with and why?

16 Check Your Understanding Mac and Tosh are arguing in the cafeteria. Mac says that if he flings the Jell- O with a greater speed it will have a greater inertia. Tosh argues that inertia does not depend upon speed, but rather upon mass. Who do you agree with and why? Tosh is correct. Inertia is that quantity which depends solely upon mass. The more mass, the more inertia.

17 Check Your Understanding Supposing you were in space in a weightless environment, would it require a force to set an object in motion? Absolutely YES!! Even in space, objects have mass. If they have mass, they have inertia. Newton s Laws rule EVERYWHERE!!

18 Check Your Understanding Fred spends most Sunday afternoons at rest on the sofa watching pro football games and consuming large quantities of food. What effect (if any) will this practice have on his inertia?

19 Check Your Understanding Fred spends most Sunday afternoons at rest on the sofa watching pro football games and consuming large quantities of food. What effect (if any) will this practice have on his inertia? Fred s inertia will increase. Fred will increase his mass if he makes this a habit. And, if his mass increases, then his inertia increases.

20 Check Your Understanding Two bricks are resting on the edge of the lab table. Shirley Sheshort stands on her toes and spots the two bricks. She acquires an intense desire to know which of the two bricks is most massive. Since Shirley is vertically challenged, she is unable to reach high enough to lift the bricks; she can however reach high enough to give the bricks a little push. Discuss how the process of pushing the bricks will allow Shirley to determine which of the two bricks is most massive. What difference will Shirley observe and how can this observation lead to the necessary conclusions?

21 Check Your Understanding Two bricks are resting on the edge of the lab table. Shirley Sheshort stands on her toes and spots the two bricks. She acquires an intense desire to know which of the two bricks is most massive. Since Shirley is vertically challenged, she is unable to reach high enough to lift the bricks; she can however reach high enough to give the bricks a little push. Discuss how the process of pushing the bricks will allow Shirley to determine which of the two bricks is most massive. What difference will Shirley observe and how can this observation lead to the necessary conclusions? The bricks, like any objects, possess inertia. That is, they resist changes in their state of motion. When Shirley pushes each of the bricks, the one that is heaviest will resist the most and vice versa.

22 State of Motion Inertia is the tendency of an object to resist changes in its state of motion. But, what is meant by state of motion? The state of motion of an object is defined by its velocity the speed and direction. So Inertia: Tendency of an object to resist changes in its velocity. An object at rest has zero velocity and, in the absence of an unbalanced force, will remain with zero velocity. An object in motion with a velocity of 2 m/s, East will (in the absence of an unbalanced force) remain in motion with a velocity of 2 m/s, East.

23 State of Motion Acceleration: the rate of change of velocity of an object EX: change in speed AND/OR direction of the object would result in a change in acceleration An object that does not change its velocity is said to have an acceleration of 0 m/s/s. So. Inertia: Tendency of an object to resist acceleration

24 An air track glider is shown moving across an air track. Air is blown through many small holes in the track in order to lift the glider off the track. The glider appears to move with constant speed/motion As they say, An object in motion.

25 Check Your Understanding A group of physics teachers is taking some time off for a little putt-putt golf. The 15 th hole at the Hole-In-One Putt-Putt Course has a large metal rim that putters must use to hide their ball toward the hole. Ms. Ave guides a golf ball around the metal rim. When the ball leaves the rim, which path (1,2, or 3) will the golf ball follow? Why?

26 Check Your Understanding A group of physics teachers is taking some time off for a little putt-putt golf. The 15 th hole at the Hole-In-One Putt-Putt Course has a large metal rim that putters must use to hide their ball toward the hole. Ms. Ave guides a golf ball around the metal rim. When the ball leaves the rim, which path (1, 2, or 3) will the golf ball follow? Why? Path 2. Once leaving the rim, the ball will follow an inertial path (straight line). The ball is moving to the right at the instant shown in the diagram. Once leaving the rim, there is no more unbalanced forces to change its state of motion.

27 Check Your Understanding A 4.0 kg object is moving across a friction-free surface with a constant velocity of 2 m/s. Which one of the following horizontal forces is necessary to maintain this state of motion? Why? a) 0 N b) 0.5 N c) 2.0 N d) 8.0 N e) Depends on the speed

28 Check Your Understanding A 4.0 kg object is moving across a friction-free surface with a constant velocity of 2 m/s. Which one of the following horizontal forces is necessary to maintain this state of motion? Why? O N. If an object is in motion, then it will stay in motion with those very same motion characteristics.

29 Balanced and Unbalanced Forces What is a balanced force? <-- This is a free body diagram There are two forces acting upon the book. One force gravity exerts a downward force. The other force push of the table on the book (AKA: normal force) pushes upward on the book. The two forces are of equal magnitude and in opposite direction they balance each other. The book is said to be in equilibrium (no unbalanced forces acting upon the object, so it maintains its state of motion.

30 Balanced and Unbalanced Forces What is an unbalanced force? The book in the visual above has been pushed to the left. It has balanced forces of gravity and normal force acting upon it. However, in addition, the book has a frictional force acting to the left that is unbalanced by a force to the right. As a result, the book changes it state of motion the book is not at equilibrium and is accelerating.

31 Check Your Understanding Which of the following dot diagrams best describes the motion of a falling box from the time that it is dropped to the time that it hits the bottom of a pool full of water? The arrows on the diagram represent the point at which the box hits the water. Support your answer with sound reasoning.

32 Check Your Understanding Tape A. The box first accelerates with a negative acceleration until it hits the water. Upon hitting the water, the box experiences a balance of forces (50 N down due to gravity and 50 N up due to water) Thus, the box will finish its motion moving with constant velocity even if that velocity is 0.

33 Check Your Understanding Several friends were watching the motion of the falling box. Being physics types, they began discussing the motion and made the following comments. Indicate whether each of the comments is correct or incorrect and support your answer. Comment 1: One the box hits the water, the forces are balanced and the box will stop. Comment 2: Upon hitting the water, the box will accelerate upwards because the water applies an upward force. Comment 3: Upon hitting the water, the box will bounce upwards due to the upward force.

34 Check Your Understanding Comment 1: One the box hits the water, the forces are balanced and the box will stop. Incorrect. Once the box hits the water, the forces are balanced which stops acceleration. However, the box does not stop moving.

35 Check Your Understanding Comment 2: Upon hitting the water, the box will accelerate upwards because the water applies an upward force and Comment 3: Upon hitting the water, the box will bounce upwards due to the upward force. Incorrect. Once the box hits the water, the forces are balanced. The box will continue in motion at a constant speed (0).

36 Check Your Understanding If the forces acting upon an object are balanced, then the object a. Must not be moving b. Must be moving with constant velocity c. Must not be accelerating d. None of these

37 Check Your Understanding If the forces acting upon an object are balanced, then the object a. Must not be moving b. Must be moving with constant velocity c. Must not be accelerating d. None of these

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