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Chapter 2: Lesson 3

Chapter 2: Lesson 3. Newton’s 2 nd Law of Motion. The acceleration of an object is equal to the net force exerted on the object divided by the object’s mass . In other words – Acceleration of an object depends on the force acting on the object and the mass of the object Force =

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Chapter 2: Lesson 3

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  1. Chapter 2: Lesson 3

  2. Newton’s 2nd Law of Motion The acceleration of an object is equal to the net force exerted on the object divided by the object’s mass. In other words – Acceleration of an object depends on the force acting on the object and the mass of the object Force = Acceleration = The acceleration due to gravity on earth = Mass x Acceleration Force Mass 9.8 m/s2

  3. Three Forms of Newton’s 2nd Law

  4. Units for solving for Newton’s 2nd Law • Identify the information you are given • Look at the units • Unit for Force – • Remember: is a force due to • If the problem asks you to solve for weight you need to identify the acceleration due to gravity for your given location. • Unit for Acceleration – • Unit for Mass – • Use the appropriate formula Newton (N) Weight gravity m/s2 Kg or g

  5. Practice Problem • A man has a mass of 66kg on Earth. What is his weight?

  6. Practice Problem • Johnny hits the baseball with 100N of force. The baseball has a mass of 14.2kg. Identify the acceleration of the baseball.

  7. Practice Problem • A girl on roller skates with a mass of 55kg accelerates at a rate of 2m/s2. What is her force?

  8. Practice Problem • Richie went fishing with his dad. He felt a bite on his line and started reeling the fish in with a force of 201N. The fish had a mass of 9 kg. What was the rate of acceleration at which he was reeling in the fish?

  9. An object at REST • Balanced Force The force exerted by the is to • Unbalanced Force The force exerted by gravity is than air resistance. HAND GREATER EQUAL THE FORCE OF GRAVITY

  10. An object in MOTION • Unbalanced forces cause objects to ACCELERATE Increase Speed Decrease Speed Change Direction

  11. Part I: Acceleration Depends on Mass decreases increases Acceleration as its mass increases Acceleration as mass decreases Acceleration and mass are inversely related Example: You are pushing a shopping cart at the grocery store. At the beginning of your shopping trip, you exert a small force on the cart to accelerate it. (smaller mass = greater acceleration) Exert the same amount of force when the cart is full and the cart will not accelerate as much. (greater mass = smaller acceleration) http://www2.hawaii.edu/~kobatake/secondlaw4.html

  12. Part II: Acceleration Depends on Force Acceleration as the force on it increases increases Acceleration as the force on it decreases decreases directly related Acceleration and force are Example: When pushing the full shopping cart, if you push harder (greater force), the cart will move faster. If you push the full shopping cart with less force, the cart will move slower. **The acceleration is always in the as the force applied. The shopping cart moved forward because the push was in a forward direction same direction

  13. We know that objects with different masses accelerate to the ground • However, because of the 2nd Law we know that they don’t hit the ground with the same • at the same rate. • force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s

  14. Newton’s second law explains why objects fall to Earth with the same acceleration (9.8 m/s) Less mass Less Gravitational force Less inertia = easier to move More mass More Gravitational force More inertia = harder to move

  15. Circular Motion • Any motion in which an object is moving along a curved path. • For example: A rider on a merry-go-round moves in a circle. This type of motion is called • If you are in circular motion, your of motion is constantly • This means you are constantly Circular motion direction changing accelerating

  16. Centripetal Force • A force that causes an object to move in a circular path • If you are constantly accelerating there must be a force acting on you • The force exerted is the and always points to the center of the circle. at all times centripetal force

  17. CIRCULAR MOTION CENTRIPETAL FORCE CENTER • All requires a • Because the force acts toward the of the circular path, the acceleration must also be toward the CENTER

  18. Who is Johannes Kepler? • Johannes Kepler came before Newton’s time. Between the years (1571 and 1630) he developed a quantitative description of the motions of the planets in our solar system • We classify these descriptions as the laws of planetary motion

  19. The Laws of Planetary Motion(also known as Kepler’s Laws) • 1. The orbit of a planet about the Sun is an ellipse with the Sun at one focus. • 2. A line joining a planet and the Sun sweeps out equal areas in equal intervals. • 3. The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit. http://www.windows2universe.org/the_universe/uts/kepler1.html

  20. Kepler’s 1st Law • A circle has the same diameter whether you measure it across or up and down. But an ellipse has diameters of different lengths. The longest is called the major axis, the shortest is called the minor axis.

  21. Kepler’s 2nd Law • The line connecting the planet and the sun sweeps out equal area in equal time. It takes the same amount of time for the blue planet to go from A to B as it does to go from C to D. BUT the distance from C to D is much larger than from A to B. It has to be so that the green regions have the SAME area. The planet moves faster between C and D than it is between A and B. They move faster when they are near the Sun.

  22. Kepler’s 3rd Law • It means that if you know the period of a planet’s orbit (how long it takes that planet to move around the Sun) then you can determine the planet’s distance from the Sun.

  23. Wait a second… what does all of this have to do with Isaac Newton? • Kepler was able to describe the motion of the planets, however he didn’t provide an explanation as to WHY the planets move this way. • Isaac Newton came along in • He provided the general explanation of the motions of planets through and 1642 (died in 1727) Newton’s Laws of Motion The Universal Law of gravitation

  24. Still not understanding? • Though Kepler’s Laws “seemed” to work he had no theory to explain why they were true. • The theory came from nearly a century later. • Newton’s can be used to prove the Kepler’s Laws do indeed describe the motion of planetary objects. Newton’s Laws Universal Law of Gravitation

  25. For an example: • Since the planets move in (as seen in Kepler’s 1st Law) they are continually . This implies a force acting continuously on the planets (which have mass) • Which one of Newton’s Laws relates? ellipses accelerating Newton’s Second Law

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