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Lesson 3-1

What is force?. A force is a push or a pull on an object. Force has both size and direction. You can use arrows to show the size and direction of a force. The unit for force is the newton (N). Lesson 3-1. What is force? (cont.).

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Lesson 3-1

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  1. What is force? • A force is a push or a pull on an object. • Force has both size and direction. • You can use arrows to show the size and direction of a force. • The unit for force is the newton (N). Lesson 3-1

  2. What is force?(cont.) • A contact force is a push or a pull on one object by another object that is touching it. • A force that one object can apply to another object without touching it is a noncontact force. Lesson 3-1

  3. Gravity—A Noncontact Force • Gravity is an attractive force that exists between all objects that have mass. • The size of a gravitational force depends on the masses of the objects and the distance between them. Lesson 3-2

  4. Gravity—A Noncontact Force(cont.) If the mass of an object increases, the gravitational force increases between it and another object. Lesson 3-2

  5. Gravity—A Noncontact Force(cont.) As two objects move apart, the gravitational force between them decreases. Lesson 3-2

  6. Gravity—A Noncontact Force(cont.) • Weight is a measure of the gravitational force acting on an object’s mass. • The weights of objects on the Moon are smaller than objects on Earth because the mass of the Moon is smaller. Lesson 3-2

  7. Friction—A Contact Force Friction is a contact force that resists the sliding motion of two surfaces that are touching. Lesson 3-3

  8. Friction—A Contact Force(cont.) • The force of friction acts in the opposite direction of an object’s motion. • The heavier an object, the more it is affected by friction than a lighter one. • Air resistance is the frictional force between air and objects moving through it. Lesson 3-3

  9. Combining Forces • When more than one force acts on an object, the forces combine and act as one force. • The sum of all the forces acting on an object is called the net force. • When two forces act on the same object in opposite directions, you must include the direction of the forces when you add them to calculate net force. Lesson 3-4

  10. Combining Forces(cont.) • If the net force on an object is 0 N, the forces acting on the object are called balanced forces. • When the net force on an object is not 0 N, the forces acting on the object are unbalanced. • Page 308 in your books Lesson 3-4

  11. Unbalanced Forces and Acceleration • When unbalanced forces act on an object, the object’s velocity changes. • Unbalanced forces can change either the speed or the direction of motion. Lesson 3-5

  12. Balanced Forces and Constant Motion • When balanced forces act on an object, the motion is constant. • The object is either at rest or moving at a constant velocity. Lesson 3-6

  13. Forces and Newton’s Laws of Motion • Issac Newton developed 3 important rules about motion called Newton’s Laws of Motion • According to Newton’s first law of motion, if the net force acting on an object is zero, the motion of the object does not change. • Sometimes called the law of inertia. • Inertia is the tendency of an object to resist a change in its motion. Lesson 3-6

  14. Forces and Newton’s Laws of Motion (cont.) According to Newton’s second law of motion, the acceleration of an object is equal to the net force exerted on the object divided by the object’s mass. Lesson 3-6

  15. Forces and Newton’s Laws of Motion (cont.) The greater the mass, the greater the force needed to accelerate the object at the same rate. Lesson 3-6

  16. Forces and Newton’s Laws of Motion (cont.) • Newton’s third law of motion says that for every action there is an equal and opposite reaction. • When one object exerts a force on a second object, the second object exerts a force of the same size but in the opposite direction on the first object. • Equal and opposite forces are called force pairs. Lesson 3-6

  17. Which is a force that one object can apply to another object without touching it? A.air resistance B. contact force C. friction D. noncontact force Lesson 3 – LR1

  18. Which refers to the sum of all the forces acting on an object? A. unbalanced forces B. net force C. friction D.force pairs Lesson 3 – LR2

  19. Which states that the acceleration of an object is equal to the net force exerted on the object divided by the object’s mass? A.Newton’s first law of motion B.Newton’s second law of motion C.Newton’s third law of motion D.law of inertia Lesson 3 – LR3

  20. Which changes when an unbalanced force acts on an object? A. mass B. motion C. inertia D. weight Chapter Review – MC3

  21. Which is a contact force that resists the sliding motion of two surfaces that are touching? A. air resistance B. displacement C. friction D. gravity Chapter Review – MC5

  22. An object is accelerated by a net force in which direction? A. at an angle to the force B. in the direction of the force C. in the direction of the opposite force D. in an upward direction Chapter Review – STP1

  23. What happens when a ball rolls uphill? A. Its speed increases. B. Its acceleration is zero. C. Its motion and acceleration are in the same direction. D. Its motion and acceleration are in opposite directions. Chapter Review – STP4

  24. Which description of gravitational force is NOT true? A. It depends on the mass of the objects. B. It is a repulsive force. C. It depends on the distance between objects. D. It exists between all objects. Chapter Review – STP5

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