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Motion and Forces

Motion and Forces. What is a force?. A push or a pull. Applied Force changes the movement of the ball when you kick a soccer ball. B alanced force?. Net force: When two or more forces act on an object. Inertia?. The tendency of an object to resist any change in its motion.

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Motion and Forces

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  1. Motion and Forces

  2. What is a force? • A push or a pull. • Applied Force changes the movement of the ball when you kick a soccer ball.

  3. Balanced force? • Net force: • When two or more forces act on an object.

  4. Inertia? • The tendency of an object to resist any change in its motion. Newton’s Laws of Motion • First Law of Motion: • An object moving at a constant velocity keeps moving at that velocity unless an unbalanced net force act on it.

  5. Newton’s Second Law • the acceleration of an object is dependent upon two variables - the net force acting upon the object and the mass of the object. The acceleration of an object depends directly upon the net force acting upon the object, and inversely upon the mass of the object.

  6. Relationship of Variables • If the mass of the object stays constant, as the force acting upon an object is increased, the acceleration of the object is increased. 1 m/s2 • 2 m/s2 • 4 m/s2

  7. Relationship of Variables • If the Force on the object stays constant, as the mass of an object is increased, the acceleration of the object is decreased. • 6 m/s2 • 4 m/s2 • 2 m/s2

  8. Solving a 2nd Law Problem A tennis ball, 0.314 kg, is accelerated at a rate of 164 m/s2 when hit by a professional tennis player. What force does the player's tennis racket exert on the ball? G: U: E: S S m= 0.314 kg a= 164 m/s2 F = ? F = 0.314 kg x 164 m/s2 = 51.5 N

  9. What units do we use? • mass • L • acceleration • m/s • m • time • N

  10. Solving a 2nd Law Problem A 68 kg runner exerts a force of 59 N. What is the acceleration of the runner? G: U: E: S S m= 68 kg F= 59 N a = ?

  11. Changing the 2nd law formula A 68 kg runner exerts a force of 59 N. What is the acceleration of the runner? G: U: E: S S m= 68 kg F= 59 N a = ?

  12. Solving a 2nd Law Problem A 68 kg runner exerts a force of 59 N. What is the acceleration of the runner? G: U: E: S S m= 68 kg F= 59 N a = ? .87 m/s2

  13. Newton’s 3rd Law • For every action, there is an equal and opposite reaction. What forces is the bird experiencing? Gravity = pulling down at 9.8 m/s2 Force of the block = pushing in an equal and opposite reaction.

  14. Newton’s 3rd Law • If you sit on a chair, what forces are acting upon you? According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

  15. Use Newton’s 3rd law to discuss what is happening in this picture?

  16. Law of Momentum • Momentum can be defined as "mass in motion." • All objects have mass; so if an object is moving, then it has momentum - it has its mass in motion. • The amount of momentum that an object has is dependent upon two variables: mass and velocity.

  17. Law of Momentum Momentum = mass • velocity • In physics, the symbol for the quantity momentum is the lower case "p". Thus, the above equation can be rewritten as p = m • v

  18. Momentum Problems Determine the momentum of a ... • a. 60-kg halfback moving eastward at 9 m/s. G: ; U: E: S S v = 9 m/s m = 60-kg p = ? p = m • v p = 60 kg • 9 m/s = 540 kg • m/s

  19. Momentum Problems Determine the momentum of a ... • b. 1000-kg car moving northward at 20 m/s. G: ; U: E: S S v = 20 m/s m = 1000-kg p = ? p = m • v p = 1000 kg • 20 m/s = 20000 kg • m/s

  20. Momentum Problems • Which has greater momentum? • A. the halfback • B. the car

  21. Relationship of Variables • If the mass of the object stays constant, as the force acting upon an object is increased, the acceleration of the object is increased. p = m • v • 44 kg x m/s 22 m/s • 66 kg x m/s • 22 m/s • 132 kg x m/s • 22 m/s

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