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Forces

Forces. In order to change the movement of an object, that is to say, to change its velocity, an acceleration must be present. We know that acceleration is the rate of change in velocity, but where do accelerations come from? Accelerations are caused by forces. Fundamental Forces.

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Forces

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  1. Forces • In order to change the movement of an object, that is to say, to change its velocity, an acceleration must be present. • We know that acceleration is the rate of change in velocity, but where do accelerations come from? • Accelerations are caused by forces.

  2. Fundamental Forces 1) Gravitational Force: Interaction between masses. Any object with mass is attracted to other masses by the gravitational force. 2) Weak Interaction: A sub-atomic force that occurs during nuclear beta decay. 3) Electromagnetic Force: A force between charged objects or particles. Any object with a charge is attracted or repelled by other charges depending on the sign of the charge. 4) Strong (Nuclear force): An attractive force that holds nucleons (protons/neutrons) together.

  3. Fundamental Forces

  4. Newton’s 1st Law An object at rest will remain at rest unless acted upon by an outside force. Corollary to the 1st law: An object in motion will remain in motion unless acted upon by a force. Inertia: The tendency of an object to maintain a constant velocity.

  5. Examples of the 1st Law in action • Something falls directly in front of your car. You try to stop, but you can’t. • Whiplash • Tablecloth/Dishes • Skydiving: an airplane is going 150 mph N. When a skydiver jumps out of the plane, he keeps moving N at 150 mph even as he plummets out of the sky.

  6. Newton’s 2nd Law • Force is proportional to an object’s mass and the net acceleration acting on that object. • F=ma • The “sigma” sign is important! Usually multiple forces act together to create the motion that we experience. • Force is a vector, due to its relationship with the acceleration vector. • Newton’s 2nd Law as a verbal statement: Forces cause accelerations which cause changes in motion.

  7. Newton’s 3rd Law: • For every force, there is an equal and opposite force. • e.g., If you push on the wall, the wall pushes back on you. • Reaction pair: A person and the wall.

  8. Weight: The gravitational force • Weight is the force of gravity acting on a mass. Most Correct O.K. Be careful…later we will have another variable, work, that uses the symbol W. Remember: Weight is a force!!!

  9. Units The Newton (N) is the metric standard unit of force.

  10. Free-Body Diagrams Using a dot to represent the center of mass, draw vectors to represent the forces acting on the object. Free body diagrams provide a visual aid to help when applying Newton’s Second Law. A hanging body A free falling body

  11. Two men (man1 & man2) pulling a laden sled. Friction acts against the motion. Looking down from above. Note that each force has horizontal (x) and vertical (y) components of motion. Together these pieces define the overall motion of the object.

  12. Net force • In it’s most rigorous form Newton’s second law applies not only to a single force on an object, but rather to the sum of all forces (the net force) on an object.

  13. Equilibrium • Equilibrium occurs when the net force on an object is zero. • This implies that the object is not accelerating in any direction. • Case 1: The object is at rest. • Case 2: The object is traveling with a constant velocity.

  14. Friction • Friction is a resistive force that always opposes motion. This means that the vector of the frictional force always points 1800 from the direction of the velocity of the object. Normal force: Describes how the objects are in contact. Coefficient of friction: Describes the roughness of the surfaces

  15. The Normal Force • For an object at rest on a horizontal surface (e.g., a car sitting on a flat road), the Normal force is equal and opposite the gravitational force.

  16. A dogsled being drawn forward (right) with constant acceleration. Note that while the Normal force and the weight are equal and opposite, the applied force of the dogs is greater than the force of friction. In what direction is the net acceleration of the sled?

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