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Force and Motion

Force and Motion. A force is something that is capable of changing an object’s state of motion, that is, changing its velocity. Warm-Up.

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Force and Motion

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  1. Force and Motion A force is something that is capable of changing an object’s state of motion, that is, changing its velocity

  2. Warm-Up • A ski jumper competing for an Olympic gold medal wants to jump a horizontal distance of 135 meters. The takeoff point of the ski jump is at a height of 25 meters. With what horizontal speed must he leave the jump? • Challenge! A marshmallow is dropped from a 5-meter high pedestrian bridge and 0.83 seconds later, it lands right on the head of an unsuspecting person walking underneath. How tall is the person with the marshmallow on his head?

  3. Force and Net Force • Force ( ) is the cause of acceleration, or change in velocity • Force is a vector quantity • There must be a net, unbalanced force acting on an object for the object to change its velocity 10 N 30 N Net Force = 20 N

  4. This figure illustrates what happens in the presence of zero and nonzero net force.

  5. Net Force • Net Force, is the vector sum, the resultant, or the unbalanced force acting on an object. The symbol, F, means the vector sum of forces. • The unit of force in SI is a derived unit called newton, N. 1 N = 1 kg*m/s2 • 1 N is the net force required to accelerate a mass of 1 kg with an acceleration of 1 m/s2

  6. Free-Body Diagram • To analyze the force acting on an object, you should draw a free-body diagram. • Isolate the object of interest (this should be the only object in the diagram) • Draw all the forces acting on the object as vectors, including directions

  7. Example 1: Draw a free body diagram of a book sitting on a horizontal desk. • First isolate the book and analyze the forces acting on the book. There are two forces acting on the book, the gravitational force, or weight, and the supporting force on the book by the desk. • First, the gravitational force is always present if we are dealing with objects on the Earth. • Second, whenever an object makes a physical contact with another object, a force results. Here the book makes a contact with the desk, so there is a supporting force. In most cases, these contact forces are perpendicular to the contact surface and therefore are called normal forces, (N). N w

  8. Free Body Diagrams Example 2 A mass at the origin is pulled by four perpendicular ropes • Find the magnitude of the resultant force. • Find the angle at which the force is acting. Now each of the ropes is pulled at 30o counterclockwise from the given axes. 3. Draw a free-body diagram and find the horizontal and vertical components of the new resultant force. 4. At what angle is the resultant force acting with respect to the x-axis?

  9. Forces • We distinguish two types of forces: • A contact force, such as a push or pull, friction, tension from a rope or string, and so on. • What are the 5 types of contact forces? • A force that acts at a distance, such as gravity, the magnetic force, or the electric force.

  10. The Sum of Forces • A net force is not required to keep an object at rest. If the sum of all the forces is zero, the object is considered “static” (not moving) • A net force is not required to keep an object in motion. Where there is no friction, objects will continue moving with constant velocity. Therefore, THE NET FORCE ACTING ON AN OBJECT WITH CONSTANT VELOCITY IS ZERO.

  11. Adding Vectors – The Parallelogram Method • Objects in Equilibrium • Nellie Newton Sketches (p.1) • Objects in Motion • Vectors and the Parallelogram Rule (p. 23)

  12. What is the normal force for a 500 kg object resting on a horizontal surface if a massless rope with a tension of 150 N is acting at a 45oangle to the normal force? The mass of the object is too large to be moved by the vertical component of the rope’s force, so it is not in motion. Fy = 0, where Fy = FN – mg + Fy Fy = FN – (500kg)(9.8 m/s2) + (150N)(sin45o)

  13. Free-body diagrams for four situations are shown below. The net force is known for each situation. However, the magnitudes of a few of the individual forces are not known. Analyze each situation individually and determine the magnitude of the unknown forces. BONUS: What is the 5th type of contact force?

  14. Newton’s First Law • An object at rest will remain at rest and an object in motion will keep moving with constant velocity if the net force on the object is zero. • Often called the law of inertia. Inertia is the natural tendency of an object to resist a change in motion, measured quantitatively by its mass. • What happens if the net force is not zero?

  15. Newton’s Second Law • Acceleration depends on the net force and on the mass of the object • Newton’s law is a vector equation. • Acceleration is in the direction of the net force, not necessarily the direction of the velocity. • Include all forces acting on an object to determine the net force. • Do not include balanced forces (such as weight and normal force) because they cancel each other

  16. Newton’s Second Law of Motion The units of force are called newtons. 1 Newton = 1N = 1 kg . m/s2.

  17. Example 4.2 A 5.0 N horizontal force pulls a 20 kg box on a horizontal surface. A 3.0N friction force slows the motion. What is the acceleration of the object? 1. draw a free-body diagram of the box 2. write the given information 3. Apply the equation F = ma to solve the problem

  18. Example 4.3 • Find the magnitude of the weight of a 3.50 kg object on the surface of the earth where w = mg

  19. Newton’s Second Law of Motion Newton’s second law applies separately to each component of the force.

  20. Newton’s Third Law • If object 1 exerts force on object 2, then object 2 exerts an equal and opposite force on object 1. • The two forces are acting on two different objects, therefore even though the two forces are equal and opposite, they do not necessarily cancel each other. • Often called the law of action and reaction.

  21. 4.4 Newton’s Third Law of Motion For every force (action), there is an equal and opposite force (reaction). Note that the action and reaction forces act on different objects. This image shows how a block exerts a downward force on a table; the table exerts an equal and opposite force on the block, called the normal force N.

  22. Example 4.4 • A large truck collides head-on with a small car and causes a lot of damage to the small car. Explain why there is more damage to the small car than to the large truck.

  23. Homework • P. 131 -132; 2, 3, 5, 6, 13, 14, 15, 16, 19

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