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Lecture 4 Force Newton’s law of motion

Lecture 4 Force Newton’s law of motion. 1.6 Force. What is a force? A force is an influence on a system or object which, acting alone, will cause the motion of the system or object to change. If a system or object at rest is subjected to a non-zero force it will start to move.

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Lecture 4 Force Newton’s law of motion

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  1. Lecture 4ForceNewton’s law of motion

  2. 1.6 Force What is a force? A force is an influence on a system or object which, acting alone, will cause the motion of the system or object to change. If a system or object at rest is subjected to a non-zero force it will start to move. We know that a force can be a push or a pull acting on an object.

  3. Force • …the agency of change. • …changes the velocity. • …is a vector quantity. • ...measured in Newton’s.

  4. There is a good chance that 2 forces can be acting on an object at any one time. • Examples: • Lifting something • Dragging something

  5. 1.7.1 Newton’s First Law • Law of Inertia • Newton’s 1st Law:“A body remains at rest or moves in a straight line at a constant speed unless acted upon by a force”.

  6. Objects tend to resist a change in motion. This is called:Inertia

  7. Inertia Example An astronaut in outer space will continue drifting in the same direction at the same speed indefinitely, until acted upon by an outside force.

  8. 1.7.2 Newton’s Second Law The Sum of theForcesacting on a body is proportional to the acceleration that the body experiences • F a • SF = (mass) a

  9. Net Force

  10. What is Net Force? F1 When more than one force acts on a body, the net force (resultant force) is the vector combination of all the forces, i.e., the “net effect.” F2 F3 Fnet

  11. 1.7.3 Newton’s Third Law • Action-Reaction • For every action force there is an equal and opposite reaction force

  12. 1.8 Using Newton’s Laws

  13. 1.8.1 Weight Weight is a force caused (on Earth) by the gravitational attraction of a mass to the Earth’s centre. The weight of a body, of mass m, is defined to be the force, W, with which it is attracted to the Earth. On Earth, W = mg, where g is the acceleration due to gravity (g ≈ 9.81 m s−2 on Earth). FW = W = m g 1.8.2 Air Resistance Acts in the opposite direction to the motion.

  14. 1.8.3

  15. 1.8.4 Tension Many mechanics problems involve objects being pulled, pushed or suspended from a string, spring, rod or something similar. The force that the string (or similar) exerts on the object in these types of problems is called tension. T

  16. 1.8.5 Friction • A force that prevents, or tries to prevent, the slipping or sliding of two surfaces in contact. • Kinetic Friction • Ff = mk FN • Static Friction • Ff ms FN • In most cases, mk< ms.

  17. Application of Friction Kinetic Friction Static Friction

  18. For more information you can see this website: http://ffden2.phys.uaf.edu/211_fall2002.web.dir/ben_townsend/staticandkineticfriction.htm

  19. 1.8.6 Normal reaction force A mass lies on a horizontal surface. The weight of the mass pulls it downwards. The reason it does not fall is because the horizontal surface exerts an equal and opposite force on the mass called the normal reaction force. The normal reaction force always acts perpendicularly to the surface that is causing it. FN = N

  20. Normal force Fnet 0

  21. Example 8:

  22. Horse-Cart dilemma

  23. Example 7: A hockey puck having a mass of 0.30 kg slides on the horizontal, frictionless surface of an ice rink. Two hockey sticks strike the puck simultaneously, exerting the forces on the puck shown in this Figure. The force F1 has a magnitude of 5.0 N, and the force F2 has a magnitude of 8.0 N. Determine both the magnitude and the direction of the puck’s acceleration.

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