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Forces & Newton 1

Forces & Newton 1. What Is a Force?. A Force is an interaction between two bodies . Convention: F a,b means “the force acting on a due to b ”. A Force is a push or a pull . A Force has magnitude & direction (vector) . Adding forces means adding vectors . Forces - 1151.

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Forces & Newton 1

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  1. Forces & Newton 1

  2. What Is a Force? • A Force is an interaction between two bodies. • Convention: Fa,bmeans “the force acting on a due to b”. • A Force is a push or a pull. • A Force has magnitude & direction (vector). • Adding forces means adding vectors.

  3. Forces - 1151 • Contact (fundamentally E+M) • Normal: Perpendicular to surface • Friction: Parallel to surface • Anything touching the object • Rope: Tension • Spring F = -kx • Person • Non-Contact or Action at a Distance (these are field forces) • Gravity Physics g

  4. Force & Mass • Force • Symbol: For F • Units: • SI – Newton (N) (~0.22 lb) • English – Pound (lb) • cgs – dyne (10-5 N) • Mass • Symbol: m or M • Units: • SI – kilogram (kg) • English – slug (~14.5 kg) • cgs – gram (g)

  5. Weight Compared to Mass • Weight is a Force • Depends on the gravitational field strength at the location • Mass is the amount of “stuff” • Mass is independent of location! (although it is often measured by comparing weights) • Mass is also the inertia of the object • Inertia is resistance to a change in motion – See Newton 1 On Earth W = (9.8N/kg) m

  6. Remember • Every force must have an agent that produces it; • Every contact force must act only at the point of contact; • The normal force acts only perpendicular to the surface in contact; • The friction force acts only parallel to the surface in contact; • The tension force from a string or rope acts only along the line of the string or rope.

  7. Tension • Spring scale reads the force exerted on each end. • Tension is force transmitted by rope and is the force exerted by each end of rope.

  8. Tension ACT A pair of tug-of-war teams are pulling on the ends of a rope, each team with a force of 1000 N. The tension in the rope is: • 2000 N • 500 N • 1000 N • 0 N • 2000 kg

  9. Tension ACT a. T1>T2 b. T1=T2 c. T1<T2 d. depends on pulley radius

  10. Free Body Diagrams

  11. Drawing a FBD • Identify all forces acting on the object. • Draw a coordinate system. Use the axes defined in your pictorial representation. If those axes are tilted, for motion along an incline, then the axes of the free-body diagram should be similarly tilted. • Represent the object as a dot at the origin of the coordinate axes. This is the particle model. • Draw vectors representing each of the identified forces. Be sure to label each force vector.

  12. Examples of Force Vectors Pull (contact force) Push (contact force) Gravity (long-range force)

  13. Identifying Forces • Identify “the system” and “the environment.” The system is the object whose motion you wish to study; the environment is everything else. • Draw a picture of the situation. Show the object—the system—and everything in the environment that touches the system. Ropes, springs, and surfaces are all parts of the environment. • Draw a closed curve around the system. Only the object is inside the curve; everything else is outside. • Locate every point on the boundary of this curve where the environment touches the system. These are the points where the environment exerts contact forces on the object. • Name and label each contact force acting on the object. There is at least one force at each point of contact; there may be more than one. When necessary, use subscripts to distinguish forces of the same type. • Name and label each long-range force acting on the object. For now, the only long-range force is weight.

  14. T w Forces on a Bungee Jumper y x

  15. n T w fk The Forces on a Skier y x

  16. D w The Forces on a Rocket Fthrust y x

  17. Inertial Reference Frame • The following statements can be thought of as the definition of inertial reference frames. • An IRF is a reference frame that is not accelerating (or rotating) with respect to the “fixed stars”. • If one IRF exists, infinitely many exist since they are related by any arbitrary constant velocity vector!

  18. Newton 1 • Newton’s First Law • An object subject to no external forces is at rest or moves with a constant velocity if viewed from aninertial reference frame. • If no net forces act, there is no acceleration.

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