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Chapter 4

Chapter 4. Forces in One Dimension. 4.1 Force and Motion. Force – A push or a pull exerted on an object. May cause a change in velocity: Speed up Slow down Any change in velocity is acceleration Therefore a force causes acceleration Forces are vector quantities Direction Magnitude.

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Chapter 4

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  1. Chapter 4 Forces in One Dimension

  2. 4.1 Force and Motion • Force – A push or a pull exerted on an object. May cause a change in velocity: • Speed up • Slow down • Any change in velocity is acceleration • Therefore a force causes acceleration • Forces are vector quantities • Direction • Magnitude

  3. Contact Forces • Caused when an object in the external world (an “agent”) touches a system thereby exerting a force upon it • System is the object of interest: • A puck hit by a hockey stick • A book pushed off the table by a hand • Without an agent and a system, a force cannot exist

  4. Free-body Diagrams • See page 89 • Sketch situation • Label agent and system • Show forces as arrows pointing in the direction that the force is applied • Indicate which way is positive (usually up!)

  5. Force and Acceleration • The greater the force, the greater the resulting acceleration • They are directly proportional • Newton’s Second Law states that • a = F/m • F = ma • F is measured in newtons (N) • 1 N = 1kg.m/s2

  6. Net Force (Fnet) • Vector sum of all forces • See Figure 4-5 page 92

  7. Newton’s First Law • An object that is at rest will remain at rest and an object that is moving will continue to move in a straight line with constant speed, if and only if the net force acting on the object is zero • Also known as inertia (the tendency of an object to resist any change in motion)

  8. Equilibrium • If no net forces are acting on an object is said to be at equilibrium

  9. 4.2 Using Newton’s Second Law • Fg = mg • Fg (Force of gravity) is weight • Fg is always (-) • Weight is always (-) • Weight changes with location because gravity differs…that’s why we weigh less on the moon • Mass does not change with location because it doesn’t depend on gravity

  10. Apparent Weight • Equal to the weight of the object plus the net force acting on it • See Problem 2 on page 99 • Weightlessness is a factor of apparent weight. This does not mean that the object has no weight, rather there are no upward contact forces acting on it. • Fscale = mg + ma (going up) • Fscale = mg – ma (going down)

  11. Drag Force • Drag force is experienced by any object moving through a fluid • Gas (like air) • Liquid (like water) • The magnitude of the drag force increases with the speed of the object moving through the fluid

  12. Terminal Velocity • Object is dropped and accelerates as gravity acts upon it • As the velocity increases, so does the drag force of the air acting against gravity • When gravity pulling down equals drag force pushing up the object stops accelerating • This constant and final velocity is called “terminal velocity”

  13. 4-3 Interaction Forces • Newton’s Third Law • All forces come in pairs • FAonB = -FBonA • Interaction pair • Also called action-reaction pair • Two forces that are opposite in direction but equal in magnitude

  14. Example: Action-Reaction Pair

  15. Forces of Ropes & Strings • Tension – a force exerted by a string or rope • Is equal to the weight of the object hanging on the rope/string • Fnet = Sum of the positive force of a rope pulling up and the negative force of gravity pulling down • Fnet = FT + (-Fg)

  16. Normal Force • Perpendicular contact force exerted by one surface on another • May be equal to the weight of an object or… • Less than the weight if an additional upward force is exerted • More than the weight if an additional downward force is exerted • See Figure 4-16 on page 107

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