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Forces (Part 2)

Forces (Part 2). Free Body Diagrams. A free body diagram is a diagram used to describe the forces acting on an object It only describes the forces acting on ONE single object, and not other objects connected to it Forces are represented by arrows.

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Forces (Part 2)

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  1. Forces (Part 2)

  2. Free Body Diagrams • A free body diagram is a diagram used to describe the forces acting on an object • It only describes the forces acting on ONE single object, and not other objects connected to it • Forces are represented by arrows. • Direction of arrow represents direction of force • Length of arrow represents magnitude of force • Each arrow must be labelled

  3. Free Body Diagrams • Objects are typically represented either using a silhouette of the object itself, or a simplification thereof. • E.g. free-body diagram of a cow in free fall COW Gravitational Force (Weight) Gravitational Force (Weight)

  4. Types of Component Forces • Forces can be classified into 2 broad categories: contact forces & long range forces. • Contact forces require an object to physically touch something else in order for the force to take effect • Non-contact do not require physical contact for the force to take effect

  5. Types of Component Forces

  6. Half-Time • Breaking The Magician’s Code - Levitation Tricks • 1) Levitating Girl 1 (https://www.youtube.com/watch?v=OAHsY_w4lcM) • 2) Levitating Girl 2(https://www.youtube.com/watch?v=F64L4CtGshw) • 3) Flying(https://www.youtube.com/watch?v=O0MOVgzSU_E) • 4) Walking on Water(https://www.youtube.com/watch?v=z0ukAescNC0)

  7. Gravitational Force • Weight is simply the gravitational force exerted by the Earth on an object • Formula: Gravitational Force = mg, • m = mass of the object (which the force is exerting on) • g = gravitational field strength. This is also identical to the acceleration due to free fall, which is 10 ms-2. • It is advisable to use “mg” instead of “W” or any other symbol to represent weight or gravitational force.

  8. Gravitational Force • To draw gravitational force of a free body diagram, locate the center of gravity of the object (typically the center of the object) • Draw an arrow downwards (towards the center of the Earth) • Label the arrow “mg” mg

  9. Normal Contact Force • Sometimes called “normal reaction force” • The normal contact force is the force objects exert on each other when they press each other • Normal contact force is always a push force • Usually represented by symbol “N” or “n” • “Normal” here refers to “perpendicular” • The direction of normal contact force is always perpendicular to the surface of contact

  10. Normal Contact Force • To draw normal contact force on a free body diagram, locate the surface of contact • Draw a force originating from the middle of the contact surface, going perpendicular and away from the surface • Label the arrow “N” or “n” N N Object on an inclined surface Object on the ground

  11. Tension • Tension is the force exerted by a taut string, rope, spring, etc. pulling on an object • Usually represented by symbol “T” • Tension is always a pull force • To draw tension in a free body diagram, locate the point where the string (for e.g.) is pulling the object • Draw an arrow originating from that point, in the same direction as the string • Label the arrow “T”

  12. Tension • Consider this scenario: • Ball suspended from the ceiling with a string • Free Body Diagram of the ball: T mg

  13. Tension • Consider this scenario: • Object being pulled along the ground by a string • Free Body Diagram: N T mg

  14. Friction • Friction is the contact force that opposes of tends to oppose motion between surfaces in contact • There is no symbol for friction. Either spell the word out in full, or use “Ffriction“ • To draw friction in a free body diagram, identify the contact surfaces, and draw an arrow parallel to the surfaces, in the direction which would oppose motion

  15. Friction moving to the right • If an object is moving in one direction, friction acts in the opposite direction • If friction is holding up an object which would otherwise slide away, friction acts in the direction opposing the sliding Friction Friction

  16. Rules for Drawing FBDs • Step 1: draw the object first. Do NOT draw other objects connected to it • Step 2: identify the center of gravity, draw and label the mg of the object • Step 3: draw all other component forces. The length of the arrows should be proportional to the magnitude of the force. • Note: NEVER draw the resultant force on a Free Body Diagram!! If there is a need to indicate the resultant force, draw it outside the FBD.

  17. Example • Draw the FBD of an object which is being pulled by a rope up a rough slope T N Friction mg

  18. Exercise 2 • An elevator weighs 1000 kg and is pulled upwards by a cable. • (a) Draw the free body diagram of the elevator • (b) If the elevator is accelerated upwards at 0.1 ms-2, calculate the tension in the cable.

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