Presentation on theme: "AP Physics B Summer Course 2012 2012 年 AP 物理 B 暑假班 M Sittig Ch 10: Free Body Diagrams and Equilibrium."— Presentation transcript:
AP Physics B Summer Course 2012 2012 年 AP 物理 B 暑假班 M Sittig Ch 10: Free Body Diagrams and Equilibrium
Force Free-body diagrams help visualize forces. What is a force? A force is an interaction between two objects (a push or pull). What is the net force? Net force is the vector sum of all the forces acting on an object.
Free-Body Diagram Practice A book is at rest on a tabletop. Diagram the forces acting on the book. A girl is suspended motionless from the ceiling by two ropes. Diagram the forces acting on the combination of girl and bar. An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the forces acting on the egg as it is falling. A flying squirrel is gliding (no wing flaps) from a tree to the ground at constant velocity. Consider air resistance. Diagram the forces acting on the squirrel. A rightward force is applied to a book in order to move it across a desk with a rightward acceleration. Consider frictional forces. Neglect air resistance. Diagram the forces acting on the book.
Free-Body Diagram Practice A college student rests a backpack upon his shoulder. The pack is suspended motionless by one strap from one shoulder. Diagram the vertical forces acting on the backpack. A skydiver is descending with a constant velocity. Consider air resistance. Diagram the forces acting upon the skydiver. A force is applied to the right to drag a sled across loosely packed snow with a rightward acceleration. Diagram the forces acting upon the sled. A football is moving upwards towards its peak after having been booted by the punter. Diagram the forces acting upon the football as it rises upward towards its peak. A car is coasting to the right and slowing down. Diagram the forces acting upon the car.
Free-Body Diagram How-To free-body-diagrams.swf free-body-diagrams.swf From http://www.wisc-online.com/Objects/ViewObject.aspx?ID=tp1502http://www.wisc-online.com/Objects/ViewObject.aspx?ID=tp1502
What is Equilibrium? When the net force on an object equals zero. An object is either at rest or moving with a constant velocity Constant velocity: speed, direction not changing.
Equilibrium Could this object be in equilibrium?
How to Solve Equilibrium Problems 1. Draw an appropriate free-body diagram. 2. Resolve vectors into x- and y-components, if necessary. 3. Write an expression for the vector sum of the left-right vectors. Then write an expression for the vector sum of the up- down vectors. Set each of these expressions equal to zero. 4. Solve the resulting equations.
Example Problem Two astronauts tug on opposite sides of a satellite. The first astronaut tugs to the left with a force of 30 N. With what force does the second astronaut tug in order to keep the satellite at rest?
Example Problem Two astronauts tug on opposite sides of a satellite. The first astronaut tugs to the left with a force of 30 N. With what force does the second astronaut tug in order to keep the satellite moving toward him a constant speed of 20 m/s?
Example Problem A man is pushing a box across the floor with a force of 15 N at an angle of 60° from the horizontal. If the box is moving at a constant velocity, what is the size of the friction force on the box?
What is a normal force? A contact force. A support force. Acts perpendicular to the surface on which an object rests. Not always equal to the weight of the object.
Friction A force that resists motion. Acts in parallel to two surfaces pressed together. Acts in the opposite direction to motion or intended motion. Static friction: when the object is at rest. Kinetic friction: when the object is moving.
Friction Force of Friction (N) Coefficient of Friction (unitless) Normal Force (N)
Force Find the force F that the man needs to push with to keep this box moving across the floor at a constant velocity. 60 ° F 12 kg μ = 0.45
Force Find the force F that the man needs to push with to keep this box moving across the floor at a constant velocity. θ F m μ
Inclined Planes Normal force is perpendicular to the plane. Smarter to choose axes parallel (∥) and perpendicular ( ⊥ ) to the plane. Draw a free-body diagram for the block.
Torque A force that causes an object to turn, not move in a straight line. Equal to the product of ① the distance from the fulcrum, and ② the component of the force perpendicular to that distance.