Bell Work: Tuesday 12/8/09 A cowboy drops his gun while riding his horse at a fast gallop. In the absence of air resistance, where will the gun hit the ground relative to the horse? Why? a. In front of the horse b. Behind the horse c. Next to the horse • (c) next to the horse • The gun will be pulled down due to the force of gravity, but will also continue to move straight forward with the same velocity as the horse (inertia- 1st law of motion). There is no air to push it any other way.
Quiz Study Guide: • Study the following words: • Instantaneous speed • Velocity • Reference point • Average speed • Potential energy • kinetic energy • Speed • Constant speed • Motion • What is the difference between speed and velocity? • What are Newton’s Laws: • 1st • 2nd • 3rd • What is the difference between positive, negative and regular acceleration?
Name our theme park… • 3rd Willie Wonka • 4th TV Shows • 5th Movie Theme • Wonka Wonderland
Force (Free-Body) Diagrams • Free-body diagrams- used to show the size (magnitude) and direction of all forces acting upon an object in a given situation • The object is normally drawn as a square, with the forces all pointing outwards from the center. • Shown is an example of a free body diagram. Notice the arrows are drawn and labeled to indicate the force they represent. • What do they mean? • F frict – Force of friction • F grav – Force of gravity • F app – Applied force • F norm – Normal force
Ffrict Friction Force • The force exerted by a surface as an object moves across it or makes an effort to move across it; two types: kinetic (sliding, rolling, fluid) and static friction • Example: a book slides across a desk, then the desk exerts a friction force in the opposite direction of its motion. • Fgrav Gravity Force • Directed "downward" towards the center of the earth • Fapp Applied Force • Force which is applied to an object by a person or another object • Example: If a person is pushing a desk across the room, then there is an applied force acting upon the object. Applied force = force exerted by the person. • Fnorm Normal Force • “Support force” - exerted upon an object which is in contact with a stable object • Example: if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book. • FnetNet force • Sum of all the forces acting on an object; in the direction the object moves • When net force = 0, there is no movement/no change in motion • Fair Air Resistance Force • Fluid Friction- This force will frequently be neglected due to its small size; It is most noticeable for objects which travel at high speeds
Find the net force by finding the sum of the forces acting on the object. 400 N 200 N 20 N 0 N 5 N 0 N 15 N
Calculate the missing forces that yield the expected net force. • A = 50 N(the horizontal forces must be balanced) • B = 200 N(the vertical forces must be balanced) • C = 1100 N (in order to have a net force of 900 N, up) • D = 20 N (in order to have a net force of 60 N, left) • E = 300 N (the vertical forces must be balanced) • F = H = any number you wish (as long as F equals H) • G = 50 N (in order to have a net force of 30 N, right)
Free Body Diagrams • A vase is at rest on a table top. Diagram the forces (2) acting on the vase. • An egg is free-falling from a nest in a tree. Neglect air resistance. Diagram the force (1) acting on the egg as it is falling.
A flying squirrel is gliding from a tree to the ground at constant velocity. Consider air resistance. Diagram the forces (2) acting on the squirrel. • A car is coasting to the right and slowing down. Neglect air resistance Diagram the forces (3) acting upon the car.
A skydiver is descending with a constant velocity. Consider air resistance. Diagram the forces (2) 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 (4) acting upon the sled.
A rightward force is applied to a book in order to move it across a desk at constant velocity. Consider frictional forces. Neglect air resistance. Diagram the forces (4) acting on the book. • 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 (4) acting on the book.
A lacrosse ball is moving upwards towards its peak after having been cleared by the goalie. Neglect air resistance Diagram the force (1) acting upon the lacrosse ball as it rises upward towards its peak.
Momentum = mass * velocityUnits = mass (kg) * velocity (m/s) = kg*m/s • All objects in motion have momentum • The more momentum an object has, the harder it is to stop • Like velocity and acceleration, described by a direction also
Can a small mass produce a large momentum? • Of course! • If so, can you think of an example? • A bullet shot from a gun • What makes it have such a large momentum? • It has a small mass, but it has a very large velocity
Law of Conservation of Momentum • The total momentum of objects in an interaction does not change, unless outside forces act on the object • The quantity of the momentum is the same before and after objects interact
The force of gravity is at all times directed downwards and the normal force is at all times directed perpendicular to the seat of the car.
Centripetal Force Back To Forces and Motion Centripetal Force = The push or pull on a moving object toward the center of its curved path. Centripetal force is always a net force. • Centripetal Force- “center seeking”; push or pull on a moving object directed towards the center of its curved path. • Centrifugal Force- “center fleeing”; push or pull on moving object directed outwards from the center of the curved path. • Newton’s 3rd Law- Centripetal and Centrifugal forces are a action-reaction pair of forces. They are equal in magnitude, but opposite in direction. DRAW THIS DIAGRAM Draw and label arrows While the coaster cars zoom around the loop, the track exerts a centripetal force toward the loop’s center and centrifugal outwards. Back To Forces and Motion