2 Force and Motion 1. Define Force. FORCE - a push or a pull 2. Distinguish between balanced and unbalanced forces. When forces acting on a mass are unequal, the forces are unbalanced. Balanced forces are forces that are equal and opposite.3. Explain how forces affect motion. Forces can cause objects to (a) start moving (b) stop moving (c) change speed (d) change direction
3 Force and MotionUse the concepts of unbalanced and balanced forces to explain the flight of an airplane.Four forces act upon an airplane:Liftweightthrustdrag.LIFT - created by the shape of the wing; lifts the plane up into the air.WEIGHT - the force caused by gravity; pulls the plane down to the Earth.THRUST - the forward force of the plane created by jet engines or propellers; pulls or pushes the plane forward.DRAG - wind resistance that pulls back on the plane and slows it down.
4 Force and MotionAirplane motionliftdragthrustweight
5 Force and MotionA. Take off: a. Plane must travel faster and faster down the runway. Thrust is greater than drag (unbalanced pair of forces). b. Plane must rise from the ground. Lift is greater than weight. (unbalanced pair of forces)B. Flight at constant elevation and constant speed: a. thrust equals drag (balanced forces) b. lift equals weight (balanced forces)C. Landing: a. Plane descends onto runway. Lift decreases and weight becomes greater than lift. (unbalanced pair of forces) b. Plane travels slower and slower down the runway. Drag increases while thrust decreases. (unbalanced pairs of forces)
6 Force and Motion4. Explain inertia. INERTIA - the tendency of an object to resist any changes in its motion. An object at rest will remain at rest until an unbalanced force is used. An object in motion will remain in motion until an unbalanced force is used. The mass of the object is one factor that determines its inertia. more mass = more inertia Force must be applied to an object to overcome its inertia.Example: If pushing a stalled car from the street, it is much more difficult to get the car moving than it is to keep it moving. Inertia is working against you to keep the car at rest (object at rest will stay at rest). Once the car is rolling, inertia is working with you and you only have to push with enough force to overcome gravity and friction.
7 Force and Motion5. Demonstrate an understanding of Newton’s First Law of Motion. NEWTON’S FIRST LAW OF MOTION (LAW OF INERTIA) - An object at rest will remain at rest and an object in motion will remain in motion along a straight path unless acted on by an unbalanced force.EXAMPLES: 1. Seat belts - you are in a car going 55 mph; If the car suddenly stops (oak tree in grille) your body will continue to travel at 55 mph.2. Head restraints - if you are at rest, all of you is at rest including your body and head. If your auto is struck from behind your body will be thrown forward by the car seat. Your head will have a tendency to remain at rest back where it was. This causes the head to appear to be thrown back (whiplash) when actually it occurs because the body is thrown forward.3. Magician that can quickly pull the tablecloth from a table full of dishes and glassware. The dishes remain at rest.4. If ice is on the roadway, you do not make the curve but instead continue in a straight path (off the roadway)
8 Force and Motion6. Demonstrate an understanding of Newton’s Second Law of Motion. NEWTON’S SECOND LAW (LAW OF ACCELERATION) - Acceleration of a body increases as the amount of force producing the acceleration increases. The larger the mass of the body, the larger the force needed to produce acceleration. F = maAn athlete can throw a ball farther than his little sister because the force applied is greater. more force = more acceleration (constant mass)The same person can throw a baseball farther than a bowling ball because the same force is applied to the smaller mass. less mass = more acceleration (constant force)
9 Force and Motion7. Demonstrate an understanding of Newton’s Third Law of Motion. NEWTON’S THIRD LAW (LAW OF ACTION/REACTION). For every action, there is an equal and opposite reaction. Forces occur in pairs. To be a true example of the third law of motion, the following statement must be true: “ ____ acting upon _____” where the terms in the blanks are interchangeable.For example, if a ball is hanging on a string from the ceiling, to say that the third law is represented by the string pulling up on the ball and the earth (gravity) pulling down on the ball is incorrect. Both statements are true, but are not an example of the third law. To be true, the statements must be “earth pulling on ball, ball pulling on earth” and “string pulling on ball” and “ball pulling on string” etc.....
10 Force and Motion8. State the unit of force in the metric system of measurement. Newton - a Newton is the unit of measurement of force in the MKS system of measurement (N).9. Describe the relationship between mass, gravity and weight.MASS - the quantity of matter in an objectWEIGHT - is the measure of the amount of gravitational pull between two objects.The MASS of an object is constant. The WEIGHT of an object varies depending upon the mass of the object and the force of gravity.An object will have the same mass whether is it on earth or on the moon. On the moon, an object will weigh 1/6 of what it weighs on earth because of the difference between the pull of gravity on earth and on the moon. The weight of the object will be different but its mass will remain the same.
11 Force and MotionWhy would you weigh more at Galveston than you would at Denver? Newton’s Second Law can be applied to calculate mass, weight and gravity. F w = mg F = Force of weight (N)m = mass (kg) g = acceleration due to gravity (9.8 m/sec2)Because Galveston is closer to the center of the earth than Denver, the pull of gravity is greater there than in Denver.
12 Force and Motion10. Define friction. FRICTION - Friction is any force that opposes motion and it occurs when two objects come into contact.11. State the two factors which determine the amount of friction between two object. (a) the type of surface/texture of the surface. rougher surface = more friction (walking on carpet) smoother surface = less friction( walking on ice) (b) the mass of the object. more mass = more friction (sliding a box full of books) less mass = less friction (sliding an empty box)
13 Graphing and Motion12.Define momentum and solve problems using the momentum formula. Momentum - property of a moving object because of its mass and velocity greater mass = more momentum greater velocity = more momentum p = mv p = momentum (kgm/s)m = mass (kg)v = velocity (m/s)13. State the Law of Conservation of Momentum. The total amount of momentum of a group of objects does not change. Example: The momentum of one cue ball is transferred and remains constant as it spreads throughout the other pool balls on the table.
14 Force and Motion 14. Centripetal Force According to Newton’s second law, an object’s acceleration is directed to the center of a curved path.Centripetal force is a force exerted toward the center of a curved path.