Types of forces we ll use Interaction Diagram (1)

Transcription

1 What is a force? Unit 4 Balanced Force Model A push or a pull is the usual answer A force is an interaction between two objects Types of forces we ll use Interaction Diagram (1) Illustrates all objects interacting in a specific situation Situation 1-hover puck off, not moving List all the objects in the scenario (this is an example) Interaction Diagram (2) Connect each object with a double arrow and list the type of force next to it Circle the object you re studying Interaction Diagram (3) Forces that cross the line are drawn in a Free Body Diagram (FBD) FBD are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation 1

2 For scenario 1 Scenario 2: on, moving Free Body Diagram Other scenarios Scenario 3: off, moving to the right Scenario 4: on, moving by being pulled by a rope to the right You can skip the interaction diagram as you get more proficient. What pattern do you see? Newton s First Law An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. Mass is a measure of an objects inertia (kg). Mass doesn t change as you go from place to place. Can mass be measured without gravity? People in space lose weight nausea, fluid buildup, muscle lost. Bathroom scaled don t work. How can you keep track of your mass? Use Inertia! Inertial Balance Lab The more massive you are, the harder it is for the springs of the chair to change your direction and the more slower you swing back and forth. 2

3 Inertia Inertia: The tendency of a body to maintain its state of rest or uniform motion Which cart is easier to start moving not considering friction, yet Cup, index card, and quarter example Dishes and table cloth example Spring Scale Lab Weight F g = m*g Where g = 9.8 N/kg Not 9.8 m/s/s a different thing! Gravitational force on an object = mass of object * gravitational field strength Slope meaning-for every 9.81 N there is 1 kg-gravitational field strength! F g = mg where g is 9.8 or 10 N/kg Y intercept- weight of zero kg Gravitational force is weight they are the same thing When you weigh yourself on a scale, you are measuring how much gravitational force the Earth is putting on you What happens to your mass and your weight when you go to the moon? Why do all things fall at the same rate? F g = m*g More mass means more gravitational force on that object More mass also means more inertia so that thing is harder to accelerate to get moving. These two concepts cancel each other out, and all things accelerate at the same rate due to gravity Free Body Diagrams (more examples) Force pictures that help figure out what is going on. 3

4 Forces are drawn: As arrows In the correct direction The proper size Labeled correctly This is the only force acting on an object in freefall. This is the only force acting on an object in freefall. The unbalanced force causes the object to speed up. 4

5 The normal force is exerted by the table on the book. Normal forces act away from the object that exerts them (away from the table) and are perpendicular to the surface of contact between the two objects. Because these two forces are balanced, there is no left over force and the object does not change speed or direction. It just sits there. What else? This is the only force acting on the moon. The unbalanced force causes the object to change direction continuously. 5

6 The normal force exerted by the incline on the box acts perpendicular to the surface and away from the incline. Friction acts in a direction opposite the direction of motion. Free Body Diagrams (Force Diagram) Each force is drawn with an arrow. Forces must be labeled with correct symbol, F type (on,by) Include the feeler-dealer or on the object by Forces must point in the correct direction. Forces must be the correct relative size (larger/smaller/same size). If the forces are balanced there is no change in speed or direction, object moves at a constant velocity and follows N1L 6

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8 Newton s Third Law For every action there is an equal and opposite reaction. True or False? For every FORCEthere is an equal and opposite FORCE. Always True! What Does it Mean? Misconception: Actions are always equal and opposite. Truth: Only forces are equal and opposite. Actual definition of Newton s 3 rd : Every force is experienced by two objects equally but in opposite directions I push you, you fall down. I push you, you push back on me. What Does it Mean? What Does it Mean? Misconception: Forces can be very large on weak materials like paper. Truth: You can only hit something as hard as it can hit you. Misconception: There are special cases where the law is not true. Truth: There are apparently no exceptions to the law. I can hit a piece of paper with a million Newtons of force. Paper is weak, so you can t hit it very hard. If I sneak up on you, you won t hit me back. You hit me back whether you mean to or not. 8

9 What Does it Mean? What Does it Mean? Misconception: Since the forces are equal and opposite, they cancel each other out. Truth: The forces ALWAYS act on different objects. Only forces that act on the same object can cancel out. Misconception: Because the forces are the same, the accelerations are the same. Truth: Acceleration depends on force AND mass. The bigger mass will have less acceleration. I hit the wall What is the reaction force? Which force is bigger? Which acceleration is bigger? 9

10 Bus hits car What is the reaction force? Which force is bigger? Which acceleration is bigger? 10