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Transcript
Chapter 6-1 Force & Motion What is force? A push A pull Contact vs Long Range Forces A Contact force acts only on an object by touching it Example: Push a crate across the floor Contact vs Long Range Forces Long Range force: force is exerted without contact Examples: Gravity Magnetism So, Push your physics book across the desk: contact force of hand on book Push your physics book off the desk: long range force of gravity takes over Forces have agents! The agent is the specific, identifiable, immediate cause of the force Agents for example contact force: • Friction of the crate against the floor • Push of your hands on the crate The agent is an object: a real, tangible thing! Agents for Examples of Long Range Force: For gravity: The mass of the Earth attracting (gravitationally) the mass of the crate The agent is an object: a real, tangible thing! Agents for Examples of Long Range Force: For magnetism: The north pole of the magnet in your hand repelling the north pole of the hanging magnet. The agent is an object: a real, tangible thing! If you can’t name an AGENT then a force doesn’t exist! Some pictorial models with vectors drawn for forces: Be able to draw these kinds of pictorial models and force vectors: Notice that force vectors are different from velocity or acceleration vectors because they are drawn from the same spot. How does acceleration depend on Force? The more Force, the more acceleration. A direct relationship How does acceleration depend on the mass of the object? The more mass, the less acceleration. An inverse relationship An unbalanced force (this is, a Net Force) is the cause of a change in velocity! BIG TOPIC Drawing Free Body Diagrams (FBD) Steps to follow in drawing a FBD: Draw the object as a dot with a box around it Drawing FBD Step 2 Draw all forces starting on the dot: These are force vectors, so forces are Represented as arrows: Length is proportional to magnitude of force Direction is drawn in the direction of the force Remember our hard working crate-pusher? The scenario is that the crate is accelerating to the right (it’s important that it is accelerating). Let’s see what a FBD of the forces on the crate looks like--- Drawing FBD of crate being accelerated to the right: Draw the force of the push as an arrow to the right Draw the friction force of the floor on the crate as an arrow to the left It is important the friction force is less than the pushing force for this scenario—Why? Drawing FBD of crate being accelerated to the right: There must be a net force, Fnet to the right Draw FBD of these scenarios: No. 1 Draw FBD Scenario 2: Draw FBD Scenario 3: Let’s use the parallelogram method to solve these vector diagrams: Solve these problems: draw the FBD and solve for the net Force What is a Newton, anyway? I know what a pound feels like but what does the force of a Newton feel like? The typical apple weighs 1N Other typical weights/Forces measured in Newtons: Remember Galileo? Well, he really came up with this concept, Newton just codified it as his first law of motion and, thus, got credit for it. “An object at rest will remain at rest or An object that is moving will continue to move •in a straight line •at a constant speed Unless acted upon by an unbalanced (net) force.” Often called the “Law of Inertia” What is inertia? It is the tendency of an object to resist change. Things want to keep doing what they’re already doing. Another BIG CONCEPT: EQUILIBRIUM • Occurs when net force = zero – The object is at rest or – The object is moving at constant velocity (Remember: no net force, no acceleration– the object is in uniform motion or the object has constant velocity) Different Types of Forces and their symbols: Note the use of “system” and “environment” above: System is a defined collection of objects—it is the subject under consideration. Environment is the rest of the world outside the System. Note: the environment can exert forces on the system. Common misconceptions about Forces: • When a ball has been thrown, the force of the hand that threw it remains on it. • A force is needed to keep an object moving • Air does not exert a force • The quantity ma is a force Fin 6-1