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Forces These are the things you are expected to remember…. ¿ Qué es una fuerza ?. A force is an interaction that CAN cause an object to accelerate. A force won’t always cause an acceleration. Only a net (non-zero) force will cause an acceleration. What is a force?. Other important Stuff.
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¿Quéesunafuerza? A force is an interaction that CAN cause an object to accelerate. A force won’t always cause an acceleration. Only a net (non-zero) force will cause an acceleration. What is a force?
Other important Stuff • First conceived by Galileo. • Inertia is the tendency for an object to resist changes in its motion. • Inertia is proportional to the mass of an object (and is often used synonymously with “mass” for translational motion). • Also a sweet idea for a roller derby name Inertia
Newton’s Laws of Motion 1st Law = Law of Inertia An object at rest will remain at rest, an object in motion remain in motion with a constant velocity unless acted on by a net external force. The “disclaimer” at the end is very important!
Newton’s Laws of Motion 2nd Law = The acceleration of a body is proportional to the net force on the body AND inversely proportional to thebody’s mass. Mathematically ΣF = ma
Newton’s Laws of Motion 3rd Law = Action-Reaction Pairs For every (action) force there is an equal but opposite (reaction) force Fhead on hand Fhand on head
Everyday Forces Weight: is the force due to gravity on an object. It is always directed (straight) downward. Fw = mg Normal Force: is the force with which a surface pushes back on an object. This force is always perpendicular to the surface no matter the orientation of the surface.
Everyday Forces Tension: The force applied by (or in) a rope. Ropes only pull. Friction: The force a surface applies that resists motion or an applied force Gravitational force: is the force on an object due to a second object. Any 2 objects will exert an equal and opposite gravitational force on each other. We’ll do more with this later.
Using Free Body Diagrams 1. Draw forces acting on object acting through the center of mass 2. Label “everyday” Forces* * you do NOT label components as part of the free body diagram. The components are only to assist you in solving for a variable, they are not “stand alone forces”. 3. Label acceleration if present 4. Apply ΣF=ma in the necessary directions FN mg
This is a free body diagram – notice the components of the weight are NOT labeled! FN The components are helpful in solving but are NOT fundamental forces. a θ mg
Friction Friction is the resistive force between two bodies as they slide past one another. Static friction must be overcome to initiate the motion of a body while kinetic friction must be overcome while an object is in motion. Both friction types are defined by the coefficient of friction, μ (either kinetic, μk or static, μs)
UCMUniform Circular Motion What is UCM? It describes an object that moves in a circle with a constant speed Implications of UCM… An object accelerates without changing its speed. The direction continuously changes as an object moves around a circle
UCMUniform Circular Motion • Newton’s first law says that an object in motion continues in motion with a constant velocity unless acted upon by a net external force. • Velocity is a vector, speed and direction. • A constant velocity means constant speed AND direction. • Direction changes, therefore a force was applied!
UCMUniform Circular Motion According to Newton’s first Law of Motion, an object’s natural path is a straight line, therefore to pull an object out of its natural path a net force must be applied. Centripetal force is the force that is responsible for an object undergoing uniform circular motion (UCM). This force is always directed radially inward (tangent to the instantaneous velocity). It causes a change in direction without causing a change in speed. The centripetal force is any “everyday force” that acts centripetally – that is acts so that it is directed toward the center of the circular path.
UCMUniform Circular Motion • What is the equation for centripetal force? • Remember that Fc is the net force toward the center of the circle.
Centripetal vs Centrifugal OMG people….you better know this! The centrifugal force is a fictional force in an inertial frame of reference. It is not real! It is usually used in place of the correct Newton’s First Law when explaining certain behaviors exhibited in circular motion. It is not real! Centrifugal translates from Latin to mean “center fleeing.” The “centrifugal effect” was named such because it feels as if you are being pushed outward when moving in a circle. You know better than that….it is just inertia that you are feeling.
Common Problem types… …that often give students trouble.
Systems • Define the direction of the motion/acceleration • Separate the system into discrete objects. • Write a F = ma equation for each object • Combine equations and solve
Inclined PlanesTeachers love them… students hate them. You have to pay attention to the direction of the acceleration AND the direction of the forces (and their components).
I’m excited to start working with forces again I hope that you are too… …too bad if you’re not, we’re doing them anyway!
