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Forces. Summarizing a few things we know…. From the Bowling Ball and Air Track activities we have evidence that…. Forces are responsible for changes in motion F same direction as v : Speed up F opposite direction as v : slow down F perpendicular to v : changing direction
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Forces Summarizing a few things we know…
From the Bowling Ball and Air Track activities we have evidence that… • Forces are responsible for changes in motion • F same direction as v: Speed up • F opposite direction as v: slow down • F perpendicular to v: changing direction • Forces cause acceleration • When there is no netforce, motion does not change in any way… a = 0 m/s2
From the Force Probe demos (with Cam and George) we have evidence that… • Forces can only exist in pairs • Action-Reaction forces are always equal in size • Action-reaction forces are always opposite in direction • Action-reaction forces always act on different objects (forces are an interaction between two things) • If A pulls/pushes B, then B pulls/pushes A equally in the other direction
From the Force of the Earth activity we have evidence that… • The force the earth on an object is directly proportional to the mass of the object. Fearth = (10 N/kg)m • The intercept is zero…zero mass feels no force from earth • The slope is called the gravitational field strength, g • All objects in the same location experience the same gravitational field strength • The “force of the earth” on an object is more commonly know as the object’s weight Fearth (N) m (kg)
Mass vs. Weight • Mass – a measure of the quantity of matter in an object, a property of the object, NOT a force. • This value is the same regardless of location • A measure of an object’s inertia • SI unit is the kilogram (kg) • Weight – a measure of the force of gravity on an object. • This value depends on the object’s location in a gravitational field. • SI unit is the Newton (N) • Weight = mass x gravitational field strength Fg = mg *Near the surface of Earth, a 1 kg mass weighs 10 N
Equilibrium-a condition of balance • We were able to measure the weight of the objects using a scale because they were in equilibrium. Since acceleration = 0 m/s2, the forces must be balanced, thus the reading on the scale was equal to the weight Force from the scale Force from the earth…weight
Equilibrium • Static equilibrium • Object at rest • Dynamic equilibrium • Object moving at constant speed • True for both… • Acceleration = zero (a = 0 m/s2) • Net force = zero (F = 0) • When the net force acting on an object equals zero, its state of motion will not change and the object is in mechanical equilibrium.
