Forces

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# Forces - PowerPoint PPT Presentation

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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## PowerPoint Slideshow about 'Forces' - jolie-davidson

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Presentation Transcript

### 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
• 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.