Force, Mass and
Momentum
Newton’s Second Law:
F = ma
• 1 newton = 1 kg ∙ 1 m/s²
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Force: 1 pound = 4.45 newtons
Your weight is the force of gravity: F = mg
g = 10 N/kg
225 pounds = 1000 newtons
Momentum = p = mv
• Units: p = mv
Units:
kg ∙ m/s
• Newton’s First Law : An object in motion continues in motion
• If there are no forces,
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velocity doesn’t change
Δv= 0
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momentum doesn’t change
Δp = 0
Δ = delta = difference = change
• F = ma = m Δv/t = m(vf – vo)/t
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= (mvf – mvo)/t = (pf – po)/t = Δp / t
• F = Δp / t
The rate of change of momentum equals force.
Impulse
• F = Δp / t
The rate of change of momentum equals force.
• If we mulitply by t
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F ∙ t = Δp
Ft is called impulse
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(F ∙ s = work = Δ Energy)
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• Impulse = Change in Momentum
• newton ∙ second = kg ∙ m / s
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(1 newton = 1 kg ∙ 1 m/s²)
The Third Law
• Newton’s Third Law:
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“If A pushes B then B pushes A”
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(in opposite directions)
FA = −FB
Time is equal t ∙ FA = − t ∙ FB
The impulse is equal and opposite
ΔpA = − ΔpB
ΔpA + ΔpB = 0
ΔP = 0
Capital P is total momentum
• Momentum is conserved
Conservation
• Energy is Conserved
• Momentum is Conserved
• Mass is Conserved