F = G Mm
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Transcript of F = G Mm
Calculation of g
• F = GMm/r2
• g = F/m = GM/r2
• Mass of Earth = 6x1024kg• Radius of Earth = 6.4 x106m• Calculate g on Earth• g = GM/r2
• = 6.67x10-11 x 6x1024/ (6.4 x 106)2
• = 9.8 Nkg-1
g is a vector
Star
planet
g from planet
g from star
Total g = Vector sum
Gravitational PE
• These magnets have no energy when they are separated• You do work when you push them together• When they are close together potential energy is stored• Let them go and the energy is released
N SS NF F
PE
Gravitational PE
• The magnets have zero energy when they are apart.• They slide together and have less energy (negative)• A force must do work to pull them back to zero• When objects attract each other they have negative potential energy
S NS N
- PE
FF
Gravitational Potential
• Gravitational potential is always negative
• The potential at a point is the amount of energy needed to move 1 kg from infinity to that point
• V = -GM/r
planet A distant object has zero PE
Attracted by gravity
Negative PE
Amount of work needed to remove object
Zero energy
Back to zero energy
Gravitational Potential Energy
planet
1 kg
Attracted by gravity
Negative PE
Amount of work needed to remove 1kg
Zero energy
Back to zero energy
2kg
Amount of work needed to remove 2 kg
The potential at a point is the energy needed to move 1 kg from infinity to that point
The potential energy of an object is the energy needed to move the object from infinity to that point
PE = mV = -GMm/r
V = -GMm/r
Escape velocity
• How fast must an object go so that it doesn’t come back?
• It must have enough KE to overcome the negative PE (-GMm/r) and get to zero energy
• 1/2mv2 = GMm/r• V2 = 2GM/r• V = (2GM/r)
planet
Calculate the escape velocity of Earth
r= 6.4 x106m m =6 x 1024 kg
v = (2GM/r) = (2 x 6.67 x 10-11 x 6 x1024 / 6.4 x106)
= 11 000 ms-1 = 11kms-1