Show picture of Henry Cavendish’s experiment on the board. 1700’s
Draw picture of girl and boy on park bench (bubble “physics”) F = 1x 10 -6 N
Ch4.5 - conceptual gravity
GravityNewton rationalized that if the earth puts a forceon an apple, then who’s to say that it doesn’texert a force on the moon?Contact vs. Non-contact Forces↓ Newer concept: forceMust be touching acting at a distance(gravity, magnets,electricity…)
Gravity must beproportional to bothmasses because ofNewton’s 3rdLaw(Equal and oppositeforces)F α M mNewton’s Math ProofForce of gravity mustdecrease by theinverse square of thedistance.F α 1/r2This is known as theinverse square law!It is repeated inelectricity andmagnetism!CombineThese!2MmFrα 2GMmFr=
Law of Universal GravitationEvery particle in the universe attracts every otherparticle with a force that is proportional to theproduct of their masses and inverselyproportional to the square of the distancesbetween them. This force acts along a linejoining the 2 particles.2GMmFr=G = gravitational constant (small or large?)G = 6.67 x 10-11Nm2/kg2It took 100 years later for this law to be proven.Henry Cavendish performed the experiment.
Sample ProblemA 50 kg girl & a 70 kg boy are 50 cm apart on apark bench. Find the force of attraction…Field Area around anobject that affectsother objects.Gravitational Field Area around amass that affectsother massesWhat represents thegravitational field around theearth?At Crown Point, the radiusof the Earth is 6.37101 x106m. If the Earth has amass of 5.98 x 1024kg.Then calculate the Earth’sgravitational field in CrownPoint.11 242 6 226.67 10 (5.98 10 )(6.37101 10 )9.83 /GM x xgr xg m s−= ==