Physics 100 Chapt 10 Newtons Universal Law of Gravitation
Very fast horizontal toss t = 0s t = 1s x= 8km 5m t = 2s x=16km t = 3s x=24km 20m 45m V=8km/s
Orbital motion is free fall V = 4 km/sV = 6 km/sV = 8 km/sV = 10 km/s Circular Orbit!Eliptical Orbit
Centripetal acceleration a = v 2 /r for a circular orbit (v = 8km/s = 8x10 3 m/s) a = (8 x10 3 m/s) x 10 6 m = 64 x10 6 m 2 /s x 10 6 m = 10 m/s 2 Toward Earths center = g
Artificial satellite a v a = v 2 /r = g
Moon-earth v a=v 2 /r
Is the Moon in free-fall around the Earth? r=3.84x10 5 km v a = v 2 /r what is v? v = dist/time = 2 r 28d = 2 x 3.14 x 3.84 x 10 8 m 28d x (24h/d) x 3.6 x 10 3 s = 24 x 10 8 m 2.4 x 10 6 s = 1.0 x 10 3 m/s
Moons centripetal acceleration a moon = v 2 /r; v = 1.0 x10 3 m/s) a moon = (1.0 x 10 3 m/s) x 10 8 m = 1.0 x 10 6 m 2 /s x 10 8 m = 2.7 x m/s 2 Toward Earths center g
Newtons dreams Hmmmmm The Moon is in free-fall around the Earth Its acceleration is only 1/3600 g (accel at the Earths surface)
Distances r=3.84x10 8 m = 60 x 6.4x10 6 m The moon is 60x further from the Earths center than objects on (near) the Earths surface = 60 x R E ()2=)2= R E = 6.4x10 6 m
Newtons big idea The force of gravity gets weaker as distance squared The moon is 60x further from the Earths center than objects on (near) the Earths surface The strength of Earths gravity near the Moon is (1/60) 2 t=1/3600 times weaker
Gravity gets weaker as 1/dist 2
Universal law of gravity m M r F m F M F 1r21r2 combine:F mM r 2 F = G mM r 2 Proportionality constant: Newtons Constant
Universal Universal: applies to all objects!!!
What is G? W= G mM E R E 2 W W= m GM E R E 2 W= m g g=g= GM E R E 2
Force of gravity between ordinary-sized objects 80kg 60kg 1m F = G mM r 2 F = 6.7x Nm 2 /kg 2 60 kg 80kg (1m) 2 F = 6.7 x 60 x 80 x N Boys weight = mg = 80kg x 10m/s 2 = 800 N 30x10 9 times bigger! F = x N = 3.2x10 -7 N
Measuring gravity force between ordinary-sized objects is very hard
Moons gravity causes tides
So does the Suns Full Moon New Moon Half Moon
Measuring Weight mg N
Weightlessness N=mg N>mgN<mg N=0
Weightlessness means =0 N compensating upward
Weightlessness in action
Floating is space is really free-falling in space
What is g on the moon? W= G mM M R M 2 W= m GM M R M 2 W= m g M gM=gM= GM M R M 2 W R M =1.7 x 10 6 m m M
g M on the Moon g = GM M R M 2 6.7x N m 2 /kg 2 x 7.4x10 22 Kg (1.7 x 10 6 m) 2 = g M = 1.7 m/s 2 1/6 x g Earth
summary Obey the law!