POWER POINT # 2 NEWTON’s LAWS GRAVITY

I: NEWTON’S LAWS OF MOTION Begin by stating Newton’s laws: Newton’s first law (N1) – If a body is not acted upon by any forces, then its velocity, v, remains constant Note: N1 sweeps away the idea of “being at rest” as a natural state.

Newton’s 2nd law (N2) – If a body of mass M is acted upon by a force F, then its acceleration a is given by F=Ma Note N2 defines “inertial mass” as the degree by which a body resists being accelerated by a force. Another way of saying this is that force = rate of change of momentum (rate of change of mv). Newton’s 3rd law (N3) - If body A exerts force F on body B, the body B exerts a force –F on body A.

II: NEWTON’S LAW OF UNIVERSAL GRAVITATION Newton’s law of Gravitation: A particle with mass m1 will attract another particle with mass m2 and distance r with a force F given by Notes: “G” is called the Gravitational constant (G=6.6710-11 N m2 kg-2) This is a universal attraction. Every particle in the universe attracts every other particle! Often dominates in astronomical settings.

Defines “gravitational mass” Using calculus, it can be shown that a spherical object with mass M (e.g. Sun, Earth) gravitates like a particle of mass M at the sphere’s center.

I:KEPLER’S LAWS EXPLAINED Kepler’s laws of planetary motion Can be derived from Newton’s laws Just need to assume that planets are attracted to the Sun by gravity (Newton’s breakthrough). Full proof requires calculus (or very involved geometry)

Planets natural state is to move in a straight line at constant velocity But, gravitational attraction by Sun is always making it swerve off course Newton’s law (1/r2) is exactly what’s needed to make this path be a perfect ellipse – hence Kepler’s 1st law.(use calculus) The fact that force is always directed towards Sun gives Kepler’s 2nd law (conservation of angular momentum) Newton’s law gives formula for period of orbit

III: FRAMES OF REFERENCE We have already come across idea of frames of reference that move with constant velocity. In such frames, Newton’s law’s (esp. N1) hold. These are called inertial frames of reference. Suppose you are in an accelerating car looking at a freely moving object (I.e., one with no forces acting on it). You will see its velocity changing because you are accelerating! In accelerating frames of reference, N1 doesn’t hold – this is a non-inertial frame of reference.

ACCELERATING MOTION Motion at constant acceleration a in meters/sec2 Start with zero velocity. Velocity after time t is v(t)=at. The average speed during this time was vav=(0+at)/2=at/2 The distance traveled s=vavt=at2/2 Suppose you accelerate from 0 to 50 m/sec in 10 secs The distance s will be given by S=(1/2)(5 m/sec2)102= 250 m The general formula if you start with initial velocity v(0) is s=v(0)t+(1/2)at2

Moon Moon falls about 1.4 mm in one sec away from straight line Apple falls 5 m in one sec Earth REM/RE=60

TIDES 1/R2 law Daily tide twice Why? Moon Earth Water pulled stronger than the earth Earth pulled the water

TIDES Twice monthly Spring Tides (unrelated to Spring) and Twice monthly Neap Tides Full moon – extra low tides Earth moon Sun Earth New moon Extra high tides moon

TIDES Twice monthly Neap Tides Sun moon at right angles First Quarter Earth Earth Last quarter

Real and fictitious forces In non-inertial frames you might be fooled into thinking that there were forces acting on free bodies. Such forces are call “fictitious forces”. Examples – G-forces in an accelerating vehicle. Centrifugal forces in fairground rides. The Coriolis force on the Earth. Fictitious forces are always proportional to the inertial mass of the body.

Non – Inertial Frames g Monkey and hunter Accelerometer a T mg ma Two real forces mg downwards and T. When car accelerates they must add to ma by Newton’s 2nd law An observer in the car feels a force that pushes everything backwards. To explain his result

He adds a “fictitious force” known as inertial equal to Fin=-ma This is known as inertial force. T Sum of forces equal to zero ma mg vt Centrifugal vs Centripetal vt a vr The only real force is the Centripetal force pulling towards the center so the ball must be accelerating o An observer sitting on the ball feels a fictitious force Centrifugal that pushes him outwards balanced by the string held at o.

Look at the rotor in amusement park. People on the outside See only a Centripetal force from the wall pushing riders inwards into circular motion. People inside the rotor feel the fictitious Centrifugal force pushing them outward with the force from the wall balancing it. Centrifugal force Fc= mvt2/R An observer in a non-inertial system with acceleration a should add to the real forces a fictitous inertial force Fin= -ma in order to describe the dynamics of the system correctly, i.e. to provide a description equivalent to an observer observing from an inertial frame. Weight-less-ness. Weight in an elevator or the shuttle.

Maybe gravity is a fictitious force… Weak equivalence principle  gravitational force is proportional to inertial mass. Maybe gravity is a fictitious force… … and we live in an accelerating frame of reference?

Vesc=(2GME/RE)1/2

IV: NEWTON’S WORRIES Newton knew that his theory has problems Gravity is “action at a distance” – he didn’t like that! A static universe would be gravitationally unstable.