Chapter 3InertiaNewton’s 1st Law: InertiaSlide 4Orbital Motion and GravityUniform MotionNon-Uniform MotionNewton’s 2nd Law: Fnet = maSlide 9Slide 10Slide 11Newton’s 3rd Law: Equal and Opposite ReactionsMore 3rd LawAstronomical MotionNewton’s Law/Theory of GravityNewton’s Law/Theory of GravitySlide 17Slide 18Measuring an object’s mass from orbital motionWeight vs MassSurface GravitySurface Gravity: The mathSlide 23Slide 24Escape VelocityEscape VelocitySlide 27Slide 28Slide 29Chapter 3Gravity and MotionInertia•Inertia is the tendency of objects to resist changes in their motion.•A body at rest remains at rest, and a body in motion stays in motion… unless acted on by an outside force.•Through simple experiments with inclined planes, Galileo demonstrated the idea of inertia and the importance of forces.•Newton’s First Law of Motion: A body continues in a state of rest or of motion in a straight line at a constant speed unless made to change that state by forces acting on it.Newton’s 1st Law: Inertia•Galileo’s experiments with balls and inclines.•If the forces are balanced, the motion does not change.•The ball isn’t traveling in a straight line because a force (tension in the string) is acting on it.•If we remove this force, the ball continues to move in a straight line in the direction it was moving the moment the string was released.NONOYESOrbital Motion and Gravity•Newton’s thought experiment to understand orbit.•If Earth were flat, the cannonballs would always hit the ground eventually, no matter how fast they were fired.•But the Earth is round, and if you fire the cannonball fast enough, it will go all the way around and hit the back of the cannon.Uniform Motion•Motion where the speed and the direction don’t change is called “uniform motion.”Non-Uniform Motion•If the speed and/or direction are changing, the motion is not uniform.•When motion is not uniform, it means that acceleration is happening.•Speeding up, slowing down, and/or changing direction are all examples of acceleration.Newton’s 2nd Law: Fnet = ma•“The acceleration of a body is proportional to the net force exerted on it, but inversely proportional to the mass of the body”•Can be written as a=F/m. Larger forces make larger accelerations. Larger masses make smaller accelerations.•m is mass, which measures how much inertia an object has.A person is riding their bicycle at a constant speed. Which of the following are true?A. The motion is uniformB. The motion is non-uniformC. There is a net forceD. The forces are balancedE. A and C are trueF. B and D are trueG. A and D are trueH. B and C are trueA car on the highway sees a speed trap and starts to slow down. Which of the following are true?A. The motion is uniformB. The motion is non-uniformC. The car is acceleratingD. There is a net force on the carE. A, C and D are trueF. B, C, and D are trueThe moon orbits around the Earth at a constant speed. Which of the following is true for the moon?A. The motion is uniformB. The motion is non-uniformC. The moon is acceleratingD. The moon is not acceleratingE. There is a net forceF. The forces are balancedG. A, D, and F are trueH. B, C, and E are trueNewton’s 3rd Law: Equal and Opposite Reactions•When two objects interact, they create equal and opposite forces on each other.•This is true for any two objects, including the Sun and the Earth!Equal forces. Equal masses.Equal accelerations.Equal forces. Different masses.Different accelerations.More 3rd Law•The forces are equal, but the masses are not.•Because a = F/m, the larger mass has the smaller acceleration.Astronomical Motion•Planets and moons don’t travel in straight lines, so there must be a new force acting on them (Newton’s 1st and 2nd Laws).Newton’s Law/Theory of Gravity•All things with mass attract all other things with mass.•How strongly do they attract? See formula.•FG = force of gravitational attractionM, m = mass of the objectsd = how far apart the objects areG = Gravitational constant (6.67 x 10-11 m3/kg*s2)Newton’s Law/Theory of Gravity•Earth and Moon pull on each other with the same force.•The moon has a smaller mass, so it has a bigger acceleration, and moves faster.In a football game, a 200 lb receiver and a 280 lb linebacker collide in midair at full speed. The receiver is knocked backwards onto the ground. Which of the following is true?A. The receiver and the lineman each pushed on each other with the same forceB. The receiver pushed on the lineman with the larger forceC. The lineman pushed on the receiver with the larger forceIf the forces are equal, why does the receiver end up on his back?A. The receiver has a smaller massB. The receiver has the larger accelerationC. Both A and B are trueMeasuring an object’s mass from orbital motion•You can do this in a simple way using physics and math, if you make some helpful assumptions (one object is way bigger than the other)•Short version: if we set the centripetal force (force acting on an object moving in a circle) equal to the gravitational force (this is where the centripetal force comes from), then we get…•M = d*V2/G•You don’t need to be able to derive this equation. Just know that it is possible to find the mass of an object if you know 1) the speed of the orbit and 2) the distance between the two objects.M = mass of big objectd = distance between themV = speed of orbit of small objectG = gravitational constantWeight vs Mass•Mass = how much inertia an object has Does NOT change when you go into space Depends on how much matter you contain•Weight = the force of gravity pulling you down DOES change when you go into space Depends on the strength of gravitySurface Gravity•You may remember hearing “all objects accelerate at the same rate” when they are dropped.•This rate is called surface gravity.•The rate at the surface (9.8 m/s2) is NOT the same if you get far away from the surface.Different forces.Different masses.SAME acceleration.Surface Gravity: The mathForce on an object=object’smassobject’saccelerationxForce ofgravityG*M*md2=m x aWe want to know how this equation works AT THE SURFACEso d has to be the radius of the planet (let’s call it R).acceleration at the surface=G*MR2=‘g’m = your mass M = planet’s massYou are expected to be able to calculate surface gravity and to compare it between different planets/moons.Knowing that surface