ASTR 1514 1st Edition Lecture 6Laws of Motion Chapter: 3.3-3.5Kepler’s Laws of Motion describe the motion of the planets very well, however they do not explain why they have the motionsObjectives:1.) Use physical laws to explain planetary motion2.) Understand Newton’s laws of motion, and the force of gravity3.) Understand how Newton’s laws of motion and gravity produce planetary orbits.Newton’s LawsNewton’s laws explain why objects move the way they move.Newton noticed that a moving object will stay in constant, at a constant speed and constant direction, motion. An object at rest stays at rest. Velocity The speedANDdirection of an objects motion.Speed: Driving 60 miles per hour. Velocity Driving 60 miles per hour in the North, South, East, West etc. Direction. Newton’s First law refers to velocity – Speed and Direction stay the same. Newton’s First Law- A body continues at rest or in a uniform motion in a straight line unless acted upon by some force- Example: You are traveling on your bike and hit a curb, the bike is stopped by the curb but you continue forward flying off of the bike with a uniform velocity. Newton’s Second Law- Unbalanced forces cause change in motion. Example: Speeding up with gas pedal in a carand slowing down with brake pedal. - A change in direction also is a change in motion. Example: Driving around a curve at constant speed, the speed is not changing but the direction is. - An unbalanced force causes acceleration, a change in velocity (speed or direction), deceleration, negative acceleration. - A larger mass requires a larger force to move the mass. - A larger force will result in a larger acceleration. - Acceleration measures how quickly a change in motion takes place. 1.) Example: Stepping on a cars gas pedal will cause you to accelerate faster2.) Example: Stepping on the cars brake pedal will cause you to slow down or decelerate. Newton’s second law can be written as F=ma F is the force, m is the mass of an object, anda is the acceleration.Force = (mass)(acceleration).Direct and Inverse Relationships- Direct relationship Increasing the size of one variable increases the size of the other. 1.) Force has a direct relationship with acceleration. - Inverse relationship Increasing the size of one variable decreases the size of the other. 2.) Acceleration has an inverse relationship with mass. Example of Newton’s second law: It requires a greater force to push a heavier shopping cart compared to a less heavier shopping cart. Newton’s Third Law When one body exerts a force on a second body, the second body exerts an equal and opposite force back on the first body. Ex: Astronaut in space is free floating, she throws a package in front of her and is pushed backwards in the opposite direction, both objects exert an equal force. - Forces occur in action-reaction pairs.- The forces are equal in size - The two forces had opposite directions. Gravity- Force we have most direct experience with- Gravity is a force so it must have an acceleration. (Newton’s 2nd law)- All objects fall with the same acceleration, g.g = 9.8 m/s2- The force that you feel on Earth depends on the acceleration (9.8 m/s2) and your mass. - Your weight is the product of mass and the force of gravity. Fweight= (mass)(gravity).- Example: If your mass is 5, then Fweight = (5)(9.8) = 49 lb. - Force of gravity is what we mean by weight. Properties of Gravity- Gravity is an attractive, mutual force between any two objects with a mass. - Depends on the scale of each mass and the distance between them. - The greater the mass then the greater the attraction and greater the force. (direct relationship). - The greater the distance between the objects, the weaker the attraction, and weaker the force. (Inverse relationship). - Forcegravity = G x (mass1)(mass2)/(radius2). G = Universal gravitational constant.m = mass of each objectr = distance separating each object. How does gravity explain orbits? - Orbits are one body falling around another due to gravity. Less massive object is a satellite of a more massive object. - Ex: A cannonball fired at just the right speed would keep falling around the Earth never hitting the