Force and Motion ISCI 2002 Force 1. Force: ‘push’ or ‘pull’ on an object 2. Objects in motion stay in motion unless enacted upon by a ‘unbalanced’ force. • Newton’s First Law Force 3. Forces: friction (drag), thrust, etc. 4. Are you telling me that if an object in motion continues at a constant velocity unless ……. • Forces change the velocity of an object in motion Law of Inertia 1. The first law relates to inertia. 2. The tendency for an object to remain in whatever state of motion that it is in is referred to as ‘inertia’ 3. Consider the following: • The greater the mass the _________ the inertia. 4. Mass – how much inertia an object has Weight and the Force of Gravity 1. Weight: (mass)(force of gravity) 2. Mass is constant. Weight depends on the force of gravity upon an object. • Is the weight of a 60-kg rocket at the surface of the Earth equal to the weight when at 2,600 kilometers above the Earth? • No. Why? The Newton 3. The ‘Newton’ • (kg)(m/s2) -------- kg x m/s2 Relationship between Force, Mass and Acceleration 1. Newton’s Second Law • The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. • a = Fnet / mass Second Law 2. What happens to acceleration when: (a). The force upon it doubles? (mass is constant) (b). The force is decreased by ½? (mass is constant) (c). Mass is doubled (force is constant) Newton’s Third Law 1. For every action there is an equal and opposite reaction. 2. Forces always come in pairs. 3. Features: • • • They are equal in magnitude Opposite in direction They act on different objects Third Law Newton’s Third Law • If you punch the wall with a 10 N force, the wall exerts a force of __________ upon your fist. • Opposite in direction? • Act on different objects? Speaking of Acceleration due to Gravity • Galileo – objects in ‘free-fall’. • Objects falling freely fall with the same acceleration (9.8 m/s2) – near Earth • So speed or velocity changes 9.8 m/s every second. • The instantaneous speed or velocity of a body dropped from rest: • V = (9.8 m/s2) time • V = at (if the initial velocity is 0) Free Fall Freely Falling Objects • At 2 seconds an objects velocity fall is ____________. • If a ball is thrown upward at an initial velocity of 29.4 m/s, what is it’s velocity at 1.0 seconds? 2.0 seconds? 3.0 seconds? 4.0 seconds? 5.0 seconds? 6.0 seconds? Free Fall • Galileo rolled balls down a ramp. • He measured the distance they moved during time intervals. • This slowed down the ball in order to take accurate measurements. • He found that: • d = ½ at2 • If an object falls (from rest) for a total of 2 seconds, how far has it fallen?