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MOTION NOTES Motion: When an object changes position over time when compared with a reference point. You can describe the direction of its motion with a reference direction. Direction: N, S, E, W, up and down. Point of Reference: buildings, mountains, etc. Speed: Speed (s) depends on distance (d) and time (t). Formulas s=d t a=v t Symbols and units s = speed (m/s) d = distance (m) t = time (s) v = velocity (m/s) a = acceleration (m/s/s) Units: speed: m/s (meter per second) distance: m (meter) time: s (second) acceleration: m/s/s or m/s2 Speed is the rate at which an object moves. Average speed = total distance total time Velocity: The speed of an object in a particular direction. The rate of change of an object’s position. Constant velocity: an object’s velocity is constant only if its speed and direction don’t change. Acceleration: The rate at which velocity changes (speed up, slow down, change in direction or any combination). Acceleration tells you how fast velocity changes. To accelerate means to change velocity. a = final velocity – starting (initial) velocity time it takes to change velocity OR a = vf – vi tf – ti f = final i = initial Momentum The property of a moving object that depends on the object’s mass and velocity. The more momentum an object has, the harder it is to stop the object or change its direction. Law of conservation of momentum: two or more objects interact, they may exchange momentum but the total momentum remains the same. 1 Why don’t planets fall? They move around so fast that their speed gives them momentum. Planets don’t fall in toward the sun because they are speeding around their orbits. The sun’s gravity stops them flying off into space. The closer a planet is to the sun the faster it orbits. They orbit in an elliptical orbital because of the sun’s gravity that pulls them towards it. Otherwise they would be moving in a straight line as Isaac Newton’s first law indicates that an object in motion will stay in motion in a straight line unless a force is applied to it. Speed Cockroach 1.25 m /s Kangaroo 15 m/s Cheetah 27 m/s Sound 330 m/s Space shuttle Light 10,000 m/s 300,000,000 m/s Force: a push or a pull F=mxa Unit: the Newton Forces in combination Forces in the same direction Forces in Different Directions Unbalanced Forces set an object in motion Balanced Forces keep an object static (not moving) Unbalanced Forces produce a change in motion Balanced Forces produce no change in motion If the net is 0, the forces are balanced. Friction: A force that opposes motion (rough surface with microscopic valleys and hills). Types of friction: Sliding ex. eraser Rolling friction ex. wheels Fluid friction ex. wet floor Static friction ex. force applied but does not cause it to move (see p. 122). Friction – harmful or helpful Ways to reduce friction – lubricants: wax; motor oil; grease. Also by switching from sliding to rolling friction. Ways to increase friction: make surfaces rougher. Ex. Textured shoe soles for sports. Gravity: A force of attraction between two objects. Gravity depends on mass (m) and the distance (r) between the objects. F = G x m1 x m2 r 2 G = constant of universal gravity G = 6.673 x 10 –11 N. m2 kg2 2