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http://commons.wikimedia.org/wiki/File:Soccer_kick.jpg Changes in motion can be measured and graphically represented. Forces cause changes in position, speed, and direction. Motion is a change in position, which is measured by distance and time. Anything moving is in motion. http://commons.wikimedia.org/wiki/File:Mercedes_In_Motion.jpg A force is a push or pull that can change motion. A force can move an object. A force can transfer energy to an object. For example, pushing on a wall doesn’t move the wall, but energy is being transferred. Force = mass x acceleration In order for a force to move an object, there must be unbalanced forces. Like tug of war…the winners are pulling the other team with more force; therefore, the pulling forces are unbalanced. http://commons.wikimedia.org/ wiki/File:Tug_of_war_2.jpg Unbalanced forces can change the position, speed, or direction of an object. Resistance forces (friction or wind) that oppose motion can slow down an object. A force in the same direction can cause an object to speed up. A force from a different direction can change the object’s direction. If we have unbalanced forces, then there must be balanced forces. When forces are balanced, an object’s motion will be constant. It will not change speed or direction. We talk about speed everyday. Speed is the distance traveled by a moving object per unit of time. Speed affects the energy of an object. The faster it moves, the more kinetic energy it has. Speed = distance time The energy of motion Objects in motion have kinetic energy. Speed and mass affect the kinetic energy of an object. If two objects are moving at the same speed, but one object has a greater mass—then the object with the greater mass will have more kinetic energy. The more mass in an object, the more force needed to move it. If I traveled 30 meters in 10 seconds, what was my speed? S = D/T o 30m/10s o 3 m/s o Distance is a description of how far an object traveled between two points. Velocity and speed are like fraternal twinsvery similar, but with important differences. Velocity is speed in a given direction. 35 mph NORTH 4 km/sec EAST 100 m/s right 1500 km/s 65 m/s north 3000 km/s Velocity Speed Velocity Speed Acceleration is a change in the rate of speed. It can be a change in speed, direction, or both. Acceleration = Change in speed (m/s) Time (s) Example: 0 to 60mph in 3s The race car accelerated from stop to 60 m/s by the finish line. It took the race car 8.0 seconds to finish. What was the acceleration? Original speed = 0 Final speed = 60 m/s Time = 8.0 seconds Acceleration = 60 - 0m/s = 7.5 m/s2 8.0s Friction is a force that opposes motion. It can be caused by wind, water, surface texture, etc. Have you ever ridden down the street on your bike and felt the wind push against you? That’s air resistance! Air resistance is the frictional force from the air that resists forward motion.