Motion is the change in position in a certain amount of time.

Frames of Reference Whenever you describe something that is moving, you are comparing it with something that is assumed to be stationary, or not moving. The background or object that is used for comparison is called a frame of reference. The most common frame of reference is the Earth, but no single frame of reference is “correct”.

Measuring Motion When you say that something has moved, you are describing motion. Speed is the rate at which an object moves. Speed = Speed is the rate at which an object moves. Speed = Distance Time ●Distance is usually measured in meters or kilometers. Time is usually measured in seconds or hours.

Constant and Average Speed When speed does not change it is called constant speed. For an object moving at constant speed, the speed at any point is the same as the average speed. For an object whose speed varies, you calculate the average speed.

Velocity Another characteristic used to describe motion is velocity. Velocity is speed in a given direction (the direction in which an object moved). Example : The dog ran eastward at 10 m/sec. The dog’s speed is 10 m/sec. Its VELOCITY is 10 m/sec east.

Combining Velocities Velocities that have the same direction are added together. Velocities that have opposite directions combine by subtraction.

If you are rowing a boat downstream at 16 km/hr and the river water is also traveling downstream, of course, at 10 km/hr. You would actually be moving at 26 km/hr. (velocities in the same direction are added together) If you are rowing a boat upstream at 16 km/hr and the water is traveling downstream at 10 km/hr, you are actually moving at 6 km/hr (opposite directions – subtract).

Changes in Velocity The rate of change in velocity is known as acceleration. If something is accelerating, it is speeding up, slowing down, or changing direction. The acceleration of an object is equal to its change in velocity divided by the time during which this change occurs. Acceleration = Final Velocity - Original Velocity Time

If there is a decrease in velocity, the value of acceleration is negative. Negative acceleration is called DECELERATION. As a car comes to a stop, it decelerates, or slows down.

Momentum All moving objects have momentum. The more momentum an object has, the harder it is to stop. Momentum depends on the mass of the object and the velocity with which it is traveling. Momentum is equal to the mass of an object multiplied by its velocity. Momentum = Mass x Velocity Momentum = Mass x Velocity

A 200 pound fullback runs up the middle of the football field. Suddenly he collides with a 175 pound defensive back running toward him. The more massive fullback is thrown back two yards. HOW can this happen? Although he has less mass, the defensive back would have to have more momentum (he would have been running faster than the fullback) His greater velocity makes up for his smaller mass. IF both players had the same velocity, who would have more momentum? Example…..

Conservation of Momentum When the momentum of one object is transferred to another object none of the momentum is lost (conservation of momentum) The total momentum of any group of objects remains the same unless outside forces act on the objects. (conservation of momentum) playing billiards {pool}; baseball; throwing an object off a boat, boat goes in opposite direction;