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August 29, 2015
Posted by **Kelsey** on Wednesday, February 10, 2010 at 6:45pm.

- 1st grade -
**Nadia Samim**, Wednesday, February 10, 2010 at 7:19pm1. Speed

We describe how fast an object is moving as the 'speed' of the object. When we can measure how much distance an object covers in a certain period of time, we know the 'speed' of the object. Say a man cycles 1000 meters in one hour, his 'speed' will be one kilometer per hour and is written as 1KMph.

2. Velocity

When we talk about the direction and the speed of the object, we are referring to its 'velocity'. So, we can describe the 'velocity' of an object as its 'speed' towards a certain place or in a certain direction. Therefore, an airplane traveling at 500 miles an hour towards the west will be said to have a 'velocity of 500 miles per hour due west. If we can measure any quantity as well as determine the direction of the quantity, we call that quantity a 'vector'. Velocity is, therefore, a 'vector.'

3. Acceleration

The speed and direction of an object is always changing. The speed of the rider on the cycle may change from 1KMph to 2KMph and back again to less than 2KMph. Generally speaking physics refers to the rate of change of 'velocity' as 'acceleration'. Let us say that the rider on the cycle changed his speed from 1KMph to 2KMph and that it took him 1 hour to attain the speed of 2KMph. This means that he increased his speed by 1KMph in 1 hour; therefore, we will say that his 'acceleration' or rate of change in velocity was 1KMph. Acceleration not only refers to speeding up, but also slowing down.

You can summarize if you'd like.

- 1st grade -
**Lori**, Wednesday, February 10, 2010 at 7:27pm1. The first law of motion states that every object will remain at rest or in uniform motion in a straight line ,unless compelled to change its state by the action of an external force.

2. The second law of motion explains how the velocity will change. The law defines a force to be equal to change in momentum ( mass times velosity ) per change in time.

3. The third law of motion states that for every action ( force ) in nature there is an equal and opposite reaction.