### BBC Bitesize - GCSE Combined Science - Describing motion - AQA - Revision 2

This equation can be used to calculate the acceleration of the object whose Since acceleration is a velocity change over a time, the units on acceleration are . When the velocity of an object changes it is said to be accelerating. Acceleration is the rate of Thus the SI unit of acceleration is the meter per second squared. Measurement and Units The units associated with numbers are important in physics. The units tie the numbers to real and measureable physical quantities.

Observe the use of positive and negative as used in the discussion above Examples A - D. In physics, the use of positive and negative always has a physical meaning. It is more than a mere mathematical symbol. As used here to describe the velocity and the acceleration of a moving object, positive and negative describe a direction. Both velocity and acceleration are vector quantities and a full description of the quantity demands the use of a directional adjective. North, south, east, west, right, left, up and down are all directional adjectives.

Consistent with the mathematical convention used on number lines and graphs, positive often means to the right or up and negative often means to the left or down. So to say that an object has a negative acceleration as in Examples C and D is to simply say that its acceleration is to the left or down or in whatever direction has been defined as negative. Negative accelerations do not refer acceleration values that are less than 0. We Would Like to Suggest Sometimes it isn't enough to just read about it.

You have to interact with it!

- What do solved examples involving acceleration look like?
- Acceleration
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And that's exactly what you do when you use one of The Physics Classroom's Interactives. We would like to suggest that you combine the reading of this page with the use of our Name That Motion Interactive.

### Acceleration – The Physics Hypertextbook

It is found in the Physics Interactive section and allows a learner to apply concepts of speed, velocity and acceleration.

The direction that something travels is important.

Telling someone, "drive one mile east" is very different than telling the person, "drive one mile south". Quantities that have a magnitude and a direction are "vectors". In formulas, letters are used in place of a specific value. To distinguish vectors from scalar values, vectors are usually written with an arrow above the letter, Often in equations it is easiest to use only the magnitude of a vector.

The magnitude of a vector can be identified by vertical lines on either side of the letter, or by the letter with the arrow removed.

The magnitude of vector is, Displacement The term "distance" is used in physics to mean a scalar measurement, such as "3 meters". The term "displacement" is used to mean a vector quantity.

Therefore, displacement has both a distance and a direction. When an object moves along a straight line, its starting position can be defined as the origin, O.

**Physics - Acceleration & Velocity - One Dimensional Motion**

The variable x can be assigned to mean any position along that line. The displacement is a vector that points from the origin to the position x. So, the displacement is the vector. To represent two or more positions along the straight line, the variables can be given numbers in subscript, for example, x1 and x2.

This change in position is a distance.

The average speed calculation shows the average speed of the entire journey, but instantaneous speed shows the speed at any given moment of the journey. A vehicle's speedometer shows instantaneous speed.

Average speed can be found using the total distance travelled, usually abbreviated as d, divided by the total time required to travel that distance, usually abbreviated as t. Sciencing Video Vault Instantaneous speed actually is a velocity calculation that will be discussed in the velocity section. Units of speed show length or distance over time. Formula for Velocity Velocity is a vector value, meaning that velocity includes direction.

Velocity equals distance traveled divided by time of travel the speed plus the direction of travel. For example, the velocity of a train traveling 1, kilometers eastward from San Francisco in 12 hours would be 1, km divided by 12 hr east, or kph east.

Going back to the problem of the car's speed, consider two cars starting from the same point and traveling at the same average speed of 50 miles per hour. If one car travels north and the other car travels west, the cars do not end up in the same place.

## Describing motion - AQA

The velocity of the northbound car would be 50 mph north, and the velocity of the westbound car would be 50 mph west. Their velocities are different even though their speeds are the same. Instantaneous velocity, to be completely accurate, requires calculus to evaluate because to approach "instantaneous" requires reducing the time to zero.