# Negative Acceleration: Understanding Movement Direction

• sallychan
In summary, the conversation discusses the concept of negative acceleration and how it can result in slowing down or going backwards while speeding up. There is confusion about how to determine the direction of the object's movement when only the acceleration is known. It is explained that the choice of positive and negative direction is arbitrary and that acceleration can either reduce the object's speed or cause its trajectory to curve.
sallychan
I know that "negative acceleration" can be slowing down, or going backward and speeding up. I read the old posts here and I am still confused.

If no velocity are given, and you only know that the acceleration is, say, a= -5 m/s^2, how do you know the object is going forward or backward?

sallychan said:
IIf no velocity are given, and you only know that the acceleration is, say, a= -5 m/s^2, how do you know the object is going forward or backward?

Hi sallychan. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

You don't know anything about the current velocity (or displacement) when all you are told is acceleration.

Last edited by a moderator:
What is "positive" direction and what is "negative"?
That is an arbitrary choice WE make!
When we use the concept of RETARDATION then that means the acceleration is in opposite direction of the velocity, where "opposite" means along the same line the velocity lies, but in the other direction (that is, 180 degrees shifted from the velocity direction). Retardation always reduces the SPEED of the object.

If the acceleration is ORTHOGONAL to the velocity, that is with a direction 90 degrees to the direction of the velocity, the acceleration will not change the speed of the object, but will make its trajectory curved, the sign of the acceleration telling us to "which side" the object will curve. We call such acceleration centripetal acceleration.

## 1. What is negative acceleration?

Negative acceleration, also known as deceleration, is a decrease in the velocity of an object over time. It occurs when an object is moving in the opposite direction of its acceleration.

## 2. How is negative acceleration represented?

Negative acceleration is typically represented by a negative sign or a downward arrow in equations and diagrams. It can also be represented graphically by a downward slope on a velocity-time graph.

## 3. What causes negative acceleration?

Negative acceleration can be caused by various factors, such as friction, air resistance, and changes in direction. It can also occur when an object is moving against a force, such as gravity.

## 4. What is the difference between negative acceleration and negative velocity?

Negative acceleration refers to a decrease in velocity, while negative velocity refers to an object moving in the opposite direction of its initial velocity. In other words, negative acceleration is a change in velocity, while negative velocity is a direction of movement.

## 5. How is negative acceleration related to Newton's Second Law of Motion?

Newton's Second Law of Motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This means that a greater negative acceleration can be achieved with a larger net force and/or a smaller mass.

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