# is deceleration always negative?

• dawgdays
5 days ago

No, deceleration is not always negative.

Deceleration is used to describe an acceleration that is directed opposite to an object’s current motion. If the object is moving in the positive direction, deceleration would be a negative acceleration. If the object were moving in the negative direction, deceleration would be a positive acceleration, which makes “deceleration” a confusing word to use.

Accelerations, on the other hand, are positive or negative in reference to the coordinate system being used. They do not depend on the current direction of motion of an object, and reversals of direction do not cause the confusion described above.

Because of this, I suggest you quit using the term deceleration in physics problems.

• Steve4Physics
5 days ago

Deceleration is a positive value when the *speed* of something is decreasing.

Do not confuse deceleration with negative acceleration – which means acceleration in the negative direction.

For example, take upwards as positive and downwards as negative. When you throw a stone up (assuming no air resisitance), the acceleration (g = -9.8m/s²) is alway downwards, always negative.

So when the stone is rising, its velocity is upwards (positive) and acceleration (g) is downwards. The stone gets slower: its decleration is positive (deceleration = 9.8m/s²).

But when the stone falls, its velocity is downwards and acceleration is downwards. The stone gets faster: deceleration is negative (deceleration = -9.8m/s²).

The general rules are:

1) If velocity and acceleration are in opposite directions (one positive, the other negative) speed decreases so deceleration is a positive value.

2) If velocity and acceleration are in the same direction (both positive or both negatie) speed increases so deceleration is a negative value.

Mathematically you can write:

deceleration = -d|v|/dt

where |v| is the magnitude of velocity (i.e. speed)

• ignoramus
5 days ago

It depends upon what interpretation you put upon the word. Most answers so far seem to deal with it as a synonym for “force”. Which is not strictly correct, and could be ambiguous. Ambiguity should be avoided always.

“Deceleration” rather refers to the EFFECT of a force on a moving object , and as such, it implies a slowing down. That is all. So, in effect, it is a negative effect. But whether you call the effect of the deceleration positive or negative depends upon how you have defined the problem, which direction you call positive, and which direction negative.

I think that you should avoid using the word. Always refer to “acceleration”, which can be either positive or negative, and refer to a slowing down as well as a speeding up.

• spot a
5 days ago

No. Deceleration is always the exact opposite of acceleration

A ball thrown upwards gradually slows due to gravity then falls faster and faster towards the ground

Relative to the ball, the acceleration due to gravity in the upwards direction is -ve and the deceleration is +ve

Relative to the ball, the acceleration due to gravity in the downwards direction is +ve and the deceleration is -ve

• Anonymous
5 days ago

deceleration is a gradual decrease of a velocity.

so, a deceleration is always a negative (inverted) value of an acceleration, because by make a velocity of a moving object slower gradually and constantly, that means the object’s velocity is having deceleration.

thanks.

• Enough Trolls
5 days ago

Deceleration is acceleration in another direction.

• Old Science Guy
5 days ago

deceleration is always opposite to the direction of travel

so

if moving in + direction then decel is –

but

if moving in – direction then decel is +

When you get a good response,