The sign of an acceleration gives the direction of which quantity?
You've already seen what an acceleration's sign means: the direction of the velocity change. Put that sign next to the velocity's sign, and together they tell you whether the object is speeding up or slowing down.
Start with a cart at v = +6 m/s and a = +2 m/s². Its velocity runs +6, +8, +10 m/s. The speed grows. Same sign: speeding up.
Flip one thing, the acceleration. With v = +6 m/s and a = −2 m/s², the velocity runs +6, +4, +2 m/s. The speed falls. Opposite signs: slowing down.
Now flip the velocity too. With v = −6 m/s and a = −2 m/s², the velocity runs −6, −8, −10 m/s. The speed grows: 6, then 8, then 10 m/s. Same sign again: speeding up.
Last flip. With v = −6 m/s and a = +2 m/s², the velocity runs −6, −4, −2 m/s. The speed falls. Opposite signs: slowing down.
'Speed' here is the size of the velocity, the sign stripped off.
An object's velocity and acceleration have opposite signs. What is happening to its speed?
One more name you will hear: slowing down while moving forward is sometimes called a deceleration.
While moving forward, a deceleration of 5 m/s² is the same thing as an acceleration of −5 m/s². The 'moving forward' matters: for an object moving backward, an acceleration of −5 m/s² speeds it up.
Here are two quick reads. An object's velocity and acceleration are both negative. Both signs are negative, the same sign, so it is speeding up.
An object's velocity is positive and its acceleration is negative. One sign is positive, the other negative, so it is slowing down.
Which sign combination means an object is slowing down?
Which sign combination makes an object speed up while it moves in the negative direction?
Which statement is the speed-up/slow-down rule?
A student claims a negative acceleration always slows an object down. Which sign combination shows the claim is wrong?