Cracking the SAT Physics Subject Test

Test will never use the

term deceleration, as this

is just a negative acceleration

that slows an

object’s speed.

Notice that an object can accelerate even if its speed doesn’t change. (Again, don’t
let the everyday usage of the word accelerate confuse you!) This is because
acceleration depends on ∆v, and the velocity vector v changes if (1) speed changes,
(2) direction changes, or (3) both speed and direction change. For instance, a car
traveling around a circular racetrack is continuously accelerating even if the car’s
speed is constant, because the direction of the car’s velocity vector is constantly
changing.

5. A car is traveling in a straight line along a highway at a constant
   speed of 80 miles per hour for 10 seconds. Find its acceleration.

Here’s How to Crack It

Since the car is traveling at a constant velocity, its acceleration is zero. If there’s no
change in velocity, then there’s no acceleration.

6. A car is traveling in a straight line along a highway at a speed of
   20 m/s. The driver steps on the gas pedal, and 3 seconds later, the
   car’s speed is 32 m/s. Find its average acceleration.

Here’s How to Crack It

Assuming that the direction of the velocity doesn’t change, it’s simply a matter of
dividing the change in velocity (32 m/s − 20 m/s = 12 m/s) by the time interval

during which the change occurred: ā = ∆v/∆t = (12 m/s) / (3 s) = 4 m/s^2.