Question

HELP ME FAST A car travels at a constant velocity of 70 mph for one hour. By the end of the second hour, the car’s velocity was 60 mph. At the end of the third hour, the car’s velocity was 80 mph. During which hour was their acceleration positive? During which hour was their acceleration negative?A truck speeds up from a velocity of 6 m/s to 14 m/s in 4 seconds. What is the truck’s acceleration?

Answer 1

Velocity in 1st hour = 70 mph

Velocity at the end of 2nd hour = 60 mph

Velocity at the end of 3rd hour = 80 mph

Now, acceleration is the rate of change of velocity

We need to determine when was acceleration positive and when was it negative. Positive acceleration means speed is increasing, and negative means decreasing.

We see that by the end of the 3rd hour the velocity has increased vs. the end of the 2nd hour, hence in the 3rd hour acceleration was positive

We see that by the end of the 2nd hour the velocity has decreased vs. the 1st hour, hence in the 2nd hour acceleration was negative

Initial velocity of the truck = 6 m/s

Final velocity of the truck = 14 m/s

Time in which the velocity changed = 4 seconds

Acceleration = $\frac{\text{Final velocity of the truck - Initial velocity of the truck}}{\text{Time}}$

⇒ Acceleration = $\frac{14-6}{4}$

⇒ Acceleration = $\frac{8}{4}$

⇒ Acceleration = 2 m/s²