Question

A 750 kg car is at rest at a red light. If the light turns green and it zooms away with an acceleration of 7.8 m/s2 how fast will it be traveling in 7 seconds?

Answer 1

Answer:

• 75 m/s

Explanation:

1) Data:

a) m =750 kg

b) V₀ = 0 (at rest at the red light)

c) a = 7.8 m/s²

d) t = 7.0 s

e) Vf (at t = 7 s) = ?

2) Physical principles and formulae

Uniformly accelerated motion is ruled by the equations:

i. Vf= V₀ + at

ii. d = V₀ t + at² / 2

iii. Vf² = V₀² + 2ad

Where:

• V₀ = initial speed
• Vf = final speed
• a = acceleration
• d = distance run

3) Solution

You just need to use the first equation:

• Vf= V₀ + at = 0 + 7.8 m/s² × 7 s = 74.6 m/s
• Round to two significant figures: 75 m/s