Kinematic Equations Formula

Kinematic Equations Formula

Kinematics is the study of objects in motion and their inter-relationships. There are four (4) kinematic equations, which relate to displacement, D, velocity, v, time, t, and acceleration, a.

a) D = vit + 1/2 at2     b) (vi +vf)/2 = D/t

c) a = (vf - vi)/t          d) vf2 = vi2 + 2aD

D = displacement

a = acceleration

t = time

vf = final velocity

vi = initial velocity

Kinematic Equations Formula Questions.

1) Bob is riding his bicycle to the store at a velocity of 4 m/s, when a cat runs out in front of him. He quickly brakes to a complete stop, with an acceleration of - 2m/s2. What is his displacement?

Answer: Because Bob is stopped, the final velocity, vf = 0. His initial velocity, vi = 4 m/s. The acceleration, a = -2m/s2. Time is not given, so use equation (d) for displacement, D, because it is not time-dependent.

vf2 = vi2 + 2aD

(0)2= (4 m/s)2 +2(- 2 m/s2)D

0 = 16 m2/s2 + (- 4m/s2)D

-16 m2/s2 = (- 4 m/s2)D

16 m2/s2 = 4 m/s2)D

(16 m2/s2) / (4 m/s2) = D

The total displacement is 4 m.

2) You travel at a constant velocity of 11 m/s for 5 minutes. How far have you traveled?

Answer: At constant velocity, vi = vf = 11 m/s. The time, t = 5 min, or t = (60 sec/min x 5 min) = 300 sec. Now use equation (b) to solve for displacement, D.

(vi + vf)/2 = D/t

D = [(vi + vf)/2] t

D = [(11 m/s + 11 m/s)/2] x 300 sec

D = (22 m/s)/2 x 300 sec

D = 11 m/s x 300 sec

D = 3,300 m The total displacement is 3, 300 m.

3) What is the acceleration of a car that speeds up from 11 m/s to 40 m/s after 10 seconds?

Answer: The Vi = 11 m/s. The vf = 40 m/s. Time, t = 10 s. Use kinematic equation c) to solve for acceleration.

a = (vf - vi)/t

a = (40 m/s - 11 m/s) /10 s

a = (29m/s)/10 s = 2.9 m/s2

4) If a car accelerates at 3.0 m/s2 from a complete stop, how long will it take to go 3000 m?

Answer: The acceleration, a = 2.9 m/s2, and the displacement, D = 3000 m. The car was at rest, so vi = 0. Use equation a) to solve for time.

D = vit + 1/2 at2

3000 m = 0t + 1/2 (3.0 m/s2)t2

3000 m = 1/2 (3.0 m/s2)/t2

3000 m/ 1.5 m/s2 = t2

2000 s2 = t2

t = 44.72 sec

Related Links:
Vectors & Scalars Quiz
Distance and Displacement Quiz
Speed and Velocity Quiz
Newton's Laws of Motion Quiz






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