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and
<math>a = \frac{\Delta v}{t}</math> or <math>a = \frac{v-u}{t}</math>
Gives
<math>\frac{F}{m} = \frac{\Delta v}{t}</math>
<math>F = \frac{m \Delta v}{t}</math> or <math>F = \frac{mv - mu}{t}</math>
Where:
<math>t</math> = The time taken for the [[momentum]] to change.
<math>m</math> = The [[mass]] of the [[object]].
<math>mv</math> = The final [[momentum]] of the [[object]].
<math>mu</math> = The initial [[momentum]] of the [[object]].
===Example Calculations===
{| class="wikitable"
| style="height:20px; width:200px; text-align:center;" |A car travelling at 20m/s collides with a lamppost and comes to a complete stop in 0.16 seconds. If the passenger has a [[mass]] of 80kg, calculate the [[force]] exerted by the seat belt on the passenger correct to two [[Significant Figures|significant figures]].
| style="height:20px; width:200px; text-align:center;" |During a crash the air bag activates. By the time the car stops the drivers 5kg head is still moving forward with a velocity of 2.4m/s and collides with the airbag. This collision lasts for 0.25 seconds. Calculate the [[force]] between the air bag and the person's head during the collision.
|-
| style="height:20px; width:200px; text-align:left;" |'''1. State the known quantities'''
Δv = 20m/s
t = 0.16s
m = 80kg
| style="height:20px; width:200px; text-align:left;" |'''1. State the known quantities'''
Δv = 2.4m/s
t = 0.25s
m = 5kg
|-
| style="height:20px; width:200px; text-align:left;" |'''2. [[Substitute (Maths)|Substitute]] the numbers into the [[equation]] and [[Solve (Maths)|solve]].'''
<math>F = \frac{m \Delta v}{t}</math>
<math>F = \frac{80 \times 20}{0.16}</math>
<math>F = \frac{1600}{0.16}</math>
<math>F = 10000N</math>
| style="height:20px; width:200px; text-align:left;" |'''2. [[Substitute (Maths)|Substitute]] the numbers into the [[equation]] and [[Solve (Maths)|solve]].'''
<math>F = \frac{m \Delta v}{t}</math>
<math>F = \frac{5 \times 2.4}{0.25}</math>
<math>F = \frac{12}{0.25}</math>
<math>F = 48N</math>
|}