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So, to calculate the acceleration of an object, you divide the change in its velocity, by the time taken for that change. In physics, change is always calculated as the final value minus the initial value. So, if Jerry is going at $$10 \, \mathrm{m/s}$$ and his velocity becomes $$30 \, \mathrm{m/s}$$, the change in velocity is $$20 \, \mathrm{m/s}$$.

If an object's velocity decreases, the final velocity will be less than the initial velocity. In this case, the acceleration will be negative. If the acceleration is negative, it is called deceleration. For example, when you apply brakes on your bicycle, your velocity decreases, and you are decelerating.
We saw that Jerry's velocity changed from $$2.5 \, \mathrm{m/s}$$ at $$2 \, \mathrm{s}$$ to $$5 \, \mathrm{m/s}$$ at $$4 \, \mathrm{s}$$. What is his acceleration in $$\mathrm{m/s^2}$$?
• $$\frac{2.5 - 5}{2}$$
• $$\frac{5 - 2.5}{2}$$
• $$\frac{2.5 + 5}{2}$$
• $$\frac{2.5 + 5}{2 + 4}$$