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Braking Car and Power Dissipation

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Unit: Work, Energy, and Power

Prerequisites

Power: Rate of Energy Transfer

Later Topics

None

Multi-Step Problem Preview

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Part 1

Consider a car moving on a level road. The car has mass m=1062 kgm = 1062\text{ kg} and initial speed vi=22.5 m/sv_i = 22.5\text{ m/s}. The brakes exert a constant horizontal braking force of magnitude Fb=4712 NF_b = 4712\text{ N} opposite the direction of motion. The acceleration due to gravity is g=9.8g = 9.8 m/s2^2 (not needed in this part).

What is the initial kinetic energy KiK_{i} of the car?

Correct!

Solution:

Solution

The kinetic energy of a body of mass mm moving with speed viv_i is

Ki=12mvi2.K_i = \frac12 m v_i^2.

Substituting m=1062 kgm = 1062 \, \mathrm{kg} and vi=22.5 m/sv_i = 22.5 \, \mathrm{m/s},

Ki=12⋅1062⋅22.52=268818.75 J.K_i = \frac12 \cdot 1062 \cdot 22.5^2 = 268818.75\,\mathrm{J}.

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