Question

An object of mass 10 kg is pulled across a rough floor at a constant speed of 4 m/s using a force of
40 N, then the coefficient of kinetic friction is (ignoring air resistance):
a) 0.4 b) 4 c) Zero d) 0.25 e) 2.5

Answer 1

Answer on Question #41030, Physics, Mechanics

An object of mass $10\,\mathrm{kg}$ is pulled across a rough floor at a constant speed of $4\,\mathrm{m/s}$ using a force of $40\,\mathrm{N}$, then the coefficient of kinetic friction is (ignoring air resistance):

a) 0.4 b) 4 c) Zero d) 0.25 e) 2.5

Solution:

The equation of motion is

$$F - F_{fr} = ma,$$

where force is $F = 40\,\mathrm{N}$, force of friction is $F_{fr} = \mu\mathrm{N}$ ($N = mg$, $\mu$ is the coefficient of kinetic friction), the acceleration is $a = 0$ (object is moved at a constant speed).

Thus,

$$F_{fr} = F$$ $$\mu mg = F$$ $$\mu = \frac{F}{mg} = \frac{40\,\mathrm{N}}{10\,\mathrm{kg} \cdot 10\,\mathrm{m/s^2}} = 0.4$$

Answer. a) 0.4