Answer:
8.04 seconds
Step-by-step explanation:
Assuming that the child starts from rest at the bottom of the hill and travels until she comes to a stop, we can use the following kinematic equation:
v_f^2 = v_i^2 + 2ad
where v_f is the final velocity (which is zero since the child comes to a stop), v_i is the initial velocity (which is the velocity at the bottom of the hill), a is the acceleration (-0.392 m/s²), and d is the distance traveled.
We can solve for d:
d = (v_f^2 - v_i^2) / (2a)
= (0 - v_i^2) / (2-0.392)
= v_i^2 / 0.784
Since the child is sliding along flat snow-covered ground, there is no change in elevation, so we can use the distance traveled from the bottom of the hill to the stopping point as the distance d.
To find the time it takes for the child to travel this distance, we can use the following kinematic equation:
d = v_it + 0.5a*t^2
where t is the time and all other variables are as previously defined.
Substituting the expression for d obtained above, we get:
v_i^2 / 0.784 = v_it + 0.5(-0.392)*t^2
Solving for t, we get:
t = (2 * v_i) / 0.392
We still need to find the value of v_i, the initial velocity of the child at the bottom of the hill. To do so, we can use conservation of energy. The child starts at rest at the top of the hill, so all the initial energy is potential energy. At the bottom of the hill, all the potential energy has been converted to kinetic energy. Assuming no energy is lost to friction, we can equate these two energies:
mgh = 0.5mv_i^2
where m is the mass of the child, g is the acceleration due to gravity (9.8 m/s²), and h is the height of the hill.
Solving for v_i, we get:
v_i = √(2gh)
Substituting this expression for v_i into the expression for t obtained earlier, we get:
t = (2 * √(2gh)) / 0.392
Plugging in the values of g, h, and a, we get:
t = (2 * √(29.820)) / 0.392 = 8.04 seconds