Respuesta :
Answer:
The time taken for the upward motion is 1 second. The same time is taken for the downward motion
It reaches a maximum height of 4.9 meters.
Step-by-step explanation:
The equation of motion is:
[tex]x(t) = -4.9t^{2} + 9.8t[/tex]
Since the term which multiplies t squared is negative, the graph is concave down, that is, x increases until the vertex, where it reaches it's maximum height, then it decreases.
Vertex of a quadratic equation:
Quadratic equation in the format [tex]x(t) = at^{2} + bt + c[/tex]
The vertex is the point [tex](t_{v}, x(t_{v}))[/tex], in which
[tex]t_{v} = -\frac{b}{2a}[/tex]
In this question:
[tex]x(t) = -4.9t^{2} + 9.8t[/tex]
So [tex]a = -4.9, b = 9.8[/tex]
Vertex:
[tex]t_{v} = -\frac{9.8}{2*(-4.9)} = 1[/tex]
The time taken for the upward motion is 1 second.
[tex]x(t_{v}) = x(1) = 9.8*1 - 4.9*(1)^{2} = 4.9[/tex]
It reaches a maximum height of 4.9 meters.
Downward motion:
From the vertex to the ground.
The ground is t when x = 0. So
[tex]-4.9t^{2} + 9.8t = 0[/tex]
[tex]4.9t^{2} - 9.8t = 0[/tex]
[tex]4.9t(t - 2) = 0[/tex]
[tex]4.9t = 0[/tex]
[tex]t = 0[/tex]
Or
[tex]t - 2 = 0[/tex]
[tex]t = 2[/tex]
It reaches the ground when t = 2 seconds.
The downward motion started at the vertex, when t = 1.
So the duration of the downward motion is 2 - 1 = 1 second.
