y /m 5
4
3 y 2 u0 1 θ
x
0
0
5
10
15
20
25
30
35 x /m
Figure 2.19 A launch at of = 30° to the horizontal with initial speed 20 m s−1.
At what point in time does the vertical velocity component become zero? Setting vy = 0 we find: 0 = u sin − gt ⇒ t=
u sin g
The time when the vertical velocity becomes zero is, of course, the time when the object attains its maximum height. What is this height? Going back to the equation for the vertical component of displacement, we find that when: t=
Figure 2.20 A real example of projectile motion!
u sin g
y is given by: ymax = u ymax =
u sin 1 u sin sin − g 2 g g
2
u2 sin2 2g
Exam tip You should not remember these formulas by heart.You should be able to derive them quickly.
What about the maximum displacement in the horizontal direction (sometimes called the range)? At this point the vertical component of displacement y is zero. Setting y = 0 in the formula for y gives: 0 = ut sin − 12 gt 2 0 = t (u sin − 12 gt) and so: t=0
and
t=
2u sin g 2 MECHANICS
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