Question

Kinematics equations:

1. An angry scaffolder throws her spanner down to the ground with a velocity of 2 ms-1
the spanner takes 2 seconds to strike the ground.
Calculate the height the scaffolder was at when she threw down the spanner.

2. An experimental rocket is launched vertically with a velocity of 5ms-1
At a certain height the tail falls off and strikes the ground after 4 seconds.
Assuming the tail falls off while the rocket is travelling at 5 ms-1, calculate the height at which it
fell off.

Answer 1

Explanation

• Motion under gravity is applied here.
• Proper knowledge of the definitions of the terms used in the 4 basic equations (v=u+at..) is required.

Neglect the projectile appearance of the 2nd sketch & consider vertically.

Calculations

1).

$\qquad \begin{aligned} \small \downarrow s &= \small ut + \frac{1}{2}at^2\\ \small h&= \small 2ms^{-1}\times2s + 0.5\times9.8ms^{-2} \times (2s)^2\\ &= \small \bold{23.6m} \end{aligned}$

2).

$\qquad \begin{aligned} \small \downarrow s &= \small ut + \frac{1}{2}at^2\\ \small h &= \small (-5ms^{-1})\times 4s + 0.5\times 9.8ms^{-2} \times (4s)^2\\ &= \small \bold{58.4m} \end{aligned}$

Good Luck