Physics / Unit 1: Motion in One Dimension
1.6 Free Fall
Notes
An object in free fall moves only under gravity (air resistance ignored). Its acceleration is $g = 9.8 \text{ m/s}^2$ downward, whatever its mass.
- Use the kinematic equations with $a = -9.8 \text{ m/s}^2$ if up is positive.
- At the top of a vertical throw, velocity is zero but acceleration is still $9.8 \text{ m/s}^2$ downward.
- Without air, a feather and a hammer fall together.
- Example: a rock dropped from rest falls for 2 s. $v = 9.8 \times 2 = 19.6 \text{ m/s}$; distance $\displaystyle = \frac{1}{2}(9.8)(2^2) = 19.6 \text{ m}$.
Practice
A ball is dropped from rest. How fast is it moving after 3 s? (Ignore air resistance.)
$v = gt = 9.8 \times 3 = 29.4 \text{ m/s}$.
A ball is thrown straight up. At its highest point, which is true?
Gravity never switches off, so the acceleration stays $9.8 \text{ m/s}^2$ down.
A stone is dropped from rest off a bridge and hits the water 2.5 s later. How high is the bridge?
$\displaystyle d = \frac{1}{2}(9.8)(2.5^2) = 4.9 \times 6.25 = 30.6 \text{ m}$.
Ready for more? Take the Unit 1 test.