v² − u² = 2as for braking distances and falling objects, and the SUVAT solver for checking your working.
What you will learn
- Use v² − u² = 2as
- Work out falling speeds with g = 9.8 m/s²
- Rearrange an equation of motion
Five letters, one equation
When the acceleration is constant, five quantities describe the motion: s (displacement), u (initial velocity), v (final velocity), a (acceleration) and t (time).
GCSE gives you this one on the equation sheet:
v² − u² = 2 a s
It has no t in it, so use it when the question does not mention time.
- A car at 20 m/s brakes at 5 m/s² to a stop. Braking distance?
- u = 20, v = 0, a = −5 (slowing, so negative)
- 0 − 20² = 2 × (−5) × s
- −400 = −10 s, so s = 40 m
Write down the three letters you know and the one you want before you touch the numbers. Most mistakes are a wrong sign on a.
The rest of this lesson and its 7 practice questions are locked.
In this lesson
- Five letters, one equation
- Falling objects
- Try it: Check your working with the solver
- Try it: Sideways does not change the fall
7 practice questions, marked as you go; about 25 minutes in all.
In this unit: Forces and motion
Speed, velocity and acceleration, the graphs that show them, the equations of motion, Newton's laws and stopping distances.