The Equations
Quite often in physics we consider objects moving with constant acceleration, or constant acceleration can be assumed. For motion with constant acceleration there are 5 equations relating acceleration, velocity, displacement and time. The quantities involved in the equations are given the following symbols:
s = Displacement
u = Initial Velocity
v = Final Velocity
a = Acceleration
t = Time
Each of the 5 equations misses out one of these quantities:
Equation 1 – Without v
Equation 2 – Without u
Equation 3 – Without a
Equation 4 – Without s
Equation 5 – Without t
If you would like a simple derivation of these equations using a velocity-time graph please let me know in the comments below.
Tips
Here are some tips on how I tackle questions.
- Check the question is about motion with constant acceleration. If the acceleration is not constant, or can’t be assumed constant, you can’t use SUVAT equations.
- Write out each of the letters of SUVAT and pick out the information you know from the question. This way you can see what information you have and which equation you should use (this is shown in the worked examples below).
- If the question says the object starts at rest that means the initial velocity is 0 ms-1, if it ends at rest the final velocity is 0 ms-1.
- If an object is in free fall the acceleration is equal to the acceleration due to gravity; 9.81ms-2 (3sf).
Worked Examples
Example 1
A car travelling at 15ms-1 increases its speed at a steady rate to 20ms-1 in 30s. Calculate the acceleration of the car.
Example 2
A student drops a pen out of a window 4m above the ground. Calculate the time it takes for the pen to hit the ground (ignoring the effects of air resistance).
Example 3
A car starts from rest and speeds up with a constant acceleration of 2ms-2. Calculate the cars velocity after 50m from its starting point.
If you have any questions please leave them in the comments below and follow me on Instagram and Pinterest to keep up to date with new posts. If you’d like advice on how to prepare for an exam click here.
where did the 9.81 in acceleration come from in example 2
Hi m! When an object is dropped, like the pen in the question, it’s in free fall until it hits the ground. An object in free fall accelerates at a rate equal to the gravitational acceleration which on Earth is 9.81 ms-2. This acceleration remains the same for any object in free fall regardless of its mass. It might be worth checking out this post on free fall! Hope that helps 🙂