Kinematics of motion in a straight line — common mistakes

Before applying a SUVAT equation, always list the variables you know and the one you need to find. This helps you select the correct equation. Remember, these equations are only valid if acceleration is constant.

When integrating to find velocity or displacement, do not forget the constant of integration, '+ C'. Use the initial conditions given in the problem (e.g., 'starts from rest at the origin' means v=0 and s=0 when t=0) to find the value of C.

Speed is a scalar quantity—it only has magnitude (e.g., 20 m s⁻¹). Velocity is a vector quantity—it has both magnitude and direction (e.g., 20 m s⁻¹ due north). In straight-line motion, we often use positive and negative signs to indicate direction.

The Cambridge International exam papers will always specify the value of 'g' to use on the front cover of the question paper. For P4 Mechanics, this is typically g = 10 m s⁻². However, always check the instructions in your specific exam. If you are doing practice problems from other sources, they may use g = 9.8 or 9.81.

First, write down the values you know for s, u, v, a, and t. Identify the variable you want to find. Then, look for the equation that contains the four variables you are working with (the knowns and the one unknown). Each of the five equations omits exactly one of the five variables.

A negative sign simply indicates direction. In a straight-line motion problem, you must first define a positive direction (e.g., upwards, or to the right). A negative displacement then means the object is on the opposite side of the origin from the positive direction. A negative velocity means the object is moving in the negative direction.