9709 · 4.2
Kinematics of motion in a straight line — FAQ
Frequently asked questions for 9709 Kinematics of motion in a straight line. Direct answers first, then deeper explanation — then practise with marking.
What is the difference between speed and velocity?
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.
When should I use g = 9.8 m s⁻² and when should I use g = 10 m s⁻²?
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.
How do I choose the right SUVAT equation?
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.
What does it mean if I calculate a negative displacement or velocity?
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.