WASSCE SHS
When acceleration is constant, the equations of motion connect speed, time and distance. A start from rest means the initial speed is zero.
Speed is distance over time and has no direction. Velocity has direction as well. Acceleration is the change in velocity over time. The equations you use at this level assume acceleration does not change. If a body starts from rest, the initial velocity u is 0. Final velocity v equals u plus at. Distance s equals ut plus half at squared. When u is 0 those shorten to v = at and s = half at squared. Write the symbols you were given before you substitute, so a marker can see that you chose the right equation.
A negative acceleration means the body is slowing down if the positive direction is the way it is moving. It is still an acceleration. Units: metres, seconds, metres per second, metres per second squared. A distance in metres per second is a unit error and it costs the unit mark. These equations do not apply when acceleration changes from moment to moment, for example air resistance growing with speed. The questions here keep acceleration constant.
The equation is a mark. The substitution is a mark. The answer with unit is a mark. u = 0 must be stated or used when the body starts from rest. A candidate who uses a constant speed formula for an accelerating body has the wrong model. Direction matters only when the question gives one. If it does not, a positive value in the direction of motion is enough.
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
5 marks
A car starts from rest and accelerates at 2 m/s² for 5 s.
(a) Find its speed at the end of the 5 s. (2 marks)
(b) Find the distance it travels in that time. (3 marks)
(a) v = u + at = 0 + 2 × 5 = 10 m/s.
(b) s = ut + ½at² = 0 + ½ × 2 × 25 = 25 m.
2 marks
State what is meant by a constant acceleration of 2 m/s². (2 marks)
The velocity increases by 2 metres per second every second, and that rate of increase does not change.
2 marks
Why can you not use s = ½at² for a falling leaf that quickly reaches a steady speed? (2 marks)
The equation needs constant acceleration. Once the leaf falls at a steady speed its acceleration is zero, and before that air resistance made the acceleration change.
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Practise Physics freePut the initial speed equal to zero. Final speed is acceleration times time. Distance is half times acceleration times time squared. For an acceleration of 2 m/s² lasting 5 s, the speed is 10 m/s and the distance is 25 m.
When the acceleration is changing. A leaf that reaches a steady speed is no longer accelerating at a constant rate, because air resistance grows as it speeds up. Use the equations only for the part of the motion where acceleration is constant. All WASSCE Physics past questions