Force, mass, acceleration: F = ma
A force is a push or a pull -- it can start, stop, speed up, slow or turn an object. Force = mass x acceleration (F = ma): force in Newtons (N), mass in kg, acceleration in m/s^2. An increase in force causes acceleration; a decrease, or a force opposing motion, causes deceleration. For a fixed force, a larger mass gives a smaller acceleration. Note: mass (matter, kg, constant) is NOT weight (gravity's pull on that mass).
Forces on a performer and a sprinter
Three recurring forces act on a moving performer. Gravity pulls them down. Air resistance opposes motion and grows with speed. Muscular force comes from the performer's own muscles. A sprinter leaving the blocks is the examinable case: gravity acts downward; the ground reaction force is the track pushing back on the driving feet, propelling the sprinter forward and up; air resistance opposes forward motion, small at first.
Forces on an object in flight
Once a performer releases an object -- a shot, javelin or ball -- three forces decide its flight. The force at release is the push given just before it leaves the hand or foot, setting its initial speed and direction. Air resistance opposes its motion, slowing it. Gravity continuously pulls it back down, so every thrown or kicked object curves downward and lands rather than travelling straight forever. A greater force at release sends it further.
Drawn from real examiner reports.
Mass defined as weight; acceleration vague
Two definition slips. Mass vs weight: mass is not an object's "weight". Mass is the amount of matter (kg), constant; weight is the pull of gravity on it. Acceleration: answers drop "increase", giving "how fast it moves", and miss that it is an increase in speed or velocity. Lock it: mass = matter (kg); acceleration = increase in speed/velocity.
Digest (### 1.13): mass is wrongly defined as the "weight" of an object, and the definition of acceleration frequently omits "increase in speed/velocity" (s23 P12 Q13a).
Effort drawn to the bone, not the muscle
On lever diagrams, forces must be drawn at the point they act. A common error draws the effort line to the forearm bones rather than the biceps muscle that generates it; components are also mislabelled or unlabelled. Mark the effort at the muscle's insertion, the fulcrum at the pivoting joint, the resistance at the load -- and label all three.
Digest (### 1.13): lever diagrams draw the effort to the forearm bones rather than the bicep muscle, and components are mislabelled/unlabelled (s23 P11 Q7b(i)-(ii); s22 P13 Q8c; w23 P13 Q2b; s22 P12 Q15).
Lever class confused with its movement
Candidates confuse a lever's class with the movement it produces -- naming what the joint does ("it bends the arm") instead of the class, set by the order of fulcrum, effort and resistance. It depends on which component is in the middle, not how it looks. Examples come easily for third-class levers (kicking) but weaker for first-class (heading).
Digest (### 1.13): lever class is confused with the movement it produces; third-class examples (kicking) are secure but first-class (heading) weaker (s23 P11 Q7b(i)-(ii); s22 P13 Q8c; w23 P13 Q2b; s22 P12 Q15).
Force answers with no unit (N)
In a force calculation, an otherwise-correct answer with no unit, or the wrong unit, loses a mark. Force is measured in Newtons (N). Always write the formula (force = mass x acceleration), substitute the numbers, and finish with N. For deceleration, enter acceleration as negative and keep the minus sign -- it shows the force opposes the motion.
Bigger mass, same force = less acceleration
From F = ma, if force is fixed then acceleration = force / mass, so a larger mass accelerates less, a smaller mass more. Candidates sometimes assume the heavier performer accelerates faster, or muddle mass with force. For two performers pushing off with the same force, the lighter one accelerates faster -- mass and acceleration are inversely linked at constant force.
Forgetting the ground reaction force
Asked for the forces on a sprinter in the blocks, candidates often name only gravity and air resistance and forget the ground reaction force -- yet that is the force that actually drives the sprinter forward. It is the track/blocks pushing back on the feet as the sprinter drives against them. Describing it vaguely as "pushing off" also loses precision marks.
The fulcrum is always in the middle
Students learn "fulcrum in the middle" as a rule for ALL levers. It fits only a first-class lever (Effort-Fulcrum-Resistance, e.g. heading a ball). In a second-class lever (calf raise) the resistance is central; in a third-class lever (kicking a ball) the effort is central. The class depends on which component is in the middle.
Digest (### 1.13): lever CLASS is confused with the MOVEMENT it produces, and components are mislabelled/unlabelled on diagrams (s23 P11 Q7b(i)-(ii); s22 P13 Q8c; w23 P13 Q2b; s22 P12 Q15).
Name the class from the middle component
Identify a lever class by which component sits in the middle: fulcrum in the middle = first class; resistance in the middle = second class; effort in the middle = third class. Work out the order along the lever rather than judging from the action.
Label a lever diagram at the exact points
Mark the fulcrum at the pivoting joint, the effort where the muscle pulls (its insertion, not a generic spot on the bone), and the resistance where the load acts. Then read off the order of the three points to name the class -- do not guess it from the movement.
Show F = ma and finish with the unit
In force calculations write the formula (force = mass x acceleration), substitute the numbers, and end with the unit Newtons (N) -- a number with no unit loses a mark. Where possible apply it to a named performer, as generic answers are a repeated Paper 1 criticism.
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