The force the ground exerts on an athlete, equal and opposite to the force applied (Newton's third law).
Friction
Force opposing sliding between two surfaces; increased by studs and spikes.
First law: a ball stays at rest until kicked. Second law: a bigger force gives a bigger acceleration. Third law: pushing back on the blocks produces a forward ground reaction force.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Impulse questions almost always come down to: same change in momentum, longer time → smaller force (or larger impulse → more speed).
B.2.2Projectile motion and fluid dynamics
Key terms
Projectile
An object in flight affected only by gravity and air resistance.
Angle of release
Angle at which an object is released relative to the horizontal.
Drag
Air or water resistance opposing motion.
Magnus effect
Spin makes a ball curve because of a pressure difference across it.
Distance depends on speed of release (most important), angle of release and height of release.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Speed of release has the biggest effect on distance: mention it first.
B.2.3Angular motion and stability
Key terms
Centre of mass
The point where the body's mass is concentrated; it can lie outside the body.
Moment of inertia
Resistance to rotation; depends on mass and how far it is from the axis.
Angular momentum
Moment of inertia × angular velocity; conserved in the air.
Stability
Resistance to losing balance; increased by a low centre of mass and a wide base of support.
A body is stable when its line of gravity falls within the base of support. Lowering the centre of mass and widening the base increase stability (wrestler's stance).
The full notes, worked example and check questions for this subtopic are in the app.
Exam tip: Use 'angular momentum is conserved' as the key phrase whenever an athlete changes body shape in the air.