Define the term centre of mass.

The diagram shows a gymnast performing a piked somersault.

www.gymdrills4profs.com.

Explain the manipulation of the moment of inertia during the flight and landing phases.

The diagram shows a speed skater.

Apply the concept of the coefficient of static friction and dynamic friction to the start of a speed-skating sprint.

the point around which the mass of a body is evenly distributed

OR

the point which the body is balanced in all directions/ OWTTE;

the manipulation of moment of inertia directly affects the gymnast's angular velocity in order to conserve angular momentum throughout the skill

OR

rotating objects have angular motion, moment of inertia and angular velocity work inversely to conserve angular momentum once an object is in motion;

the moment of inertia of a rotating object can be changed by redistributing the mass of the object about the axis of rotation <enabling the gymnast to perform a somersault>;

at the start of the flight phase, the gymnast begins flexes their hips to reduce their moment of inertia;

the reduction in moment of inertia increases angular velocity, this allows the somersault to be executed;

prior to landing they extend their hips to increase moment of inertia;

increasing moment of inertia reduces rotation / slows the gymnast for landing;

when a force is applied by the skater to attempt to move from stationary to skating / overcome inertia, this is considered the coefficient of static friction;

at some point, the force applied is sufficient to overcome the inertia / static friction and the skater will begin to move;

once the skater is in motion, sufficient force is applied to overcome static friction, this is considered the coefficient of dynamic friction;

more force is required to overcome static friction than dynamic friction;

Most candidates attempted this question and were able to provide a suitable definition of centre of mass. Some candidates confused centre of gravity with centre of mass and further review of this concept would be beneficial for candidates.

Fewer candidates attempted this question in comparison to 5a. It appeared that candidates either knew the concept or didn't either scoring 4 marks or 0 marks. Candidates who struggled to access marks confused the relationship between angular velocity and moment of inertia or contradicted themselves. In terms of understanding the requirements of the question most candidates referred to the image and used this to break down their response to the question which was pleasing to observe.

Many candidates understood that static friction was greater than dynamic friction and were able to correctly apply both static and dynamic friction appropriately to demonstrate a good understanding of the concept. However further review of this area is required.