forces acting on a performer during linear motion

• what are the types of force

  A force changes a bodies state of motion.

  • An internal force – is applied when our skeletal muscles contract for example, the force generated as the quadriceps contract concentrically to extend the knee in a jump

  •An external force – comes from outside the body. E.g., friction, air resistance, and weight. Gravity is also an external force and is often described in terms of weight, as weight is the gravitational force that the earth exerts on a body to pull it down.
• vertical forces acting on a performer during linear motion -> weight

  Weight is a gravitational force that the earth exerts on a body, pulling it towards the centre of the earth (or effectively downwards). The greater the mass of the body the greater the weight pulling it downwards.

  Weight (N) = mass x acceleration due to gravity
• vertical forces acting on a performer during linear motion - reaction force

  Whatever the force acting on a performer during linear motion, a reaction force will be generated. Newtons 3rd law of motion – for every action force there is an equal and opposite reaction force. There is always a reaction force whenever two bodies  are in contact with one another.
• horizontal forces acting on a performer during linear motion - friction

  There are 2 types of frictional force: static and sliding.

  It is the force exerted on one surface by another when there is no motion between the two surfaces, Static friction occurs before an object starts to slide.

  When friction acts between two surfaces that are moving relative to one another sliding friction occurs.
• Friction acts in opposition to motion. Friction resists the slipping and/or sliding motion of two surfaces. An arrow is therefore drawn in the opposite direction of this slipping (usually in the same direction as motion).

  In skiing the friction arrow opposes motion as the slipping occurs in a forward direction.
• friction can be affected by the following factors:

  • The surface characteristics of the two bodies in contact: 100m runner wears running spikes -> increase friction, as the spikes make contact with the track & therefore maximise acceleration.

  • temperature of the two surfaces in contact: in curling, the ice is swept in front of the curling stone. The sweeping action slightly raises the surface temperature of the ice which reduces friction between the stone & the ice allowing the stone to travel further.

  • The mass of the objects that are sliding: a larger mass results in greater friction
• air resistance

  Air resistance is commonly referred to as ‘drag’ – most commonly used when describing resistance in water. There is a much greater drag force in water due to its greater density.
• air resistance opposes the motion of a body travelling through air and depends upon:


  • velocity of the moving body – faster performer moves = greater the air resistance

  • cross-sectional area of moving body: the larger the cross-sectional area the greater the air resistance. E.g., tour de France competitors crouch low over the handlebars, rather than sitting upright.

  • The shape & surface characteristics of a moving body: a streamlined shape = less air resistance so does a smooth surface e.g., elite swimmers shave off all body hair or wear half/full suits so they create a smooth surface.