Frictional Force

The effect of air resistance on falling objects does much to muddy our understanding of gravitational force because the effect of one impacts on the other. Air resistance is a type of frictional force and it is worth considering some general points about friction first.

The suggested sequence of ideas associated with the concept of friction is that the children are moved from an understanding of ‘grip’ as a property of an uneven-even surface to an appreciation of friction as a force acting to prevent relative movement of two surfaces in contact.

A rough surface impedes the movement of an object across it

Different surfaces impede this movement to different extents

The experience that children have of slippery surfaces such as frozen puddles (i.e. low friction) and ‘gripping’ surfaces such as deep pile carpets (i.e. high friction) and the impact they have on movement means that many of them bring a well established ’life-view’ of friction. In the context of science, this life-view needs to be clarified and extended.

Friction is the scientific term for the force that changes movement as a result of surface/surface interaction (all changes of movement require the action of a force).The direction of the frictional force is opposite to that of the movement and should be represented as such when using diagrams to show forces in action. Therefore, a diagram of a pulled object being dragged along the floor would have an arrow pointing in the direction of movement representing the ‘pull’ and an arrow in the opposite direction (not down) representing the drag of friction. If the speed that the object is moving is constant, the arrows should be drawn of equal length (i.e. the forces are balanced: see next section).

Gravitational force contributes to the amount of friction through its effect on the downward force of the object on the surface it is standing. An object of large mass is pulled down onto a surface with a greater force than an object of low mass and, as a consequence, there is greater friction between the surface of the heavy object than between the surface and the light object.

Both of the surfaces in contact contribute to the magnitude of the frictional force

The characteristic of a high friction surface tends to be coarseness. There is more friction between two coarse materials rubbing against each other than between a coarse and a smooth surface. Equally, there is relatively little friction exerted between two smooth surfaces.

Friction can occur even in the absence of movement

Sometimes, the frictional force is so great that it prevents movement. Unless enough pull or push is exerted on a stationary object to overcome the friction between it and the surface it stands on then it will not move. Covering ramps with different materials, placing objects on them and then inclining them until the object slides off is a good way of demonstrating static/dynamic frictional states.

Perhaps one of the best and thought provoking demonstrations of how the degree of downward force impacts on the amount of friction is the ‘Trick with the Stick’. If a metre stick (although this works with any length, even a pencil) is supported by both hands as shown and the left hand slides underneath the stick in an attempt to slowly meet the static right hand the instinctive response would be to say that the stick will not be balanced and fall off, tumbling to the left.

However as the downward force of the stick on the left hand increases as the hand moves towards the centre, the friction between stick and hand becomes so great the stick ‘locks’ and starts to slide over the right hand instead.

The process then repeats itself as the downward force on the right hand reaches frictional ‘locking point’. The stick continues to slide first over one hand then over the other until the left hand meets the right hand with the stick lying with its middle (centre of gravity) positioned directly over the two touching hands.

Try adding a lump of plasticine to one end of the stick thereby shifting the centre of gravity towards the weighted end of the stick . . . what happens now?

The Physics of Falling
Gravitational Force
Air Resistance
Balanced and Unbalanced Forces
Self Assessment