CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.3
Different types of forces ESBFA
Tension
Tension is the magnitudeof the force that exists inobjects like ropes, chains and struts that are providing
support. For example, there are tension forces inthe ropes supporting a child’s swing hanging from a
tree.
Contact and non-contact forces
In this chapter we have come across a number ofdifferent types of forces,for example a push or apull,
tension in a string, frictional forces and the normal force. These are all examples of contact forces where
there is a physical pointof contact between applying the force and the object. Non-contact forces are
forces that act over a distance, for example magnetic forces, electrostaticforces and gravitational forces.
When an object is placed on a surface, two types of surface forces can be identified. Frictionis a
force that acts betweenthe surface and the object and is parallel to thesurface. The normal force is a
force that acts betweenthe object and the surface and is perpendicularto the surface.
The normal force
A 5 kg box is placed on a rough surface anda 10 N force is appliedat an angle of 36,9◦to the
horizontal. The box does not move. The normal force (N or FN) is the force between the box and
the surface acting in thevertical direction. If this force is not present the box would fall through the
surface because the force of gravity pulls it downwards. The normal force therefore acts upwards. We
can calculate the normal force by considering all the forces in the vertical direction. All the forces in
the vertical direction must add up to zero because there is no movementin the vertical direction.
N + Fy+ Fg = 0
N + 6 + (−49) = 0
N = 43N upwards5 kgFg= 5 x 9,8 = 49 NN 10 N Fy= 10 sin 36,9◦= 6 NFf Fx= 10 cos 36,9◦= 8 NFigure 12.8: Friction andthe normal forceThe most interesting andillustrative normal forcequestion, that is often asked, has to do with a scale
in a lift. Using Newton’sthird law we can solve these problems quite easily.
When you stand on ascale to measure yourweight you are pulleddown by gravity. Thereis no
acceleration downwardsbecause there is a reaction force we call the normal force acting upwards on
you. This is the force that the scale would measure. If the gravitational force were less then the reading
on the scale would be less.