Forces | 9702 Physics Summary Notes

• 5. Forces | 9702 Physics Summary Notes

· Force: rate of change of momentum

· Finding resultant (nose to tail):

o By accurate scale drawing

o Using trigonometry

· Forces on masses in gravitational fields: a region of space in which a mass experiences an (attractive) force due to the presence of another mass.

· Forces on charge in electric fields: a region of space where a charge experiences an (attractive or repulsive) force due to the presence of another charge.

· Upthrust: an upward force exerted by a fluid on a submerged or floating object

· Origin of Upthrust:

Pressure on Bottom Surface > Pressure on Top Surface

 Force on Bottom Surface > Force on Top Surface

 Resultant force upwards

· Frictional force: force that arises when two surfaces rub

o Always opposes relative or attempted motion

o Always acts along a surface

o Value varies up to a maximum value

· Viscous forces:

o A force that opposes the motion of an object in a fluid;

o Only exists when there is motion.

o Its magnitude increases with the speed of the object

· Centre of gravity: point through which the entire weight of the object may be considered to act

· Couple: a pair of forces which produce rotation only

· To form a couple:

o Equal in magnitude

o Parallel but in opposite directions

o Separated by a distance

 

· Moment of a Force: product of the force and the perpendicular distance of its line of action to the pivot 

· Torque of a Couple: the product of one of the forces of the couple and the perpendicular distance between the lines of action of the forces.

· Conditions for Equilibrium:

o Resultant force acting on it in any direction equals zero

o Resultant torque about any point is zero.

· Principle of Moments: for a body to be in equilibrium, the sum of all the anticlockwise moments about any point must be equal to the sum of all the clockwise moments about that same point.