Physics AS unit -2

Vector - any physical quantity that has a magnitude and direction e.g. velocity, displacement, force,momentum, acceleration.

Scalar - any physical quantity that has a magnitude only e.g. temperature, time, speed, distance, mass, energy, length.

Adding Vectors:

• Closed triangle
• Lines in same direction
• Arrow to end

Resultant force = magnitude, (pythagoras) Remeb: square root!!!

Angle = direction, feta = tan-1 (O/A)

Horizontal = cos

             horizontal component = vcos(feta)

Vertical = sin

             vertical component = vsin(feta)

Equilibrium = at rest or moving at constant velocity.

Conditions: the 2 forces are equal & opposite to each other, therefore is zero. - balanced.

If 3 forces acting:

Example:

If we have a hinged or pivoted body, any force applied changes the rotation of that body about the pivot, the turning effect is called a moment.

moment = force x perpendicular distance

moment units newton metres Nm - rembr- change any cm distances to metres!

moments have a direction, clockwise or anticlockwise moment about any point.                     moments must be balanced or the object will turn.

For the object to be in equalibrum the sum of the clockwise moments must = the sum of the anticlockwise moments about any point, or the object would rotate.

A couple = a pair of forces of equal size which act parallel to each other but in opposite directions.

produces a turning force. the size of the turning force depends on the size of the forces and the distance between them.

units - newton metres Nm

Turning force of a couple = size of 1 of the forces x perpendicular distance between forces (distance in metres)

Will rotate if the downwards force or upwards force is greater than the other opposite force.

Objects as point masses referring to a single point - centre of gravity.

In regular objects i.e. a cube the centre of gravity is in the middle, but for objects with varying masses i.e. a lorry the centre of gravity can be shifted off the middle and towards the heavier mass.

Resultant force - the vetor sum of all the forces acting on an object

The forces on an object are balanced when the vector sum at all forces acting on the object is zero. (a closed triangle)

No movement/rotation.

• Resultant force = zero

• Sum of anticlockwise = sum of clockwise about any point

If theres 3 forces acting on the object then the line of action must intersect at the same point.

Stability,   Stable Equilibrium - when the object is displaced then released it returns to its equilibrium position.

Unstable Equilibrium - if it is displaced slightly from equilibrium then releaed it doesnt return to previous position.

Q1.)Define the moment exerted by a force about a point?

(ANS - a force x by a distance, perpendicular distance from line of action of the force to the point.)

Q2.)State the priniciple of moments for a body in equilibrium?

(ANS - the sum of the clockwise moments = the sum of the anticlockwise moments about any point)

Q3.)State 2 conditions for the object to be in equilibrium?

(ANS - reaultant force = zero, the sum of the anticlockwise moments = the sum of  the clockwise moments about any point - the resultant force moment = zero)