*FORCE AND PRESSURE

*Know what the turning effect of a force is

*Know that the moment of a force depends on force and the distance from the pivot

*Know the principle of moments

*What happens when you try to open a door with

one finger?Where do you push?

Hi, my name is Mr Stick

hinges

*Do you push near the hinges?

I can’t do it!

*Do you push far from the hinges?

That’s easier!

*The turning effect of a force depends on two

things;The size of the force

Obviously!

*The turning effect of a force depends on two

things;The distance from the pivot (axis of rotation)

Not quite so

obvious!

Axis of rotation

The turning effect of a force is called the moment of the force

The moment is calculated by multiplying the force by the distance from the pivot

*Turning effect of a force – moment of a

forceMoment (Nm) = Force (N) x distance from pivot (m)

Note the unit is Nm, not N/m!

The force which you apply to a spanner to undo a tight nut is calleda TURNING FORCE.The longer the handle of the spanner then the greater is the turningeffect of the force.

Force producing a turning force to loosen the nut.

A spanner is an example of a FORCE MULTIPLIER.

Another example of a force multiplier is a crow bar.

Effort

LoadFulcrum

Another example of a force multiplier is a wheelbarrow.

Effort

Load

Fulcrum

A spanner is an example of a FORCE MULTIPLIER.

Another example of a force multiplier is a crow bar.

Effort

LoadFulcrum

Here it is being used to lift a heavy load and theforce being applied is called the effort.The effort required to lift the load is only afraction of its weight.A turning force is being applied about the PIVOT Or FULCRUM

The MOMENT of a TURNING FORCE is defined as“the product of the force and the perpendicular distanceof its line of action from the pivot”.

d d

F F

s

Moment of F = F x d Newton Metres(Nm)Moment of F = F x s Newton Metres (Nm)

Line of action of F

*Balancing unequal masses are again about centre of mass

*For example,

*The bus is heavier than the car so they’re not in balance.

d1

M1

M2

d2

*In order to balance the system, we should change their distances to the support.

*Now they’re in balance.

m1.d1=m2.d2

d2d1

When more than one force is applied to a pivoted object it is possibleFor a small force to balance a large force provided that each force hasEQUAL but OPPOSITE moments.

The following experiment allows us to investigate the PRINCIPLEOF MOMENTS:

“For an object in a state of equilibrium the sum of the clockwisemoments taken about a point is equal to the sum of the anticlockwisemoments taken about the same point.”

W2 W3 W4W1

d1

d2 d3

d4

W1 & W2

produce anti-clockwisemoments

W3 & W4

produce clockwisemoments

From the results we can see that the sum of the clockwise momentsand the sum of the anticlockwise moments are equal when the leveris balanced (in equilibrium).

Example:

Find the value of the missing force in the following example if thelever is in equilibrium.

Find the value of the missing force in the following example if thelever is in equilibrium.

5N W10N

0.5m

0.25m

0.8m

Find the value of the missing force in the following example if thelever is in equilibrium.

5N W10N

0.5m

0.25m

0.8m

If the object is in a state of equilibrium then:

Anti Clockwise Moments = Clockwise Moments

(10 x 0.5) + (5 x 0.25) = W x 0.8 5 + 1.25 = 0.8W

6.25 = 0.8WW = 6.25/0.8

= 7.8 Newtons (2 sig figs)

*

CENTER OF MASS

*

*The small forces act like a single force at G. This means that they have the resultant force at G.

*The resultant force is the stick’s weight.

*G is the centre of mass.

*What is centre of mass?

*What is centre of mass?

*Centre of mass of a body is the body that moves as though all of the mass were applied there.

*If the object is uniform, for example a meter stick, the center of mass will be at the exact geometric center.

*But, if the shape is irregular we use plumb line.

*Draw lines from each vertex along the plumb line by assuming that the point is on that line

*After drawing the lines the centre of the mass will be on the intersection point of these lines.

Intersection point-Centre of mass

G

*But, if the shape is irregular we use plumb line.

*Draw lines from each vertex along the plumb line by assuming that the point is on that line

*After drawing the lines the centre of the mass will be on the intersection point of these lines.

Plumb line Intersection point-centre of mass

*Stability –Equilibrium

*If an object isn’t moving and remainig same, the object is stable.

* State of rest or balance due to the equal action of opposing forces is called equilibrium.

*When the object is stable it’s also in equilibrium which means the forces acting on the object and the forces turning effects are in balance.

*Stability-Equilibrium Will turn to

its original position

Small force will turn it over

Will definetely fall over before another force is applied

*Types of equilibrium *There are three states of equilibrium,

*Stable equilbrium *The objects that have wide base also have the lower

centre of mass.

*The lower the centre of mass is, the more stable object is.

*So it’s more difficult to fall over an object with wider base.

*Unstable equilibrium*The objects that have very little base, have high

centre of mass.

*And if these objects are tilted the centre of mass immediately passes beyond the base.

*So they’re not stable and it’s easy to fall these objects over.

*Neutral equilibrium

*The objects that are in neutral equilibrium stay where they’re lefted.

*When moved, it changes the position.

*Wherever they stay, their center of mass is always over their base.

*Examples*The center of mass of a two-particle system lies on the

line connecting the particles (or, more precisely, their individual centers of mass). The center of mass is closer to the more massive object

*The center of mass of a ring is at the center of the ring (in the air).

*More Examples

*The center of mass of a solid triangle lies on all three medians and therefore at the centroid, which is also the average of the three vertices.

*Applications

*Objects rotate around their center of mass

*In a uniform gravitational field, the center of mass and center of gravity are the same.

*A projectile’s center of mass will follow a parabolic path.

*If an object’s center of mass is outside it’s base of support, it will topple.

*An applied force that is not through an object’s center of mass will cause rotation.