3.2 and 3.3 Forces in action, Work energy and power

Friction in general terms

the force that arises when two surfaces rub against each other.

Drag in general terms

the resistive force on an object travelling through a fluid (e.g. water or air).

Weight in general terms

the gravitational force acting on an object, through its centre of mass.

Tension in general terms

the force within a stretched cable or rope.

Normal Contact Force in general terms

– the force arising when an object rests against another object acting at a 90° angle to the plane of contact.

Up-thrust in general terms

– the upward buoyancy force acting on an object when it is in a fluid.

Find mass definition

an objects resistance to being accelerated

What is centre of mass definition?

The centre of mass of an object is the point where the entire weight of the object appears to act.

The centre of mass is the point through which the application of an external force produces only motion in a straight line, with no rotation.

When do centre of mass and centre of weight coincide?

when object is in external gravitational field

What is centre of gravity?

the point where resultant gravitational force will act

How do you find centre of mass/gravity?

need: plumb line, pencil, string,

1) suspend object on one point

let it hang

draw line of suspension using plumbline

2) Repeat at a second position

• where lines of suspension intersect is centre of mass

What is mass described as?

• Measure of an objects resistance to change in velocity

When will an object fall in terms of centre of mass?

when line of force falls outside of the contact point with the surface.

What is relation between drag and speed?

drag is proportional to speed squared

Principle of moments

when in rotational equilibrium, sum of clockwise moments is equal to sum of any clockwise moments.

What is pressure difference due to a fluid?

P = roe*h*g

P = F/Area

what is upthrust?

roe*V*g

same as weight of displaced fluid

How can density/density relate to height in floating water?

• Find upthrust of both

• use f as fraction of volume

• cancel out to get

• ROEs/roe

What is atmospheric pressure?

101kPa

what equation describes work done constantly providing constant velocity.

work done/ time

force x velocity