p3.3 making things work

?

Turning forces

A turning force is used every time you open a door. The turning effect of a force is called a moment.  The moment of a force depends on how big the force is and where the force is applied. The moemnt can be calculated  with M = F X d . M is the moment measured in newtons metres (Nm), f is the force in newtons (N) and d is the perpendicular distance in metres (m).

1 of 11

Moments and balancing

If an object is not turning the moments on it are balanced. The clockwise moments (Nm) are equal to the anticlockwise moments (Nm). 

Cranes have a counterweight on the end of the jib, so the moment of the counterweight does not balance the load but helps to reduce the forces on a tower - this stops it toppling over.

2 of 11

Changing the size of forces

A lever is a simple mahcine that makes work easier to do. Levers reduce the force needed to do tasks. When someone uses a lever, the exert a force (ie the effort) around a pivot used to move an object. A pivot is a point at which rotation occurs. Levers rely on the principle of moments to act as force multipliers - they reduce the efforts needed to move the load by increading the distance over which it is acting. this means a small effort has a greater effect. When the effort distance is greater than the load distance, the force is multiplied. 

3 of 11

Centre of mass

The mass of an object is spread throughout it, as every part contributes to the total mass. All object behave as if the mass is concentrated at one point called the centre of mass. For a symmetrical object, the centre of mass is along the axis of symmetry. When a shape is hung by the corner it will adjust itself so the COM is directly below the pivot. The imaginary line across the shape through these point is the line of action. To find the COM draw lines straight down from each croner and where they all cross. 

4 of 11

Toppling objects

If an object is tilted so its centre of mass is outside of the base of the object there will be a moment that causes it to topple. Stability is the tendancy of a body to stay in a particular position. An object is stable is it returns to its original position after a small movement. If a small movement causes the object to topple then it is unstable. the stability of an object can be increased by making its centre of mass lower (weight nearer pivoting edge so a larger moment needed to rotate) and giving it a wider base (larger rotation required to move COM to unsupported side). If the base is wider of the COM is lower then it has to be titlted further before the line of action of its weight lies outside the base. 

5 of 11

Pressure in liquids

Particles in a liquid are moving slowly all the time. Liquids are difficult to compress as the particles are already almost as close together as they can be - they are virtually incompressible. The force of particles hitting the container and anything in the liquid causes pressure. In a liquid pressure acts equally in all directions. The pressure depends on the force exerted over a particular area. A big force concentrated in a small area gives a higher pressure. P = F / A where p is the pressures in pascals (Pa which is equal to 1 N/m2), f is the force in newtons (N) and a is the area in metres squared. 

6 of 11

Multiplying forces

Liquids are used in hydraulic systems to transmit forces form one part of a machine to another. Hydraulic machines consist of a sealed system with 2 cylinders containing a fluid connected by a tube. they are used to multiply the size of a force to make jobs easier. In a hydraulics system the operator pushes down the master piston (the effort), the liquid is forced theough the connecting tube into the toher cylinder and the forces push the slave piston upwards so it can exert a force on somehting else. Fm / Am = Fs / As.Car braking systems use hydraulics to multiply the force the driver applies to the brake pedal. 

7 of 11

Period and frequency

A simple pendulum consists of a weight (called a bob)  which swings on a string. The amplitude is how far from the vertical the string moves, the time period is the time taken to complete one swing and the frequency is the number of swings per second. T = 1 / f where t is the time period is seconds and f is the frequency in hertz. An oscillation is a mathod of transferring energy using a period motion. 

8 of 11

Factors affecting the time period

The factors that could affect the time period of a pendulum are the mass of the bob, the length of the string and the amplitide. To ensure a fair test only one of these variables should be tested at a time during an experiment.  A number of factors will cause an oscillations amplitude to become smaller over time - this is called damping. The amount of damping will determine how quickly the amplitude will decay and the oscillation stops.

9 of 11

Moving in circles

Acceleration is the chnage is velocity, whcih depends on both the speed at which the car is moving and the direction. A resultant force is needed to cause any acceleration. For motion in a circle this is called the centripetal force and it always acts towards the centre of the circle. The force needed to produce circular motion depends on both the mass and the acceleration. The greater the mass the larger the force needed to keep it moving in a circle, the higher the speed the larger the force and the smaller the circles radius the larger the force. F = mv2 / r . An object moving in a circle is always accelerating as it is constantly changing direction and therefore velocity. 

10 of 11

satellites

A satellite is an object that has an orbit. The pull of gravity between the 2 objects acts as a centripetal force. All objects with mass have their own gravitational field. There are 2 types of orbit: geostationary (the satellite orbits the earth, always fixed above the equator) and circumpolar (the satellite orbits the earth, passing the north and south poles). Geostationary are used for communication and they appear not to move, and all are at the same height whereas circumpolar take 90 mins to make each orbit, are used to monitor weather and observing th earths surface. 

11 of 11

Comments

No comments have yet been made

Similar Physics resources:

See all Physics resources »See all Forces resources »