Forces

Force is a vector, a physical quality having both direction and magnitude, measured in newtons. Other vectors include velocity, diplacement, acceleration, momentum, ect.  Vectors are shown with an arrow, the length of the arrow shows the magnitude and the direction shows the directtion of the quantity.

Some scalar quantities include speed, distance, mass, temperature, time, ect.

When two objects touch to cause a push or a full, they are contact forces. For example, friction,air resistance, tension in ropes, ect.

If the objects do not touch, they are non-contact. For example, magnetic force, gravitational force, electrostatic force, ect.

An interaction pair is a pair of forces that are equal and opposite and act on two interacting objects. For example, the sun and the earth are attracted to each other by gravitational force, which is a non-contact force, in an equal and opposite way.

Forces can change the shape of an object, the direction its going in, or its state state of rest.

Gravity gives everything a weight and causes things on the surface of a planet to fall towards the gound. An object acted on only by gravity accellerates at about 10m/s^2.

Mass is the amount of matter an object has, it never changes. It is measured in Kg.

Weight is the force acting on the object due to the attraction of gravity and varries on location. For example a 1kg mass will weigh about 9.8N on the earth and 1.6N on the moon. 

The centre of mass of an object is the point at which you can assume the mass is concentrated. For a uniform objects, this will be at the centre. When an object is suspended, it comes to an equillibriom, at rest, with its center of mass directly below the point of suspension. If the object is swung or turned it will reeturn to equillibrium.  For a flat object that is symetrical, the centre of mass is along the line of symmetry, if it has more than one it is where the axis meet.

Weight = mass x gravitational field strength

Mass and weight are directly poportional.

A resultant force is the overall force on a point or object and is found adding forces going in the dame direction and subtracting any going in the opposite direction. If the resultant force is 0, the object is either stationary or moving at a contsant speed. If two force act in the same direction the resultant force is their sum, if they act in opposite directions the resultant force is their difference.

When a force moves an object through a distance, energy is transfered and work is done on the object, either usefully or it is wasted. The thing producing the force must have a source of energy.

work done = force x distance

The arrows in a free body diagram show the relative magnitudes of the forces and the directions in which they are acting. It only shows one object and the forces acting on that object.

Scale dawings are used to find the resultant force:

1) Draw all the force acting on an object "tip to tail".

2) Draw a straight line from the start of the first force to the end of the last, this is the reultant force.

3) The lenth of the reultant force is its magnitude and the angle is the bearing.

If the reultant force is 0, the object is at equilibrium. The end of the last force will reach the begining of the first force. This will have no effect (including turing effect)

Forces with awkward angles can be split into components (resolved) by adding the horizontal and vertice components at right angles on a grid and measuring the distance between them.

Hookes law

An object has been elastically deformed if it can go back to the original shape and length after the force has been removed, these are elastic objects. In this case, all the energy from the work done is transferred to the object's elastic potential energy store.

An object has been inelastically deformed if it can not go back to the original shape and length after the force has been removed. This relationship becomes non-linear.

Extension = length at stage - original length

Force = spring constant x extension           -  The sping constant depends on how stiff the object is.

The extension of a stretch elastic object is directly proportional to the load or force applied, until it reaches its limit of proportionality as long as the limit of proportionality has not been exeeded, this is non-linear.

Set up the apparatus as shown:

1) Clamp the ruler to the stand and measure the natural length of the spring.

2) Add a mass to the spring and record the new length of the spring. The extension is the change is length.

3) Repeat until you have at least 6 measurements.

4) Plot a force-extension graph. The force and extension should be directly proportional until it reaches its level of prooportianality. The area under the graph is the energy in the elastic potential store.

Elastic potential energy = 1/2 x spring contant x extention (until it reaches limit)

Distance = scalar. Displacement = vector. Therfore if you walk 5m north then 5m south your diplacement is o and your distance is 10m.

Speed = how fast you're going (scalar). Velocity = speed in a given direction (vector).

Therfore you can have objects traveling at a contrant speed with a changing velocity if the direction is changing, for example a car when travelling around a roundabout).

Distance = speed x time                                        -   the average is usually given for the speed.

Average speads:

Acceleration is the change in velocity in a given time.It is measure in m/s^2

acceleration = change in velocity / time

Uniform acceleration is contrant acceleration. Due to gravity, objects in free fall are in uniform acceleration. They have the same acceleration as the gravitaional field strength (9.8m/s^2).

uniform accelaration:

final velocity - initial velocity = 2 x acceleration x distance   (v^2 - u^2 = 2as)

or    a = v - u / t

deacceleration is when an oject slows down, it is still the change invelocity per second.

Required practiacal: A motion sensor can show you the velocity changes of an object (toy car). This is then drawn onto a velocity-time graph and the gradient can be found. You would place a toy car at the top of a ramp and measure its veloicy as it travels down. The steeper the sloe, the higher the velocity