Chapter 1 part 4: Forces & Moments

                                         General Physics

 

• 1.5: Forces & Springs 

Forces act all around us all the time , these forces may be due to the weight of an object , the thrust of an engine or the pulling of a lever. A force is a push or pull and can be calculated as Force equals Mass.Acceleration , F=M.A. Forces applied to objects can cause them to change shape, to deform.
When an object deforms due to a force , it can either extend or compress , get longer or shorter.
Take a spring for example , the amount a string stretches is proportional to how much force we put into stretching it , This can be written as F∝X where X means extension. Every spring made of different materials stretches differently , and has a vale known as a spring constant. Hence exists Hooke's Law , F=K.X where K is the spring constant  

Metals tend to extend uniformly , meaning the above equation applies very well to metals. this means that when we plot a graph of force/extension , we get a straight line , until the metal is close to reaching its breaking point , it is proportional , a metal will not stretch proportionally near its breaking point as shown in the diagram.

• 1.6: Forces & Motion

In motion, forces cause objects to accelerate , since F=M.A , But rarely is there a case where only one force acts on a body , There are usually several forces acting , but how do we deal with all of these forces ? , The answer is that forces acting in the same direction can be considered to act as one force , and you can add their magnitudes together, So if a car is moving along a road with a thrust of 5000 newtons and is at the same time experiencing friction with the road , of magnitude 1000 newtons , since these forces are considered to act in opposite direction , the resultant force is 4000 Newtons in the forward direction.

If there is no resultant ,that means that all forces are balanced in all directions, then there is no acceleration , so no change in velocity , this means that a body with no resultant force actin will move at constant speed or stay at rest (since rest is a constant 0 speed).

Friction such as that in the diagram is due to the roughness of the road and resists the motion of the car , there is also air resistance, as the car moves it is moving through the air as well , which blocks the motion of the car . Submarines experience water resistance, since water is much denser than air it resists the motion of objects within it much more than air does.

The types of resistance you need to know are : Air resistance, Water resistance and Drag (general term for resistance)

• 1.7: Moments & Turning effects

The moment of a force is the magnitude of its turning effect. This means that if a force acts about a pivot , causing an object to turn about that pivot , then this force has created a moment about the pivot.

E.G.
If i am pushing open a door that swings sideways , i am creating a moment about the door hinges. If i push hard against the door it will turn faster as i will have created a larger moment. if i push further away from the pivot the door also becomes easier to push , as this too creates a larger moment
This is due to the fact that a Moment equals Force.Perpendicular Distance from the pivot.
So by increasing the magnitude of the force or the distance from the pivot , we can create a larger turning effect.

The principle of moments
This is a rule that if the moments about a pivot are balanced , This means moment in the clockwise direction are equal in magnitude to moments in the anticlockwise direction, the object can then be in equilibrium 

For an object to be in equilibrium : Sum of clockwise moments are equal to the sum of Anticlockwise moments. And that no resultant force acts upon the object . If these two conditions are satisfied then the object is in equilibrium