A force can do one of four things to an object: - Make it speed up - accelerate.
- Make it slow down - decelerate.
- Change its direction.
- Change its shape.
If something is doing one of these four things there must be a
net force acting upon it.
Newton's First Law
'Every body continues in a state of rest or uniform motion unless acted upon by an external force.' Something without a
net force acting on it will either
stay still or
move at a constant speed in a straight line until you apply a force to it.
F = ma
Newton's Second Law: - F is the force in Newtons, N.
- m is the mass in kilograms, kg.
- a is the acceleration in m/s2.
This shows that if you keep the mass constant and double the applied force the acceleration will double.
Hooke's Law, elastic and plastic behaviour
F = kx An
elastic material is one that will return to its original shape when the force applied to it is taken away.
A
plastic (or inelastic) material is one that stays deformed after you have taken the force away.
If you apply too big a force a material will lose its elasticity.
In solids
If a force is applied over a smaller surface area you get a larger pressure. Pressure can be calculated using the following equation:
Force will be in newtons, N.
Area will be in either m2 or cm2.
If the area is in m2 then the pressure will be measured in Pascals or N/m2.
If the area is in cm2 then the pressure will be in N/cm2.
In liquids
- Pressure increases with depth.
- Pressure acts equally in all directions.
- Pressure is transmitted through liquids.
Hydraulics All hydraulics systems work because the pressure is the same throughout the system.
A really good example of this is a
car brake system. You need to know all about this for your exams.
In gases Although gases are compressible (squashy) they exert a pressure because of the gas particles bouncing off things.
Boyle's Law
For a fixed mass of gas the pressure x the volume of the gas stays the same. In other words, as you squeeze a gas its pressure will go up and its volume will get less. Important point: The temperature and mass of gas must stay the same for this to be true!
We can write this as: Pressure x volume = constant
or P1V1 = P2V2
Moments Moments make things turn or rotate. They are caused by forces but are not forces themselves. Like forces, moments have a direction. We say they are either clockwise or anti-clockwise, to show which way they will make something turn.
The bigger the force causing the turning effect the bigger the moment will be. The further the force is from the pivot the bigger the moment will be. The size of a moment can be calculated using: Moment = Force x Distance
Force is measured in newtons, N.
Distance is measured in either m or cm.
If the distance is in m then the moment will be measured in
Nm. If the distance is in cm then the moment will be measured in
Ncm. Distance As we all know, the distance between two points is how far apart they are. In science, we normally use metres as our unit.
We often represent how the distance between two points changes using a distance:time graph.
Speed Speed is how fast something is going. It is how quickly something covers a certain distance and can be worked out using the equation:
Acceleration This is how quickly something gets faster. So if you were running and getting 1m/s faster every second you would have had an acceleration of 1 m per second per second. We normally write this 1 m/s2.
We work out by the equation: