Forces

A force is a push or pull that act on an object that is caused by it interacting with something. It can change the object's shape, speed or direction.

All forces are either contact or non-contact forces.

• A contact force is when objects have to be touching for a force to act. E.g: friction, upthrust
• A non-contact force is when objects don't have to be touching for a force to act. Eg: magnetic, electrostatic 

Balance forces are two forces acting in opposite directions on an object, and equal in size. Anytime there is a balanced force on an object, the object stays still or continues moving continues to move at the same speed and in the same direction.

When two forces acting on an object are not equal in size, we say that they are unbalanced forces. If the forces on an object are unbalanced the object starts to move, changes speed or changes direction.

Scalar quantities have magnitude (size) only. Examples of scalars are:

• Mass
• Power
• Distance
• Speed
• Energy

Vector quantities have both magnitude and direction. Examples of vectors are:

• Force
• Acceleration
• Velocity

Mass:

• How much of something there is
• measured in grams and kilo grams
• measured using a balance not a scale

Weight:

• A measurement of force
• Therefore it is measured in Newtons

Weight (N) = Mass (Kg) X gravitational field strength (N/Kg)

The centre of mass of an object is the average position of all the mass in the object.

To find the centre of mass of a 2d shape, put a pin in the shape at any point and let it settle naturally. Put a string with a weight on it and allow it to dangle straight down from that point and draw a line down the piece of string. Repeat from another point on the shape. Rep 

To find the centre of mass for a 3d shape, repeat a 3rd time.

To make an object more stable:

• lower the centre of mass
• make the base wider 

• Friction

• Magnetic

• Electrostatic

• Drag (air resistance)

• Weight (gravity)

• Pull

• Thrust

• Upthrust (water)

An elastic object regains its original shape when the forces deforming it are removed. When it is deformed, it stores elastic potential energy.

Elastic potential energy = 1/2 x spring constant

An inelastic object doesn't regain its original shape when the forces deforming it are removed.

Force of spring = spring constant (N/m) x extension (m)

Spring constant =  --       Force                             Spring constant = 1 / gradient                                                                   displacement

Hooke's law states that the force needed to stretch a spring is directly proportional to the extension of the spring, provided it doesn't exceed its elastic limit.

If an object has no force propelling it along it will always slow down and stop because of friction (Unless in space or vacuum). To travel at a constant speed the force and friction need to cancel out.

Drag is the resistance you get in gases and liquids. Air resistance is an example of this. To reduce drag the most important factor is to make the object streamline. A  parachute works in the opposite way.

Terminal velocity is when either the weight or thrust of an object is the same as the drag or friction are the same.Therefore there is no acceleration or deceleration.

When the resultant force of an object does 'work' to make it move, energy is transferred from one store to another. Work is still done if the energy is transferred to a wasted store. Work done and energy transferred are the same thing.

When something moves along a rough surface you are doing work against friction. Energy is being transferred to the kinetic energy store because it is moving. Some is also transferred to the thermal store due to friction. This increases the overall temperature of the object.

Work done (J) = Force (N) x distance moved (m)

1 Joule = 1 Newton metre

A number of forces acting on an object may be replaced by a single force which has the same effect on the object as all the original forces acting together. This force is called the resultant force.

The resolved force is the horizontal and vertical component. Draw a right angle triangle using the line that represents the force as the hypotenuse. 

Moments create a turning force around a pivot

The pivot is the location where an object wants to turn around.

moment (Nm) = force (N) x distance from pivot (cm or m)

A lever is a simple machine that makes work easier to do. Examples of simple levers include cutting with scissors or lifting the lid on a tin of paint with a screwdriver. Levers reduce the force needed to perform these tasks.

Force multiplier : Moving around pivot - less force + large distance > large force + small distance 

Distance multiplier : Moving from pivot - more force + small distance >small force + large distance

Gears are like rotating levers.

They increase the force applied to wheels.

When the cog at the back is smaller it takes less force to turn it and vice-versa.