Section 1 c) Summary

Forces
Balanced forces are forces that are equal is size and opposite in direction, and cause no change.
A force is a vector quantity; it has both size and direction.
Force is measured in newtons (N)

When forces are added together, they form a resultant force. Balanced forces always have a resultant force of 0N. If the resultant force is not 0N, the force is unbalanced, and the shape, speed, size or direction of the object will change.
There are different types of forces- they can be contact or non-contact.
Contact forces require particles to touch, for example friction, but non-contact forces act over a distance and don't require the particles to make contact to act, for example magnetic force.

Contact:

• Friction
• Drag
• Upthrust
• Tension
• Normal
• Air resistance

Non-Contact:

• Magnetic
• Electrostatic 
• Gravitational/weight
• Nuclear

Newton's laws of motion state that:
1. An object with balanced forces will not change in velocity.
2. A resultant force means acceleration
3. Every force has an equal opposing force.

The third law means that as you stand on the Earth, you are exerting a force on the ground, but the Earth exerts an equal force on you. This is called normal force.

As a boat travels through the water, a number of forces act on it. The upthrust from the water opposes the pull of gravity, and they are balanced. The force exerted by the boat as it moves through the water is opposed by water resistance, and above the water level motion is opposed by air resistance. These are both forms of drag; they are forces that oppose motion.

Friction is another form of drag, it happens when an object moves along a solid and is a force in direct opposition to motion.

Vectors and Scalars
Vectors and scalars are measures of quantity. A scalar is a measure of magnitude, while a vector is a measure of both direction and magnitude.

Terminal Velocity
Terminal velocity happens when the vertical force on an object are balanced. For example, a ball is dropped from a height. Initially, the ball will be accelerating because the weight is greater than the air resistance, but soon the weight and air resistance will become equal as the resistance builds up as velocity increases. Equal forces mean that the ball is no longer accelerating and is now travelling at a constant speed: it has reached terminal velocity. We can test terminal velocity using experiments with parachutes or sycamore seeds:

Sycamore seeds:
Seeds should be collected and the length of wing measured. They should be dropped and timed, and a graph drawn to show the relationship between length of wing and speed.

Parachutes:
Dropping same-weighted objects attached to parachutes of different sizes from the same height, and measuring the time it takes for the object to reach the ground. This is investigating the air resistance on the parachute, and should show that with increased surface area, the object will move more slowly.

Principle of Moments
If an object is balanced its clockwise and anticlockwise moments are equal.
If the moments are not equal, there is a resultant moment and the object will turn.

Examples of Balanced moments:

In the example above, we can clearly see the clockwise and anticlockwise moments are equal. On both sides, we multiply:

Force x Distance from pivot = Moment

50N x 2m = 100Nm

In this example, the blocks are in different positions and exert different forces, which must both have the same moments:

Force 1 x Distance 1 = Force 2 x Distance 2 

Moment 1 = Moment 2

50N x 2m = 100Nm

100N x 1m = 100Nm

The clockwise and anticlockwise moments are equal.

In the above example, a light beam is supported by two blocks at either end. The block placed in the centre exerts a force of 900N, which is spread equally between the two supports as they are at equidistance - the ratio is 1:1

450N + 450N = 900N

The sum of the force exerted by the supports must always equal the force exerted by the block. 

In this example, the block is closer to the left support. There is more force exerted on the closer support, and the distance can be split in a ratio of 1:2. We known there is more force on the closer support, so we know the force is distributed 2:1. We can divide 900N by 3, giving 300N to find the value of 1 in the ratio. 

900N / 3 = 300N

From this, we can put the values in the ratio to find the forces exerted on the supports

2 x 300N = 600N

1 x 300N = 300N

600N : 300N

Centre of Gravity

An object's centre of gravity is the point through which its weight acts. This point can be placed on a pinpoint and it will not overbalance because the weight acting on every side of it is equal. 

The centre of gravity can be found in a 2D object, for example a piece of paper in any shape, by suspending it and marking the vertical line below the point of suspension. If this is repeated, one point can be found where all the lines cross. This is the centre of gravity. 

Vehicular Safety

Forces and moment play a big part in ensuring driver and passenger safety in moving vehicles. Many safety features designed to prevent injury in the event of a crash use principles of momentum in their design. 

Force Felt = Momentum / Time

So while the momentum can't be altered, the time can be. Safety features increase the time over which momentum decreases, therefore decreasing the impact force. These include:

• Air Bags
• Seat Belts
• Crumple zones

Moving vehicles all have a stopping distance (made of thinking distance and braking distance) that is important to maintain to avoid dangerous collisions. The faster or heavier a vehicle is, the more momentum it has, making it more difficult to slow down in a short period of time and a short distance. If stopping distances are not maintained, it can lead to collisions and pile-up.

Stopping distances in cars can be affected by:

• Sobriety
• Old age
• Tiredness
• Inexperience
• Speed
• Mass of vehicle
• Brake quality 
• Weather conditions
• Road surface
• Tyre condition

Hooke's Law and Elasticity

Hooke's law states that the extension of an elastic object is proportional to the force acting on it. 

An elastic object is an object that can be stretched by a force, but will return to its original physical state once the force is no longer acting on it. However, all objects have an 'elastic limit', the point at which so much force has been applied that it loses elasticity and is unable to return to its original shape. After the elastic limit has been reached, the principles of Hooke's law are no longer relevant. 

Formulas

Weight = Mass x Gravity

Force = Mass x Acceleration

Momentum = Mass x Velocity

Force = Change in Momentum / Time taken

Moment = Force x Perpendicular distance from pivot