# Forces

## Newton's First Law

- The
**resultant**force can either be**positive**for**negative**.

5N←o→7N *(7+5 = 12N)* **12N**

5N←O→7N *(7-5 = 2N)* **2N**

- Resultant force =
**0**→**no**change in**velocity***(direction & magnitude)*. - Resultant force =
**not**0 →**change**in**velocity***(direction & magnitude)*.

## Momentum

**Momentum**is the**product**of an objects**mass**&**velocity.**

- Momentum = mass x velocity

- kgms-1 = kg x m/s

- p = m x v

## Inertia

**Inertia**is the**reluctance**to**change**:

-stationary → to move

-moving → change velocity

**Mass**is the**measure**of an objects**inertia**.

**Momentum**is the**expression**of a**moving**objects**ineria**.

- Greater
**mass**= greater**inertia** - Greater
**momentum**= greater**inertia**

## Newton's Second Law

**Acceleration**is**proportional**to the**force***(constant mass)*.

**Acceleration**is**inversly**propotraional to the**mass**.

Force = mass x acceleration

Resultant force = mass x acceleration

Resultant force = change in momentum/time

## Weight (force)

**Weight**is a**result**of**gravity**fom a planet**pulling**on an objects**mass**.

- On
**Earth**- every**kg**of mass**experiences**a graviational**pull**of 9.81N.

Weight = mass x gravity

W = m x g

- Gravity
**depends**on the**mass**of the**planet***(proportional)*.

- The
**further**the object is from the**centre**of the planet*(core)*, the**smaller**the gravitational**pull**is.

## Air Resistance

**Resistance**is**friction**.

**Resistance**increase with**speed***(speed x2 = friction x4)*.

- As the
**speed**of a falling**object**increase = air**resistance**increases

**Resistance factors:**

-resistance & friction act **opposite** to **movement**

-resistance increases as speed2 increases

-**greater surface** area = **greater** friction / air resistance

-fluid **resistance** *(air / liquid)* depends on **collision** of paricles with moving object

-**denser** fluid = **greater** resistance *(more paricles per volume)*.

## Falling Vertically (1)

- Resultant force = weight - air resistance

**Speed increases:**

-air resistance increases

-weight stays the same

**1. At start** - *not moving*

- air resistance = 0
- resultant force = max.
- acceleration = max.

**2. Moving / Falling / Speeding up**

- air resistance = increases
- resultant force = decreases
- acceleration = decreases
- weight = unchanged

## Falling Vertically (2)

**3. Resultant is 0**

- air resistance increase until it = to weight
- acceleration = 0
- weight = air resistance
*} terminal velocity* - velocity = constant
*} terminal velociy*

**4. Parachute opens**

- air resistance = increases / greater than weight
- resultant force = negative → upwards
- surface area = increases
- deccelerating force - acceleration = negative → slows down
- air resistance = max.
- resultant force = max. negative value

## Falling Vertically (3)

**5. Slowing down**

- air resistance = decreases
- resultant force = less negative
*(increasing)* - acceleration = less negative
*(increasing)* - weight = stays the same

## Falling Horizontally

reaction force ↑

weight ↓

drive ←

air resistance / drag →

**Resultant force:**

-drive - drag

-mass x acceleration

-**positive** → accelerating

-0 → velocity constant

-**negative** → deccelerating

## Velocity & particle collision

**As velocity increases:**

**collisions**with air resistance increases- there is a greater
**rate**of**collisions** - there is greater
**momentum** - there is greater air resistance

## Falling with negligible air resistance

- The
**moon**has**no**air resistance.

- A
**brick**is a**dense**object.

- In the
**absense**of air resistance, all**objects fall**with at the**same**rate.

**Acceleration**is**due**to**gravity***(9.81 m/s/s)*.

## Related discussions on The Student Room

- intermolecular forces or forces of attraction? »
- Armed Forces »
- Forces q »
- Physics forces question query »
- Armed Forces First time buy house/ let »
- Electrostatic vs Intermolecular forces »
- MS Missing some forces »
- Forces »
- Forces and Newton?s Laws »
- Gcse aqa unit 2 intermolecular forces/ electrostatic charges »

## Comments

No comments have yet been made