# Forces

HideShow resource information
• Created by: Katy **
• Created on: 03-01-14 14:49

## 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 = 0no change in velocity (direction & magnitude).
• Resultant force = not 0 → change in velocity (direction & magnitude).
1 of 12

## Momentum

• Momentum is the product of an objects mass & velocity.
• Momentum = mass x velocity
• kgms-1 = kg x m/s
• p = m x v
2 of 12

## 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
3 of 12

## 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

4 of 12

## 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.
5 of 12

## 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).

6 of 12

## 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
7 of 12

## 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

8 of 12

## Falling Vertically (3)

5. Slowing down

• air resistance = decreases
• resultant force = less negative (increasing)
• acceleration = less negative (increasing)
• weight = stays the same
9 of 12

## Falling Horizontally

reaction force ↑

weight ↓

drive ←

air resistance / drag →

Resultant force:

-drive - drag

-mass x acceleration

-positive → accelerating

-0 → velocity constant

-negative → deccelerating

10 of 12

## 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
11 of 12

## 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).
12 of 12