# Forces and their effects

Revision note cards on the topic of Forces

- Created by: rm715
- Created on: 15-03-16 20:09

## Speed and Velocity

Distance time graphs

**Speed = gradient**of graph**= vertical / horizontal**- If object is
**accelerating**or**decelerating,**find the gradient of the tangent of the curve **Flat**= object is**stationary****Straight**uphill or downhill =**steady speed****Downhill =**Moving back to starting point**Curves = acceleration**or**deceleration**- Finding the
**gradient of a tangent**drawn just touching an acceleration or deceleration curve will give the speed.

Velcotity time graphs

**Acceleration =****Change in velocity / time taken**- A = (v-u) / t
**Gradient = acceleration****Uphill = acceleration****Downhill = deceleration**

## Resultant Forces

**Gravity or Weight**acts**DOWNWARDS****Reaction Force**from a surfance acts**UPWARDS****Thrust, Push or Pull****speed**something**up****Drag, Air Resitance or Friction****slow**something**down****Lift**holds something**up****Tension**is held in a rope or cable

**Forces** are measured in **Newtons (N) **

**Add **forces in one direction and **subtract** forces acting against to give the **resultant force.**

**Resultant force = Change in Velocity. **

- An object will remain
**stationary**if the**resultant force**is**zero.** - An object will
**accelerate****resultant force.** - An object will continue to move at the
**same velocity**if there is**no resultant force.**

## Forces and Acceleration

A **non-zero resultant force** = acceleration

**a = F/m**

**Acceleration (m/s^2) = resultant force (N) / mass (kgs)**

**Reaction forces **are **equal **and **opposite.**

## Momentum and Collision

**p = m x v**

**Momentum (kg m/s) = Mass (kgs) x Velocity (m/s)**

- The greater the mass of an object the
**greater its velocity**and the**more momentum**it has.

**Conservation of Momentum = **The total momentum **before** a collision is the same after the event, in a closed system [no external forces]

**/\ p = F x t**

**Change in momentum (kg m/s) = Force acting (N) x Time taken (s)**

**Seat belts**increase**time taken**for the passenger to stop,**decreasing the rate of change of momentum,**therefore,**reducing force**on body.**Cycle helmets, cushioned playground surfaces, airbags and crash mats**also reduce**time taken**for impact to take place,**reducing force**acting on you and**decreasing rate of change of momentum.**

## Friction and Terminal Velocity

**Friction slows you down, **acting in the opposite direction to movement.

Most resistive forces are caused by **air resistance (drag). **Making an object **streamlined **reduces drag.

**Drag** increases with **speed.**

Terminal Velocity

**Falling **objects reach a **terminal velocity** when they have reached their maximum speed. At first **gravity** is larger than the **frictional force** so they **accelerate.** **Friction increases with speed** so acceleration reduces.

**Terminal velocity is the speed **at which the object has reached **maximum, **falling from then on at a **steady speed.**

**Terminal velocity **of a falling object depends on its **shape **and **area.**

## Stopping Distances

**Stopping distance = thinking distance + braking distance**

This depends on:

**Speed**before braking**Tiredness**of driver**Drugs**and**alcohol****Condition of car**- maintenance of brakes etc**Tyres****Road surface**and weather**conditions**

## Weight, Mass and Gravity

**Weight (N) = mass (kgs) x gravity (N/kg)**

**W = m x g **

**Gravity **on Earth is **10 N/kg**

On the Moon it is **1.6 N/kg**

## Energy

**Work done**

**Work done (J) = Force (N) x Distance (m)**- W = f x d
**Work done/Energy transfer**is when a**force**moves an object across a**distance.**

**Gravitational Potential Energy**

**Gravitational Potential Energy (J) = mass (kg) x gravity (N/kg) x height (m)**- Ep = mxgxh

**Kinetic Energy**

**Kinetic Energy (J) = 0.5 x mass (kg) x speed^2**- Ek = 0.5 x m x v^2
**Kinetic energy gained = Potential energy lost**

## Elasticity

An object that returns to its original shape after being **stretched** is called **elastic**. Energy is stored by the object as **Elastic potential energy.**

**Extension (m) = Force (N) / Spring constant (N/m)**

**E = f / k **

**Hookes Law**

**Extension **of an object is **directly proportional ** to the **force applied **until it reaches its **limit of proportionality** when it can no longer extend. It has taken the **maximum force** that it can.

## Power

**Power** determines the **rate of energy transfer**. It is not the same thing as force or energy.

A powerful machine is one that transfers a **lot of energy **in **a short space of time.**

**Power (W) = Work done (or energy transferred) (J) / Time Taken (s)**

**P=W/t**

One watt = One Joule/second

## Moments

**Moment (Nm) = Force (N) X perpendicular distance from the line of action of the force to the pivot (m)**

**M = F x d**

Example:

Moment = 20 x 0.2 = 4Nm

## Balanced Moments

If **Total anticlockwise moment = Total clockwise moment,** the object **will not turn**

Example:

A: 1x1000 = 1000

B: 500x2m = 1000

Levers use the idea of **balanced moments**. The force that is needed to produce a moment depends on the **distance from the pivot**.

Increasing the **distance from the pivot** where the force is applied means **less force** is needed.

## Stability

If the **total anticlockwise moment **does **not equal the total clockwise moment** there will be a **resultant moment**.

The most **stable** objects have a **wide base** and **low centre of mass. **If the object's **centre of mass** moves beyond the **edge of the base,** it will tip over.

This is because of the **resultant moment.**

## Pendulums

Suspending a weight from a piece of string creates a **pendulum. **It will swing back and forth when pulled and let go. Time taken for the pendulum to swing from one side to the other is called the **time period. **The **longer **the pendulum, the **greater** the time period.

**Tim****e period (s) = 1 / F (Hz)**

**T=1/F**

## Circular Motion

- If an object is travelling in a circle, it is
**constantly changing direction.**It's**velocity**is changing (not speed) so the object is**accelerating.**

- A
**resultant force**must be acting on the object to cause**acceleration.**It is called the**centripetal**(sen-tree-peet-al**)****force**and it acts**towards the centre of the circle.**

- The
**centripetal force**could be: Tension, gravity or friction.

**More speed, More mass**and a**smaller circle radius**require a**larger centripetal force.**

## Hydraulics

**Pressure **in **liquid** is **transmitted equally** in **all directi****ons**. It has the same **volume **and **density **when compressed.

**Pressure (Pa) = Force (N) / Cross sectional area (m^2)**

**P = F/A**

**Hydraulic systems** use liquid to increase an applied force. One piston will have a **smaller cross sectional area****. Force **is applied to the **smaller piston, increasing pressure **which is then transmitted to the **larger piston. Larger area = larger force.**

Hydraulic systems can be **used** in:

- Car breaking systems
- Hydraulic car jacks
- Manufacturing and deployment of aircraft landing gear

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