# Physics - Explaining Motion (P4)

**Revision cards for P4 (21st Century OCR)**

- Created by: Sabiha
- Created on: 31-10-12 21:26

## Calculating speed

The **speed** of something tells us how far it will travel in a certain time,

The **velocity** of something is its speed in a certain direction.

**Speed (m/s) = distance (m) time (s)****Velocity (m/s) = displacement (m) time (s)**

If you measure **average speed** overy a very year short time interval you get very close to a value for **instantaneous speed**.

**Displacement:** the distance for an object from its starting point in a straight line.

Displacement and velocity are both **vector quantities** because they use size (magnitude) and direction.

## Graphs - Distance-time & Displacement

**Distance-time graphs** are used to visualise a journey.

- The
**time**for the journey is plotted on the x-axis. - The
**distance travelled**for the journey is plotted on the y-axis.

- A
**straight line**means that the vehicle is travelling at a**constant speed**. - A
**horizontal line**means that the vehicle is**stationary**(speed is 0). - A
**curved line**means that the speed is**changing**.

- If the curved line is getting steeper, it means the speed is
**increasing**. - The
**gradient**of the line is the speed. - The steeper the line is, the
**faster**the speed.

**Displacement graphs** are used to visualise return journeys.

When the vehicle returns to its starting point, the displacement is **zero**.

- The displacement is
*negative*if the vehicle stops**behind**its starting point. - There
**is**displacement if the vehicle moves**past**its starting point.

## Accleration

**Acceleration** is the rate at which the speed increases. It is measured in m/s

Acceleration (m/s ) = change in speed (m/s) time taken (s)

*For example:*

- If a car speeds up from
**rest**to 25m/s in 5 seconds, the**acceleration**is:

** (25 - 0) 5 = 5m/s**

**When something slows down it has a negative acceleration (deceleration).**

**There has to be a net force acting on an object for it to accelerate or decelerate.**

**When the net force or overall force is zero, the acceleration is also zero.**

A vehicle travelling at a **constant speed** around a corner is **changing its velocity**.

So you use this equation to work out acceleration when using velocity:

Acceleration (m/s ) = change in velocity (m/s) time taken (s)

## Graphs - Speed-time & Velocity-time

**Speed-time graphs** are used to show the changes in speed during a journey.

**Speed**is plotted on the y-axis.**Time**is plotted on the x-axis.**Horizontal line**means speed of jobject is**constant/steady**.- If horizontal line is along the
**x-axis**, speed =**zero (stationary).**

A straight line going **up** shows **acceleration.**

A straight line going **down** shows **deceleration.**

**STEEPER THE LINE, THE GREATER THE SIZE OF ACCELERATION.**

**Instantaneous velocity** = the instantaneous speed of a vehicle in a certain direction.

**Velocity-time graphs** are used to show direction in which an object is travelling.

**Positive velocity**= object is travelling in a certain direction.**Negative velocity**= object is travelling in the opposite direction.

**Gradient of velocity-time graph = acceleration.**

## Forces

**Force** = push/pull which acts between 2 objects. Forces act in pairs.

Forces are **vector quantities** because they have a magnitude (size) & a direction.

**Repulsive force** = pushes objects apart.

**Attractive force** = pulls objects towards eachother.

**Reaction force** = reaction on the surface.

The reaction force increases when you add more force on one object than the other. For example, **if you're jumping upwards, you need to push harder on the floor - the reaction force increases.** (Forces are unbalanced).

Force is represented by an **arrow** on diagrams.

On a scale diagram, length of arrow represents **magnitude (size)** and **direction** of the arrow shows which way the force **acts**.

## Friction

**Friction:** force that acts between 2 surfaces.

Size of friction depends on:

- Roughness of surfaces (the rougher, the more friction there is).
- How hard the surfaces are pushed together (heavier objects = more friction).

When you try to push an object along a surface, friction will be **equal** so the object will **not** move.

**Increase applied force = friction increases too.**

**Limiting friction:** when the friction reaches a **maximum** and the object starts to **move**.

You need friction to walk. The **resultant force** (overall force) acting between your foot & the floor when walking is a **combination of friction** and the reaction of the surace.

## Adding the effects of forces

Resultant force = overall/total force acting on an object.

When adding forces, make sure you add the size and direction of the force.

When the resultant force is zero, the forces are **balanced** and the **acceleration** is zero.

In a space with **no friction**, once the object starts moving it should keep moving at the **same speed**.

When the forces are **unbalanced**, there is a **net force** on the object and it will **speed up, slow down** or **change direction**.

**Example:**

## Terminal Velocity

**Terminal velocity:** the constant maximum speed reached by a falling object.

**EXAMPLE:**

If an object was falling and the drag became equal to gravity, there's no acceleration so the object carries on dropping at **terminal velocity**.

**Air resistance/drag:** an upward force the atmosphere creates that slows down falling objects.

Drag acts in the **opposite direction** to the speed/velocity of the object.

Drag **increases** as the speed of the object increases.

Larger surface area of the object = larger the drag.

## Momentum & Collisions

When 2 objects collide, **large forces** are exerted. The size of the force depends on:

- The
**mass**of the object (heaver mass = larger force). - The
**speed/velocity**of the object (faster object = larger force) - The
**duration**of the collision (longer time to stop = lower force)

**Momentum (kg m/s) = mass (kg) x velocity (m/s)**

Resultant force changes object's **momentum**. (larger force exerted = larger change in momentum).

**Change of momentum (kg m/s) = resultant force (N) x duration of impact (s)****Force (N) = change in momentum (kg m/s) time taken (s)**

Force is **equal** to the rate of the change of momentum. For example:

**A car of mass 1200kg craches at the speed of 20m/s. The collision stops the car in a time of 1.5seconds. SO:**

**Change in momentum = (1200 x 20) - 0 = 24000kg m/s**

**Force = 24000 1.5 = 16000N**

## Work

**Work done:** the energy used by the movement of a force,

**Energy:** the ability to do work.

Energy and work done are both measured in **joules (J).**

**Work done (J) = Force (N) x distance moved in direction of the force (m)**

When work is done * on* an object, the energy is transferred to it.

When work is done *by** an object,* the energy is transferred from it to something else.

**Amount of energy transferred (J) = Work done (J)**

- All forms of energy have the potential to
**do work**. - Not all energy is transferred as work - some is always
**dissipated**as heat. (wasted)

## Potential Energy

When you lift an object, you do **work against gravity**.

**1 joule of work will lift 1 Newton with a distance of 1 metre.**

The work is transferred to **GPE** of the object. **(gravitational potential energy).**

- Object raised = GPE increases.
- Object falls = GPE decreases.

**Change in GPE (J) = weight (N) x vertical height difference (m)**

**FOR EXAMPLE:**

If the weight is 700N and you climb a tress with a height of 3m:

**Gain in GPE = weight x height = 700N x 3m = 2100J**

## Kinetic Energy

When you push an object for it to move (increase its velocity), you do work. The work is transferred to the moving object as **kinetic energy (KE).**

Greater mass of object & faster its speed = greater **kinetic energy.**

**Kinetic energy (J) = x mass (kg) x [velocity] ([m/s] )**

**For example, car with mass of 800kg travelling at 12m/s:**

KE = x mass x velocity = x 800 x 12 = 400 x 144 = 57600J

The work done by applied force = same as change in KE of object

**For example, driving force of car is 8kN & it moves a distance of 7.2m:**

**Work done = force x distance moved = 8000 x 7.2 = 57600J (change in KE)**

## Energy transfers

As roller coaster travels round its track, it goes up and down. So energy changes from KE to GPE. **Total energy = GPE + KE.**

**Conservation energy:** when there are no resistive forces and the total energy remains constant.

**ENERGY CANNOT BE CREATED OR DESTROYED. IT CAN ONLY TRANSFER BETWEEN OBJECTS OR CHANGE ITS FORM.**

When an object falls, its potential energy is **transferred** to **kinetic energy.**

**Loss in GPE = gain in KE.**

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