# Motion in one direction, Types of Speed, Average Speed and Relative Speed

**Motion happens all around us. Every day, ****we see objects such as cars and motorbikes ****move in different directions at different speeds. ****When the position of an object (as a bike) changes as time passes according to the position of another fixed object (as a traffic light), we can say that the first object (the bike) is in a state of motion.**

## Motion

**Motion is the change of an object position (location) as time passes according to the position of another fixed object, ****The simplest type of motion is the motion in a straight l****ine in one direction, such as t****he motion of the train or the Metro. ****The motion of a train (or metro) is considered an example of motion in one direction b****ecause the train (or metro) moves forward or backward in a straight path or curved path or a combination of both.**

### Speed

**You can describe the motion of some objects around you as fast and some others as slow, Speed is a physical quantity that is used to describe and compare the motion of objects, ****If there are two cars, one of them is Red and the other is Blue, which of them is faster in the following two cases?**

**The first case: If the two cars move for the same period of time which is 5 sec. The red car covers a distance of 100.m, the Blue car covers a distance of 50 m, we find that t****he Red car is faster than the Blue car b****ecause it covers a large distance (100 m.) at the same time (5 sec.).**

**The second case: If the two cars cover the same distance which is 100 m, ****the Red car takes a time of 5 sec, and the Blue car takes a time of 10 sec, ****we find that t****he Red car is faster than the Blue car b****ecause it takes less time (5 sec.) to cover the same distance (100 m).**

**The factors necessary for the description of the speed (motion) are:
**

**The distance is covered by the moving body.**

**The time is taken by the moving body to cover this distance.**

**Speed is**** the distance moved through a unit of time or it is the rate of change of distance, If an object covers a distance (d) in a time span (t), so, the object speed (V) can be expressed as follows: Speed (V) = Distance (d) / ****Time (t).
**

**The amount of the object speed is equal to the amount of distance covered ****When the object covers this distance through a unit of time (1 hour or 1 minute or 1 second).**

**The object speed increases by decreasing the time taken to cover a certain ****distance because V= d/t, so the object speed is inversely proportional to the time taken ****when the distance is constant.**

**The object speed increases by increasing the covered distance at a certain time because V= d/t, so the object speed is directly proportional to the covered distance ****when the time is constant.**

**The measuring units of speed: ****The speed measuring unit is different according to the units of distance and time: m/sec, m/****min, ****km/h and ****km/sec.**

#### Conversion of some measuring units:

**Conversion of distance measuring units: ****1 kilometre= 1000 metres, 1**** metre= 100 centímetres, 1 kilometre= 1000 x 100= 100000 centimetres.**

**Conversion of time measuring units: ****1 hour= 60 minutes, 1**** minute = 60 seconds, ****1 hour= 60x 60= 3600 seconds.**

**Conversion of speed measuring units: ****1 km/h.= (5/18) m/sec.
**

**Car (A) which covers 50 metres in 5 seconds is faster than car (B) which covers 64 metres in 8 seconds b****ecause the speed of car (A) = 50/5 = 10 m/sec., while that of the car (B) = 64/8= 8m/sec.**

#### What is meant by…?

**A car moves at a speed of 120 km/h, the car****covers a distance of 120 kilometres in one hour.**

**A train covers a distance of 150 km in two hours, This means that the train moves at a speed equal to 75 km/h,****Where, V= d/t = 150/2= 75 km/h.****When the speed of a car equals zero, so the car is at rest.**

**The cars and planes are usually provided with ****a group of counters such as a speedometer, ****mileage, hour timer, and compass, If the car speedometer pointer points to ****72, this means that the car speed is 72 km/h. (20 m/sec.). ****Cars and planes are provided by a speedometer t****o help us in identifying the speed of cars and planes directly.**

#### Types of Speed

**Speed can be described as Regular (uniform) speed and Irregular (non-uniform) speed:**

**Regular (uniform) speed is the speed by which the object moves when it covers equal distances at equal periods of time.****Irregular (non-uniform) speed****is the speed by which the object moves when it covers unequal distances at equal periods of time,****or i****t is the speed by which the object moves when it covers equal distances at unequal periods of time.**

**Most of moving cars cannot move inside crowded towns all the time by uniform speed ****because the speed of the car changes according to the conditions of the road & ****the traffic, ****The Metro moves at an irregular speed b****ecause the Metro covers unequal distances at equal periods of time, or it covers equal distances at unequal periods of time.
**

#### What is the thing that moves at a constant speed in space?

**All electromagnetic waves such as light transfer through space with a constant speed equal to 3 x 10 ^{8} m/sec. **

**The time needed for the sunlight to reach the Earth is determined by the relation:**

**speed of light =**

**Distance/Time, t**

**he distance between the Earth and the Sun**

**equals 149 million km (approximately), The regular speed of the light in space**

**equals 300 000 km/sec.**

**So, ****The time needed for the sunlight to reach ****the Earth (t) = d/ v= 149 × 10 ^{6}/ 300 000 = 497 sec. =**

**8 minutes and 17 seconds.**

**If the time of sunrise is six o’clock. This means that the light traveled from the Sun at five o’clock and fifty-one minutes and 43 sec.**

### Average Speed

**It is difficult to determine the amount of irregular speed of the object, so we use another term which is called average speed, The a****verage speed ****is the total distance covered by the moving ****object divided by the total time taken to cover ****this distance, ****Average speed can be calculated as follows: ****Average speed (V¯) = ****Total covered distances (d) /Total periods of time (t).**

**If t****he average speed of a moving car is 40 km/h. This**** means that the total covered distances (40 km) by the car through a total period of time (one hour), ****Average speed ****represents the regular speed by which ****the object moves to cover the same distance at the same period of time, ****When a body moves at a regular speed, its average speed equals the regular speed.**

**If the value of the average speed of a moving body equals its speed at any moment, the body moves at a regular speed. If the value of the average speed of a moving body differs from the value of ****its speed at any ****moment, the body moves at an irregular speed.**

#### Relative Speed

**If there is a person standing on the side of the road (called the observer) to observe two cars (A & B), car (A) moves at 100 km/h, and car (B) moves at 80 km/h in the same direction.**

** Therefore, ****The speed of the car (A) relative to the static observer= 100 km/h. ****The speed of the car (B) relative to the observer= 80 km/h. ****the speed of the car (A) relative to an observer in the car (B)= 100- 80 = 20 km/h. **** You notice that the speed of the car (A) relative to the static observer differs from its speed relative to an observer in the car (B).
**

**Relative speed ****is the speed of a moving object relative to a static or a moving observer. ****The relative speed of a moving object in a certain direction differs according to the observer state and the direction of its movement:**

**When the observer is at rest,****The relative speed =****The real (actual) speed of the object.****The relative speed of the car = 100 km/h as the****relative speed is equal to the real speed.****When the****observer is****moving in the opposite direction of the moving object. The relative speed = The real speed of the object + The observer’s speed.****So,****The real speed of the object = The relative speed of the object − The observer’s speed.****The relative speed of the car = 100 + 60= 160 km/h., so,****The relative speed is more than the real speed.****When the observer is moving in the same direction at a different speed. The relative speed = The real speed of the object − The observer’s speed.****So,****The real speed of the object = The relative speed of the object + The observer’s speed.****The relative speed of the car = 100 − 60= 40 km/h. , so,****The relative speed is less than the real speed.****When the observer is moving in the same direction at the same speed.****The relative speed = The difference between the two speeds = Zero.****The relative speed of the car****= 60 − 60= Zero. The object seems static.**

**The moving car seems stable (at rest) to an observer who moves at the same speed ****and the same direction b****ecause the relative speed equals the difference between their speeds equals zero.**

#### What is meant by…?

**The relative speed of a moving car is 90 km/h,****so the speed of the car relative to an observer equals 90 km/h.****The relative speed of a moving object equals its real speed, so the observer is at rest****The relative speed of a moving car equals 70 km/h to an observer moving in the opposite direction at a speed of 20 km/h, so****the real speed of the car equals 50 km/h.**

**T****he relative speed of a moving ****object relative to an observer equals ****double its real speed. ****when the observer moves in the opposite direction at the same speed of the moving object. ****The relative speed of a moving ****object relative to an observer is equal to zero. when ****the observer moves in the same direction at the same speed of the moving object.
**

**You can subscribe to Science Online on YouTube from this link: Science Online **

**You can download the Science Online application on Google Play from this link: Science Online Apps on Google Play**

**Physical Quantities, Scalars, Vectors, Distance, Displacement, Speed & Velocity**

**Acceleration types, units, importance & Graphic representation of moving in a straight line**

**Types of motion, Relative motion, Applications of Mechanical waves & Electromagnetic waves**

**Role of waves in transferring energy, Wave Motion, Transverse waves & Longitudinal waves **

**Laws of circular motion (Centripetal Acceleration, Tangential linear Velocity & Centripetal Force)**