What is motion?

Motion is the change in the position of an object.

To describe the motion of an object we use terms like: Distance, displacement, speed, velocity and

acceleration

Distance vs. displacement

• What is distance? Distance is the total path travelled by an object.

- Distance has magnitude (value) only.

• What is displacement? Displacement is the shortest distance between the start point and finish point with direction

- Displacement has magnitude (value) and direction.

- The SI unit for both distance and displacement is meter (m)

Example: A car moves from point A to point B then to point C as shown in the opposite diagram. Find the distance and displacement travelled by the car

Answer: Distance = 800 + 300 = 1100 m

Displacement = 1100 m to right

Distance vs. displacement

A B C800 m 300 m

A B C800 m 300 m

Example: A horse runs from point A to point C then back to point B as shown in the opposite diagram. Find the distance and displacement travelled by the horse.

Answer: Distance = 800 + 200 = 1000 m

Displacement = 600 m to right

Distance vs. displacement

A B C600 m 200 m

A B C600 m 200 m

Example: A man moves from point A to point B then to point C and finally to point D as shown in the opposite diagram. Find the distance and displacement travelled by the man

Answer: Distance = 50 + 30 + 50 = 130 m

Displacement = 30 m to north

Distance vs. displacement

A B

CD

50 m

30 m

A B

CD

50 m

30 m

Speed

What is speed? Speed is the travelled distance per unit time.

- speed can be calculated by the formula:

or

- The SI unit of speed is meter per second (m/s).

- Another unit for speed is (km/h).

time

distancespeed

Speed

- Formula of speed can be manipulated using:

ds t s

d t where d = s x t or

Speed

Example: A horse runs a distance of 1200 m in 40 s with constant speed. Calculate the speed of the horse.

Answer: Given: d = 1200 m t = 40 s s = ?

t

d s

401200

m/s 30

Speed

Example: A car travels with a constant speed of 20 m/s. How much distance the car will travel in 8 s?

Answer: Given: s = 20 m/s t = 8 s d = ?

tsd 8 x 20 m 160

Speed

Example: How long will it take a man running with a constant speed of 8 m/s to travel a distance of 400 m?

Answer: Given: s = 8 m/s d = 400 m t = ?

s

d t

8400

s 50

Speed

Example: what does it mean to say that the speed of plane is 80 m/s?

Answer: It means that the plane flies a distance of 80 m every

second.

Distance-time graph• Speed could be represented by graph called distance-

time graph.

• In distance-time graph: distance is represented on the vertical axis and time is represented on horizontal axis

Time (s)D

ista

nce

(m)

Distance-time graph• Distance-time graph could have different

shapes, two of them are:

Time (s)

Dis

tanc

e (m

)

Time (s)D

ista

nce

(m)

Distance-time graph• When the distance-time graph has the opposite shape then it means that the object is: moving with constant speed (speed is not changing)

• The slope (steepness) of the of the line equals the speed.

• The steeper the line (more slope) the greater is the speed

Time (s)

Dis

tanc

e (m

)

Distance-time graphExample: The opposite graph represents the motion of two objects A and B. Which of the two objects have more speed. Explain why.

Answer: Object A has more speed because the slope of its line

is steeper (has more slope).

Time (s)

Dis

tanc

e (m

)

A

B

Distance-time graph• When the distance-time graph has the opposite shape then it means that the object is: at rest, not moving, speed = 0 or slope = 0

Time (s)

Dis

tanc

e (m

)

Average speed and instantaneous speed• When an object is moving with changing speed (speed

is not constant) then its motion is described by: 1- Average speed. 2- Instantaneous speed.

Average speed is the total distance travelled divided by the total time of travel.

Instantaneous speed is the speed at a given moment (point of time)

In a moving car instantaneous speed could be determined from the speedometer

Velocity

Velocity is the speed of an object and its direction.

Speed has magnitude (value) only.

Velocity has magnitude (value) and direction.

5 m/s

5 m/s

5 m/s

5 m/s

5 m/s

5 m/s

5 m/s

5 m/s