# Speed displacement and velocity

## Speed displacement and velocity

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# Speed displacement and velocity

The second is the unit of time.
The metre is the unit of distance.

## Speed is the rate of change of distance with respect to time.

The unit of speed is the metre per second (m s-1 or m/s)

Instantaneous Speed
It is important that you understand the difference between average speed and instantaneous speed.
When your speedometer indicates that you are doing 120 kph, it’s telling you that your instantaneous speed is 120 kph, not (obviously) your average speed for the journey.
But to calculate instantaneous speed, you just take your average speed over a very short time.
We come across this concept again in the next chapter.

Make sure all answers are in seconds, metres, m s-1 etc.

Displacement is distance in a given direction.
This can be a little tricky to grasp; it’s like saying the straight-line distance between the two points.
If you travel half-way around a circular race track of radius 100m, your distance travelled will be half the circumference of the circle (2πr/2) which is this case would be 100π metres, but your displacement would be the straight line distance, in this case the diameter of the circle; 200 metres.

## Velocity is the rate of change of displacement with respect to time.

The unit of velocity is the metre per second (ms-1, or m/s).

So if you travel through one full circuit (finishing back where you started) your average velocity will be zero, because the displacement was zero.
Now try Exercise 6.2, page 60

Distance–Time Graphs (for an object travelling at constant velocity)

Slope =

If you plot a graph of Distance (on the y-axis) against Time (on the x-axis), the slope of the graph will be the speed of the object.

Now try Exercise 6.3; pages 64 and 65.

Mandatory Experiment
Measuring Velocity using a Ticker –Tape-Timer
See page 61 of the textbook, problem 6. Then try page 25 and 26 of the workbook, Section D.

Leaving Cert Physics Syllabus: Velocity

 Content Depth of Treatment Activities STS Linear Motion Units of length and time. Definitions of units not required. Displacement and velocity. Definitions and units. Measurement of velocity using any suitable apparatus. Use of distance-time graph. Sports, e.g. athletics.

## MEASUREMENT OF VELOCITY USING A TICKER-TAPE TIMER

APPARATUS: Ticker timer and tape, low-voltage a.c. power supply, trolley, runway.

DIAGRAM

PROCEDURE

1. Set up the apparatus as in the diagram.
2. Connect the ticker timer to a low-voltage power supply.
3. Give the trolley a small push to start it moving.
4. Adjust the angle of inclination of the runway until the trolley moves with constant velocity, i.e. the spots on the tape are all equidistant.
5. The ticker timer makes 50 spots per second. Therefore the time interval between two adjacent spots is 0.02 s.
6. Measure the length s of ten adjacent spaces.
7. The time t is 10 × 0.02 = 0.2 s.
8. As the trolley was travelling at constant velocity we can say that velocity = distant/time
9. Repeat using pushes of varying strengths.
10. Tabulate results as shown.

RESULTS

 s (m) t (s) v (m s-1)

CONCLUSION
The dots were evenly spaced throughout, illustrating that the velocity remained constant.

SOURCES OF ERROR / PRECAUTIONS

1. Dust the runway to reduce friction.
2. Raise the trolley to offset friction such that the trolley moves at constant velocity.
3. Ignore the initial five or six dots on the tape as this shows the initial acceleration due to the push.

QUESTIONS

1. For ten spaces we need eleven dots. What would be the effect (if any) of using five spaces instead of ten?
2. What would be the effect of using twenty spaces?
3. What would be the effect of giving the trolley a high initial velocity?

NOTES

1. Ensure that the voltage rating of the timer is not exceeded.
2. Ignore the initial five or six dots on the tape as this shows the initial acceleration due to the push.
3. If the paper is turned the wrong way around it will be hard to spot any pattern.

Source : http://www.thephysicsteacher.ie/LC%20Physics/Student%20Notes/6.%20Velocity.doc

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