NOTES PREPARED BY
ASHAQ HUSSAIN BHAT
GL TEACHER SCHOOL EDUCATION
DEPARTMENT JAMMU AND KASHMIR
MOTION AND TIME
Points to Remember
- Motion of the objects can be either along a straight line or in a circle or it can be periodic.
- The motion of some objects may be slow while that of others fast.
- We can decide as to which object is moving faster than the other only if, the two objects are moving in the same direction.
- The distance moved by the object in a unit time is called speed of the object.
- Speed of objects helps us to decide as to which object is moving faster than the other.
- The speed of an object is measured by dividing the distance travelled by the time taken to travel that distance.
- The S.I. unit of speed is metre per second (m/s or ms-¹).
- If the speed of the object keeps changing while moving in a straight line, the motion is called non-uniform motion.
- Periodic events are used for the measurement of time.
- The object moving along a straight line with a steady speed is said to have uniform motion.
- The time interval between one sunrise and the next sunrise is called a day.
- The time taken by the earth to make one revolution around on the sun is called a year.
- Periodic motion of pendulum has been used to make clocks and watches.
- Clocks and watches are the devices used to measure short time intervals.
- The most common example of periodic motion or oscillatory motion is simple pendulum.
- The time during which the pendulum completes vibration is called time period.
- The to and fro motion of the particle about its mean position is called vibration.
- Speed= Distance x Time.
- Speedometer is a device used to measure the speed of vehicles.
- Odometer is a device which is used to measure distance moved by a vehicle.
- Distance time graph represents the motion of an object in a pictorial form
- The distance-time graph for the motion of an object with constant speed is a straight line.
SOME IMPORTANT DEFINITIONS
Graph. The pictorial representation of two variables interdependent of one another.
Non-uniform motion. If an object covers unequal distances in equal intervals of time, the motion of the object is said to be non-uniform.
Uniform motion. If an object covers equal distances in equal intervals of time, then the object is said to have uniform motion.
Oscillation. The motion of a vibrating particle from one extreme position to another extreme position about its mean position is called oscillation.
Speed. It is the distance covered by an object in a unit time. It is measured by dividing the total distance covered by an object, by the time taken to travel that distance.
Simple Pendulum. It consists of a heavy particle, suspended from a fixed point with the help of inextensible and flexible thread.
Time Period. It is the time taken by a vibrating body (Pendulum) to complete one vibration.
TEXT BOOK EXERCISES
Q. 1. Classify the following as motion along a straight line, circular or oscillatory motion :
(2) Motion of your hands while running.
(ii) Motion of a horse pulling a cart on a straight road.
(iii) Motion of a child in a merry go round.
(iv) Motion of a child over see-saw.
(v) Motion of the hammer of an electric bell.
(vi) Motion of a train on a straight bridge.
Ans. (i) oscillatory (ii) along a straight line
(iii) circular (iv) oscillatory (v) oscillatory
(vi) along a straight line.
Q. 2. Which of the following are not correct :
(i) The basic unit of time is second.
(ii) Every object moves with a constant speed.
(iii) Distances between two cities is measured in kilometres.
(iv) The time period of a given pendulum is not constant.
(v) The speed of a train is expressed in m/h.
Ans. (i) The basic unit of time is second. (Correct)
(ii) Every object moves with a constant speed. (Incorrect)
(iii) Distances between two cities is measured in kilometres. (Correct)
(iv) The time period of a given pendulum is not constant. (Incorrect)
(v) The speed of a train is expressed in m/h. (Incorrect)
Q. 3. A simple pendulum takes 32 s to complete 20 oscillations. What is the time period of the pendulum?
Ans. Number of oscillations = 20
Time taken to complete 20 oscillations = 32 s
Total time taken
Total number of oscillations
Time period of pendulum =Total time taken/Total number of oscillations
=32/20
=8/5
=1.6s Ans
Q. 4. The distance between two stations is 240 km. A train takes 4 hours to cover this distance. Calculate the speed of the train.
Ans. Distance covered by train between two stations = 240 km.
= 3/4
= 45 km / h Ans.
Q. 7. Show the shape of the distance-time graph for the motion in the following cases (i) A car moving with a constant speed. (ii) A car parked on a side road.
Ans.
(i) When speed is constant, the graph is represented by a straight line inclined at 45° to the x-axis
(ii) When the car in standing, the graph will be represented by a line parallel to X-axis.
Q. 8. Which of following relation is correct :
(i) Speed Distance x Time
(ii) Speed= Distance/Time
(iii) Speed= Time/Distance
(iv) Speed= 1/ Distance x Time
Ans. (ii) Speed = Distance / Time
Q. 9. The basic unit of speed is: (i) km/min (ii) m/min (iii) km/h (iv) m/s
Ans. (iv) m/s.
Q. 10. A car moves with a speed of 40 km/h in 15 minutes and then with a speed of 60 km/h for the next 15 minutes. The total distance covered by car is (i) 100km (ii) 25 km (iii)15km (iv) 10km
Ans. Time taken = 15 min
= 15/60
= 1/4 h
Distance covered in 1st case =40/4 = 10km
Distance covered in 2nd case =60/4 = 15 km
Total distance covered in 30 min. = 10 + 25 25 km Ans
Q. 11. Suppose two photographs shown in fig. (a) and fig. (b) had been taken at an interval of 10 seconds. If a distance of 100 metres is shown by 1 cm in these photographs calculate the speed of the blue car ?
Ans. Since scale given is 1 cm = 100 m. So, blue car has moved only 1 cm in picture. Therefore, distance covered by blue car is 100 m.
Q. 12. Figure shows the distance-time graph for the motion of two vehicles A and B. Which one of them is moving faster ?
Ans. A is moving faster.
Q. 13. Which of the following distance-time graphs shows a truck moving with a speed which is not constant.
Ans. (iii) graph will show the movement of truck with a speed which is not constant.
ADDITIONAL IMPORTANT QUESTIONS
Q. 1. List the different units of time.
Ans. Units of time :
1. Nano second,
2. Micro second.
3. Second.
4. Minute.
5. Hour.
6. Day.
7. Week.
8. Month. mas
9. Year.
Q. 2. Which devices were used for measuring time before pendulum clocks ?
Ans. (i) Sundial. (ii) Water clock. (iii) Sand clock.
Q. 3. Compare the speed of a tortoise to a rocket launching satellite.
Ans. Tortoise is a slow moving animal having speed of about 8 cm/s. while rockets have a high speed of about 8 km/s.
Q. 4. What important points are to be kept in mind while choosing the most suitable scale for drawing a graph ?
Ans. Points to be considered while choosing the scale for drawing a graph :
(i) The difference between the highest and the lowest values of each quantity.
(ii) The intermediate values of each quantity.
(iii) To utilize the maximum part of the graph paper.
Q. 5. What information can be obtained from a distance time graph?
Ans. Information given by distance-time graph ir
(i) distance moved by an object at any instant of time.
(ii) distance covered in any time interval.
Q. 6. Who discovered that time period of a given pendulum is constant? How?
Ans. Galileo discovered that time period of a given pendulum is constant. Once, Galileo was sitting in a church when he noticed the oscillations made by lamp suspended from the ceiling with the help a chain. He noted his pulsebeat and found same number during one oscillation completed by lamp. He then experimented with different pendulums and verified that each pendulum completed one oscillation in a fixed time.
Q. 7. What is a simple pendulum?
Ans. Simple pendulum. It consists of a small metallic ball or a piece of stone suspended from a rigid support by a thread. The metallic ball is called the bob of the pendulum.
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