Chapter 13 Motion and Time

(i) Oscillatory motion                                                                                                                                                                                                                  Motion of our hands while running is oscillatory because while running our hands does to and froth motion in the same interval of time, is called oscillatory motion and is also called Periodic motion. The swinging of a pendulum is an example of periodic motion.

(ii) Straight line motion                                                                                                                                                                                                                      If an object changes its position with respect to its surroundings with time, then it is called in motion. It is the straight line motion because in this the object changes its position along the straight line with respect to time. For example: car is running on the straight road.  

(iii) Circular motion                                                                                                                                                                                                                          When an object moves along a circular path without changing its position, it is called circular motion. For example: the earth moves around the sun in a circular path in its particular orbit, so the motion of the earth around the sun is circular motion; ceiling fan, wheel of the bicycle.

(iv) Oscillatory motion                                                                                                                                                                                                                   The motion which repeats itself after regular intervals of time, is called oscillatory motion and also called Periodic motion. The swinging of a pendulum is an example of periodic motion; swinging of the swing, vibration of the mobile phone etc.

(v) Oscillatory motion                                                                                                                                                                                                                    The motion which repeats itself after regular intervals of time, is called oscillatory motion. When the hammer hits the electric bell then it vibrates repetitively, so this repetitively motion is oscillatory motion.

(vi) Straight line motion                                                                                                                                                                                                                      If an object changes its position with respect to its surroundings with time, then it is called in motion. In this, the train is moving on the straight bridge, it shows the change in position of the train along the straight line that shows straight line motion.  

(i) Correct                                                                                                                                                                                                                                        The SI (Systeme international) unit of time is second according to the metric scale.

(ii) Not correct                                                                                                                                                                                                                             Every object has its own speed and variable from others. For example; in comparison to: a running horse and a moving tortoise.

(iii) Correct                                                                                                                                                                                                                                      The distance between two cities is very large so we cannot measure it in meters; so kilometres is a bigger unit thus it is used to measure the distance between the two cities.

(iv) Not correct                                                                                                                                                                                                                                The time period of the pendulum is constant, because it shows to and froth motion which is an example of periodic motion (occurs in the same interval of time).

(v) Not correct                                                                                                                                                                                                                                 We cannot express the speed of train in m/h because meter is a too short unit for measuring the distance covered by the train per hour. So, we use km/h to measure the distance covered by train.

The time taken to complete one oscillation is known as time period of the pendulum.

Time period = Total time taken
                     Number of oscillations

Given - 20 oscillation taking 32 s to complete
Therefore, 1 oscillation will take = 32 sec = 1.6 second.
                                                       20
Therefore, 1.6 is the time period of the pendulum.

Given;
Speed = Distance travelled
                           Time
= 240 km/ 4 h          (Given)
= 60 km/h

Therefore, the speed of the train is 60km/hr.

Initial reading of the odometer of the car = 57321.0 km
Final reading of the odometer of the car = 57336.0 km

The car starts at 8:30 AM and stops at 8:50 AM
Distance covered by car = (57336 - 57321) km = 15 km

Time taken between 8:30 AM to 8:50 AM = 20 minutes =20/60 hour = 1/3 hour

So speed in km/min
Speed = Distance travelled
                           Time
=15 km / 20 min
0.75 km/ min

Speed in km/h
Speed = Distance travelled
                          Time
= 15 km/ 1/3 h
= 15 x 3 km/ 1 h
= 45 km/h

Speed = 2 m/s
Time taken to reach school = 15 minutes = 15 x 60 seconds
                                                                  = 900 seconds
Speed = Distance travelled
                         Time
Distance= speed x time
               = 2 x 900
               = 1800 m
Also, 1 km = 1000 m

Therefore, 1800 x 1/1000 = 1.8 km.
Hence, the distance between her house and the school is 1.8 km.

(i) When a car is moving with the constant speed it means the car covers the equal distance in equal interval of time then the graph shows straight line.

(ii) A car parked on the road side, it means that there is no change in the position of the car with respect to time. Then the distance time graph represents the parallel line along with the time.

(ii) The speed of an object is given by the relation:

Speed = Distance travelled
                        Time

The speed is defined as the distance covered by an object per unit time. The SI unit of time is m/s.

(iv) m/s
The basic unit of distance is meter (m) and of time is second(s).

Speed = Distance travelled
                           Time
Therefore, the basic unit of speed is m/s.

(ii) 25 km

Case 1.
Speed = 40 km/h
Time = 15 min = (15/60) hour
Distance (d₁) = speed x time = 15/60= 10 km

Case 2.
Speed = 60 km/h
Time = 15 min = (15/60)hour
Distance(d₂) = speed x time = 60*(15/60) = 15 km
Total distance (d) = (d₁ + d₂) = 10 km + 15 km = 25 km

Therefore, the total distance covered by the car = 25 km.

With the help of the scale we will first measure the distance:

Suppose the distance measured is 2 cm
So, the distance covered = 2*100 = 2 m (because 1 m = 100 cm)
Time taken = 10 seconds

Speed = (distance/ time) = (200 m/10 s) = 20 m/s
Therefore, the speed of the blue car = 20 m/s.

This graph shows that vehicle A is moving faster than B.

Speed     =    Distance travelled                                                                                                                                                                                                                        Time

Vehicle A is faster that can be explained by drawing a perpendicular line along to the time. This perpendicular line cuts the B line at point b and A line is cut by the point a.

• Now compare point a and point b. Point a shows that more distance covered in time t. Point b shows less distance covered with respect to time. Now it is evident that, from the graph in the given time t, distance covered by A is more than B.

• Graph 1^st is not correct because this shows the constant speed of the truck which is represented by the straight line.
• Graph 2^nd is not correct because the distance/ time graph shows a line that is parallel along the time axis. This shows that truck is not moving.
• Graph 4^th is not correct because the distance/ time graph indicates that the truck is moving with a constant speed with the same interval of time.
• Graph 3^rd is correct.

Because in the distance/ time graph, a curved line indicates that the truck is moving with a variable speed with respect to time.