Class 7 Science Chapter 9 MCQ

The two extremely small units of time which are used for expressing very, very small-time intervals are “microsecond” and “nanosecond”. One microsecond is one-millionth of a second. A nanosecond is one-billionth of a second.

The movement of an objects is called motion, not be “stationary-motion”.

The motion which repeats itself after regular intervals of time, is called periodic motion.

Here, all the statements are correct. Hence, your answer will be option [D].

Here, all the statements are correct. Hence, your answer will be option [D].

Here, all the statements are correct. Hence, your answer will be option [D].

When the distance travelled is measured in kilometers and the time taken is measured in hours then the speed will come in the unit of kilometer per hour.

Here, all the statements are correct. Hence, your answer will be option [D].

Here, all the statements are correct. Hence, your answer will be option [D].

Here, all the statements are correct. Hence, your answer will be option [D].

Here, all the statements are correct. Hence, your answer will be option [D].

Here, all the statements are correct. Hence, your answer will be option [D].

Different units of time are used depending on the need. For example, it is convenient to express your age in years rather than in days or hours. Rockets, launching satellites into earth’s orbit, often attain speeds up to 8 km/s. On the other hand, all units (given in options) are not suitable for expressing the speed of a rocket. Hence, your answer will be option [B].

We know that odometer is an instrument for measuring the distance travelled by a vehicle. Odometer records the distance travelled by a vehicle in kilometers.

Pendulum clocks are good to use among these options. To measure time intervals shorter than one day, a number of times measuring devices were invented in ancient times. Some of the important time measuring devices used in ancient times were: sundial, sand clock and water clock.

We should first convert the time of 15 minutes into hours by dividing it by 60 = 15 /60 hours = 0.25 hour i)In the 1st case: Distance travelled = Speed x time = 40 x 0.25 km = 10 km ii)In the 2nd case: Distance travelled = Speed x time = 60 x 0.25 km = 15 km Total distance: 10 km +15 km = 25 km

The option [B] is not relevant to unit of speed. Rest of all could be the unit of speed, where necessary. Hence, your answer will be option [B].

If the distance time graph of an object is a straight line, it indicates that the object is moving with a uniform speed or constant speed i.e., its speed is not changing at all. The slope of a distance-time graph indicates the speed of the objects. If the distance time graph has a low slope then the object has low speed. On the other hand, if the distance time graph has a highest slope then the object has a higher speed.

The time measured by “Quartz clocks and watches” is much more accurate than that measured by clocks and watches which were available earlier. Thus, very accurate measurement of time is not possible by using “Quartz clocks and watches”. Quartz is a hard mineral consisting of silica. These days, the regular of oscillations or vibrations of a tiny quartz crystals when joined into an electric circuit, are used for measuring time very accurately.

In this problem, the speed is given in metre per second and the time is given in minutes. So, before putting in the formula for speed, we will have to convert time from minutes into seconds by multiplying the minutes by 60. Time taken = 15 minutes = 15 x 60 seconds = 900 s Speed = 2 m / s Distance travelled? (To be calculate) Now, Speed = Distance travelled / Time taken 2 = Distance travelled / 900 By cross multiple: Distance travelled = 2 x 900 m = 1800 m

We know that the fact that the time period of a given pendulum is constant was discovered by a scientist named “Galileo Galilei” just by chance. The periodic motion of a pendulum is utilized in pendulum clocks for measuring time. Remember, the pendulum clocks were big and bulky so they could not be carried everywhere is easily. In order to make small, light and portable clocks and watches a kind of modified pendulum was developed which is much smaller and compact than the traditional pendulum used in pendulum clocks, which is called balance wheel.

In order to solve this problem, we have to calculate the speed of both the trains separately. The train having higher speed will have travelled faster: We know that, Speed = Distance travelled / Time taken Distance travelled by train “A” = 150 km Time taken by train “A” = 3 h Speed of train “A” = 150 km / 3 h = 50 km/h Again, we know that: Speed = Distance travelled / Time taken Distance travelled by train “B” = 180 km Time taken by train “B” = 4 h Speed of train “B” = 180 km / 4 h = 45 km/h

Different units of time are used depending on the need. For example, it is convenient to express your age in years rather than in days or hours. Similarly, it will not be wise to express in years the time taken by you to cover the distance between your home and your school. Rockets, launching satellites into earth’s orbit, often attain speeds up to 8 km/s. On the other hand, a tortoise can move only with a speed of about 8 cm/s. Although snail is very slower animals comparing to tortoise, earthworm. The most appropriate unit for expressing the speed of a snail is mm/s.

We know that the distance time graph is also called “Line graphs”. This line graph shows the variation of distance travelled by a moving object with time. A line graph can also be drawn to show the variation of any other related quantities such as a variation of weight of a man with age. So, here option [C] is the correct answer.

Explanation: The distance time graph for an object which stops moving is a straight line parallel to the time axis. When an object becomes stationary its speed is zero. So, a straight-line graph parallel to the time-axis shows that the speed of object is zero.