Test yourself: -

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1. If a toy car travels 6 metres in 2 seconds, how can its speed be calculated?

2. A toy car travelling at 10m s-1 slows down to 2m s-1 in 2 seconds. Calculate the acceleration of the toy car.

3. A static object takes 5 seconds to reach an acceleration of 15ms-2. What is its final velocity?

Measuring velocity and acceleration

Acceleration may have a positive or negative sign. A negative sign means the object is moving the opposite direction.

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1. A variety of methods can be used to determine the velocity and acceleration of a moving object.

2. One of the most common techniques is using a ticker timer.3. A ticker timer has a vibrating arm that vibrates with a frequency

of 50 Hz and produces a series of dots on a piece of ticker tape that is being dragged along by a moving object.

4. When the power is supplied to ticker timer, the vibrating arm will print dots onto a ticker tape attached to a moving object.

5. Since the vibrating arm vibrates with a frequency of 50 Hz, it will print 50 dots onto the ticker tape in a second. Hence, the time between two dots (called tick) is 0.02 s.

6. The velocity of a moving object such as a moving trolley can be measured by attaching the ticker tape to the trolley.

7. When the trolley moves, it pulls the ticker tape through the ticker timer which marks dots on the tape.

8. The faster the trolley moves, the further apart the dots marked on the tape.

9. The tape is then cut into 10-stick strips and pasted side by side to form a tape chart (Figure 5.12).

10. W can then measure the length of the tape to calculate the velocity and acceleration of the trolley.

11. We can obtain various patterns of dots printed on the tape by a ticker timer, depending on the type of motion of an object.

12. Figure 5.13 shows the various patterns of dots on the tape, produced by various types of motion.

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0

2

4

6

8

10

12

14

16

18

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Figure 5.12 A tape chart

Calculation:The velocity of the first strip

=6 cm

10 x 0.02 s= 30 cm s-1

The velocity of the last strip

=18 cm

10 x 0.02 s= 90 cm s-1

The acceleration of the trolley

=(90-30) cm s-1

(6-1) x 0.2 s= 60 cm s-2

Number of ticks

Distance (cm)

10 30 40 50 6020

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Test yourself:

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. . . . . . . . . . . .

a) If a car travels 300 m in 15 s, what is its average speed in m s-1?

b) The speed limit on a highway is 110 km h-1. In order to detect speeding, the police set up two photo–detectors 80 metres apart on the roadside. A car drove past the two detectors in 3 seconds. Calculate the average speed of the car between the two detectors, in km h-1. Would the driver be prosecuted for speeding?

c) A motorcycle traveling on a straight road takes 4 s to increase its speed from 20 m s-1 to 40 m s-1. What is its acceleration?

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The Concept of InertiaInertia1. Figure 5.18 shows two men trying to push two wheelbarrows, one is

empty and the other one is fully loaded. Obviously it is easier to push the empty wheelbarrow.

Figure 5.18 It is more difficult to push a fully-loaded wheelbarrow than an empty one.2. Now imagine they are pushing the wheelbarrows down a slope. It is

very difficult to control the one with the full load. It tends to slide down the slope very fast.

3. The above scenario actually involves he concept of inertia.4. All objects possess inertia.5. The inertia of an object refers to

a) The inertia of an object refers to the reluctance of an object to start moving if it is at rest.

b) The reluctance of an object to stop moving if it is moving in the first instance.

6. Inertia is defined as the tendency of an object to maintain its state of rest or uniform motion.

7. This means if an object is at rest, it tends to remain at rest unless a force makes it move.

8. On the other hand, an object which is moving at a constant velocity will continue to do so unless other forces act on it.

A B

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9. In other word, we can also define inertia as the resistance of an object to change its state of rest or uniform motion.

Test yourself:1. What is inertia?________________________________________________________

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