MCQ 9702/11/O/N/10


Dynamic Motion

1. In order that a train can stop safely, it will always pass a signal showing a yellow light before it reaches a signal showing a red light. Drivers apply the break at the yellow light. This results in an uniform deceleration.

The distance between the two light is ‘x’.

If the speed of the train would be 20% faster at the yellow light, what must be the minimum distance between the two lights. The deceleration does not change.

How to solve:

From the equations of motion we select v² = u² + 2as

In the case of the speed = 100%,  0 = u² + 2ax ⇒ x = u²/2a

In the case of the speed = 120%, 0 = (1.2u)² + 2ax ⇒ x = (1.44u²)/2a

Ratio of distance in case one to distance in case two is 1 to 1.44 therefore the distance new distance must be 1.44x

 

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s