school zones. Example . Reaction time : When a situation demands our immediate action, it takes some time before we really respond. Reaction time is the time a person takes to observe, think and act.
For example, if a person is driving and suddenly a boy appears on the road, then the time elapsed before he slams the brakes of the car is the reaction time. Reaction time depends on complexity of the situation and on an individual. You can measure your reaction time by a simple experiment. Take a ruler and ask your friend to drop it vertically through the gap between your thumb and forefinger (Fig.
. ). After you catch it, find the distance d travelled by the ruler. In a particular case, d was found to be .
cm. Estimate reaction time. Answer The ruler drops under free fall. Therefore, v o = , and a = – g = – .
m s – . The distance travelled d and the reaction time t r are related by Or, Given d = . cm and g = . m s – the reaction time is Fig.
. Measuring the reaction time. SUMMARY . An object is said to be in motion if its position changes with time.
The position of the object can be specified with reference to a conveniently chosen origin. For motion in a straight line, position to the right of the origin is taken as positive and to the left as negative. The average speed of an object is greater or equal to the magnitude of the average velocity over a given time interval. .
Instantaneous velocity or simply velocity is defined as the limit of the average velocity as the time interval ∆ t becomes infinitesimally small : lim v lim The velocity at a particular instant is equal to the slope of the tangent drawn on position-time graph at that instant. . Average acceleration is the change in velocity divided by the time interval during which the change occurs : = ∆ . Instantaneous acceleration is defined as the limit of