Worked Examples
Impulsive forceSuppose the force F is very large and acts for a very short time. During this time the distance moved is very small and under normal analysis would be ignored. Under these condition the only effect of the force can be measured is the impulse, or change I momentum - the force is called an impulsive force.
In theory this force should be infinitely large and the time of action infinitely small. Some application where the conditions are approached are collision of snooker balls, a hammer hitting a nail or the impact of a bullet on a target.
A nail of mass 0.02 kg is driven into a fixed wooden block, Its initial speed is 30 m/s and it is brought to rest in 5ms. Find
From Equation 4.1
A football of mass 0.45 kg travels in a straight line along the ground reaching a player at 10m/s. The player passes it on at 8m/s altering its direction by 90° .
Find the impulse given to the ball by the player.
Solution
Choose the co-ordinate system like that in Figure 4.1
Figure 4.1: Co-ordinate system and path of ball in Worked Example 4.1
Initial velocity in the direction Ox is 10 m/s. Final velocity in the direction Ox is zero. So change in velocity in the direction Ox is -10m/s.
Initial velocity in the direction Oy ia zero and final velocity in the direction Oy is 8m/s. So change in velocity in the direction Oy is 8m/s.
The resultant change in velocity is
impulse is change in momentum is mass times change in velocity
