Final answer:
When an object is projected vertically upwards from the ground at speed u, its speed when it reaches the ground can be determined using the equations of projectile motion. The final velocity when the object reaches the ground will be -u - gt, where u is the initial velocity, g is the acceleration due to gravity, and t is the time taken.
Step-by-step explanation:
When an object is projected vertically upwards from the ground at speed u, its speed when it reaches the ground can be determined using the equations of projectile motion. In this case, we can use the equation v = u + gt, where v is the final velocity, u is the initial velocity, g is the acceleration due to gravity (approximately 9.8 m/s^2), and t is the time taken.
Since the object is projected vertically upwards, its final velocity will be negative when it reaches the ground. This is because the object is moving in the opposite direction to its initial velocity. So, the speed at which the object reaches the ground is -u - gt.
For example, if the object is projected upwards with a speed of 10 m/s, the final velocity when it reaches the ground will be -10 - 9.8t, where t is the time taken for the object to reach the ground.