A boy of mass 40 kg jumps from rest from a platform of height 3 m. He lands by bending his knees and stops his body in 0.5 s after landing. What is the force exerted by his legs?

**Solutions: 620 N**

To solve this question, you need to apply conservation of energy (COE) to find the kinetic energy (KE) that the boy possesses just before he reaches the ground.

After which, you can solve the question using (1) Conservation of energy or (2) Kinematics to solve.

Using COE to find the KE just before he reaches the ground.

**Method 1: Using Conservation of Energy (COE)**

**Method 2: Using Kinematics**

You must know these 2 methods and always think along this 2 directions when dealing with such questions.

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October 23, 2010 at 5:10 AM

When the man is in mid-air, it has both KE and GPE as you fixed ground as reference height. So at ground height is considered 0m. At the instant just before he touches the ground, all the energy he possesses is only KE. GPE is zero as at that instant, the height of man from ground is 0m. Get it? =)For the latter question on ball, it depends where is the lowest point. It is always good to fix lowest point as reference height. If the ball returns to the hand, then the point where the ball leaves the hand is reference height (lowerst point). Then you can consider all the energy when ball leaves the hand is max KE. And when it reaches highest point (from the hand), it has the max GPE. When it is about to reach the hand once again, just before it reaches, it is the max KE.If the ball thrown up and then fall over a cliff and final point is lower, then the lowest point will be reference height. Then the energy of the ball just after release will have KE + GPE. Understand?