# A question

Show the steps very clearly and explain it!

1. A boy jumps up and his feet are 0.5m above the ground.

(a) Find the hang-time of the boy.

(b) Find the jumping speed of the boy.

Why u = 0 m/s?

Why u = 0 m/s? <------ part (a)

v cannot be 0 m/s as well!

Help me!

### 2 Answers

- 1 decade agoBest Answer
List out your known and unknown values first:

delta y = 0.5m

u initial = ?

u final = 0 m/s

delta t = ?

acceleration = gravitational acceleration = - 9.8 m^2

Then use these two equations to solve the problem:

1) delta y = u initial * t + 0.5 a * t^2

2) (u final - u initial) / t = a

Since you have two unknowns in two equations, you should be able to solve this problem. But remember: the t you got from the above equation is just the time it takes for the boy to jump up because you use u final is zero. That means you treat the maximum point is the end point. So if you need to know the totle hang time, you need to double the t you obtained because the boy is still "hanging" when he is going down.

We use the gravitational acceleration as the accelaration value is because while the boy is jumping, the only one force acts on the boy is gravity. Since F(net) = ma, if the only net force acts on the boy is gravity, the acceleration should be -9.8 m/s. We put minus sign in front of 9.8 because the gravitational force always accelerate an object in a downward direction. But why we still put the minus sign in front of 9.8 when the boy is jumping up? Do you remember the minus sign for the acceleration has two meanings:

1) it represents the direction 2) it represents deceleration. What about for velocity? the minus sign only implies the direction. So when the boy is jumping up, the velocity is in the positive direction. The minus sign in this case is only means the boy is decelerating. Anytime when you see the sign of the acceleration is the opposite to the velocity, the object is decelerating, which makes sense because when you are jumping up, you are speeding down. If you see the sign of the acceleration is the same as the velocity's sign. The object is speeding up. So when the boy is going down, the velocity is in the negative direction, and it is speeding up. So the sign is also negative for the acceleration. That's why 9.8m/s^2 is always negative.

u = 0 m/s^2 is because when the boy is in the maximum point, it has to "stop" and then change its direction. Just like when you are walking on a street, you can't turn back if you don't stop. Your feet will not keep moving when you are trying to turn back.