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Math Student

Guest

Kinetic Energy and Speed

I'm a little stumped by this question.

A dummy is dropped 60m. The dummy's wight is 950N. And we take g=10m/s².

I need to work out:
the mass - which I got to be 95kg
the gravitational potential energy - which I got to be 5700J
the kinetic energy - but how do you work it out without the speed?
the dummy's speed as it hits the ground - but how do find the time?

Thank you in advance.

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Kinetic Energy and Speed

Mass and potential energy are correct.

I'm assuming the question wants you to find the kinetic energy of the dummy as it hits the ground. To find this, remember that the total energy is conserved. Also note that when it hits the ground, it won't have any potential energy anymore.

The speed as it hits the ground is easy enough to find out once you answer the previous question.
You could then use that to work ot the time taken to fall, if you wanted.

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Why did the vector cross the road?
It wanted to be normal.

Math Student

Guest

Re: Kinetic Energy and Speed

Oh, so would the kinetic energy just be 5700J?

But don't you need to use the formula K.E=1/2*mass*velocity²?

If it is 5700J then the speed would be: √(11400/95) which is 10.95m/s²?

mathsyperson

Moderator

Registered: 2005-06-22

Posts: 4,900

Re: Kinetic Energy and Speed

Exactly.

The 1/2 mv² formula is one way of finding kinetic energy, but it is by no means the only way. In this question you use a different method.

(You could actually find the dummy's speed when it hits the ground directly from the distance it travels and its acceleration, then find the KE your way. It's more work though, overall.)

---

Why did the vector cross the road?
It wanted to be normal.