Momentum

[hshphyss]

Can anyone help me with this problem? Thank-you

A 87 kg pole-vaulter falls from rest from a height of 6.8 m onto a foam-rubber pad. The pole-vaulter comes to rest 0.30 s after landing on the pad.
(a) Calculate the athlete's velocity just before reaching the pad ( in m/s downward).
(b) Calculate the constant force exerted on the pole-vaulter due to the collision (in N upward).

Would you incorporate kinematics in here or not? {x=1/2(vi+vf)t}

[Diane_]

Part (a) is a kinematics problem, so you're on the money there. The equation you mention will work just fine.

For part (b), think impulse.

[hshphyss]

For part a i got 6.8=1/2(0+vf)0.30 using kinematics and i got 45.33 m/s and i also tried -45.33 but neither worked. Did I do the work correctly?

[Diane_]

I'm sorry, hshphyss. I lied. The equation you mentioned will work, but you need the time of fall. What you have is the stopping time.

Go back and refigure it as a free fall problem. How fast will something be falling if it starts from rest and falls a distance of 6.8 m? That's the kinematics problem that you solve for the first part. I'm getting 12 m/s.

After that, the time you have for it to stop plus the concept of impulse should give you the average force. I'm getting 3300 N. Remember that minus and plus signs here just indicate direction - and you already know the directions involved.

[hshphyss]

thank-you so much