Physics Chap 9 Review

Physics Chap 9 Review

Things you need to know, and will be provided with

• p= momentum

• p=mv. OR....

   $\Delta$  p = m  $\Delta v$     or p= m(v final - v initial)

• So, impulse theorem is F $\Delta$  t =  $\Delta$  p   or Ft=m(v final - v initial)

• In a collision where the objects stick together.

• MaVa + MbVb = Ma+b Vf

• And last, recoil.   MaVa = -MbVb

Lets look at a few

• A 5500-kg freight truck accelerates from 4.2 m/s to 7.8 m/s in 15.0 s by the application of a constant force

• What change in momentum occurs?

• so, p=m(Vf-Vi). or 5500(7.8 - 4.2)

• How large a force is exerted?

• Ok took 15 secs, so..

• Ft = m(Vf-Vi). or. F=m(Vf -Vi)/t. 5500(7.8-4.2)/15

What will a level 4 look like?

• Small rockets are used to slightly adjust the speeds of spacecraft. A rocket with a thrust of 35 N is fired to change a 72,000-kg spacecraft’s speed by 63 cm/s. For how long should it be fired?

• Ok still pretty simple, but where is the catch?

• Ft=change in p. so t=M9Vf-Vi)/F.

• catch is, what is the velocity in???

Another example

• SpongeBob throws a spatula in a fit of anger at Mr. Krabs standing at the counter who catches it and flies backwards at some speed. How fast did Spongebob throw the spatula?(

• Assume MsVs + MkVk = Ms+k x Vf

• (s= spatula and k = mr krabs, and Vf is velocity final)

• Rearrange to Vs = (Ms+k)(Vf)/Ms

• remember, Mr. Krabs wasn't initially moving

Last one

• Diego and Keshia are on in-line skates. They stand face-to-face and then push each other away with their hands. Diego has a mass of 90.0 kg, and Keshia has a mass of 60.0 kg. Keshia flies away at 5.5 m/s. How fast then did Diego go?

• MdVd = -MkVk

• So, Vd= -(MkVk)/Md

• 
  

• 
  

Things to remember

• Just use the right equation, and rearrange

• Keep in mind V final or V initial, but if at rest, it is 0!

• Watch units!!! kg, m/s.....