Momentum & Impulse Flashcards
What is the equation for impulse?
Impulse (Ns) = Ft = final mv - initial mv
Note impulse is a vector
What is the equation for momentum?
momentum (Kgms-1 or Ns) = mv
Note momentum is a vector
When doing an impulse, momentum or coefficient of restituion equation what must you do?
Unless vectors are used pick a positive direction
Note for impulse questions the +ve direction is in the same direction as the impulse
What is the law of conservation of momentum?
total mv before = total mv after
This is always true for a system
How do you deal with two balls colliding with one unknown force or mass?
- Draw your diagram labeling up the masses and the velocities
- Pick a +ve direction and do a momentum equation
- Solve for the only unknown
How do you deal with two balls colliding when there is two unknowns e..g two speeds or mass?
- mv before = mv after
- e = sp sep / sp apr
- Solve simultatneously
When asked to describe an objects motion what must you say?
- The direction it is traveling in relative to its initial direction
- The speed in which its traveling at
- Any other relevant information such as decaying amplitude bounces
When doing impulse equations on a system with multiple moving obejects what is the case?
- If the objects collide the impulse acts equally and oposite for either object
- The impluse equation is only relevant for each object seperately not combined
What is the equation for kinetic energy?
KE = 1/2 mv2
Note KE is a scalar not a vector so you must use speed not velocity (can’t have a negative KE but can have a loss in KE)
When working with vectors in mechanics what can be helpful?
Always leave the vectors in (xi+yj) notation
Note sometimes if an object changes direction one or both of the components of its vector velocity must be multiplied by -1
What is the coefficient of restitution equation and what does it represent?
e = speed of separation / speed of approach
0 ≤ e ≤ 1
This represents how ‘elastic’ a collision is. If e=1 speeds before and after a collision would be equal and if e=0 collisions would cause objects to combine
What is the case if you have an object colliding with a fixed wall?
If the speed before the collision is V then the speed after the collision is eV
How do you deal with two balls colliding with three or four unknowns e.g e, speed or mass?
- mv before = mv after
- e = sp sep / sp apr
- rearrange e equation for unknown speeds
- sub into mv equations to get the after speeds in terms of the before speeds and e
Note if you are now asked to find a range of values for e then you must set your ‘after speeds’ equations > or < 0 and solve for an innequality with e
What is the equation for friction?
Fmax = uR
Friction can’t be the resultant force it’s only maximum at the point of moving or when moving and it always opposes motion
If you are given sin(x)=a and you need cos(x) or tan(x) what do you do?
Sin-1(a) = x
cos(x) = b
tan(x) = c
If b and c aren’t exact you must draw a triangle
