Mechanics 3 - Newton's Laws, Terminal Velocity, Momentum Flashcards
What is Newton’s 1st Law of Motion?
If no resultant force acts on a body, then it will either remain at rest, or continue moving with constant velocity (no acceleration)
What is Newton’s 2nd Law of Motion?
The rate of change of momentum (acceleration) of a body is directly proportional to the resultant force acting on it
Fres ∝ a
What is Newton’s 3rd Law of Motion?
When two objects interact, they exert an equal and opposite force on each other and the forces are of the same type
If the forces acting on an object are balanced what can you say about its motion?
There is no resultant force so it will continue moving at a constant velocity. It won’t accelerate.
What’s wrong with this?
In F=ma, F must be the resultant force!!!
How does an object reach terminal velocity?
As it speeds up, air resistance increases, decreasing the resultant force.
Eventually air resistance = driving force, Fres=0 so a=0.
What two things are the case for tension?
- Tension always acts away from the contact points
- Tension is constant throughout the rope/wire/ cable
Why are objects never truly in freefall?
There will always be air resistance opposing the weight
(Apart from when v=0)
What is the condition for terminal velocity?
The drag force = driving force (or weight) so Fres=0 and so a=0
What factors the drag force on an object?
- Fluid density
- Shape of object
- Cross sectional area of object
- Velocity of object
Why does air resistance increase with velocity?
The object is colliding with more air molecules per second
What does the velocity time graph of an object reaching terminal velocity look like?
What’s wrong with this?
The acceleration is not constant so you cannot use SUVATs
Instead use area under graph
How is momentum calculated?
What’s wrong with this?
Direction must be taken into account (as momentum is a vector)
What two things is impulse equal to?
- Rate of change of momentum
- Impact force x impact time
What are the units of impulse and momentum?
What does the area under a force-time graph represent?
The change of momentum or impulse
What is the conservation of momentum?
For a system of interacting objects, the total momentum remains constant…
…provided no external resultant force acts
In any interaction, what is conserved?
Total momentum is always conserved
Total energy is always conserved
Kinetic energy is only conserved if collision is elastic
What is an elastic collision?
A collision where kinetic energy is conserved
(as well as momentum)
What is wrong here?
You have to calculate kinetic energies separately for each object
In physics terms what is an explosion?
The total momentum = 0
How do you answer flow rate questions? (momentum of a flowing liquid)
Consider the cylinder made by a liquid’s flow after 1 second
Where the length of the cylinder = velocity of the fluid
And use density equation to get volume of cylinder
How do you work out the area of a curved graph?
- Split into boxes
- Count the boxes (pairing up incomplete boxes)
- Multiply number of boxes by area of each box
