Mechanics 3 - Newton's Laws, Terminal Velocity, Momentum

Question 1

What is Newton’s 1st Law of Motion?

Answer 1

If no resultant force acts on a body, then it will either remain at rest, or continue moving with constant velocity (no acceleration)

Question 2

What is Newton’s 2nd Law of Motion?

Answer 2

The rate of change of momentum (acceleration) of a body is directly proportional to the resultant force acting on it

F_res ∝ a

Question 3

What is Newton’s 3rd Law of Motion?

Answer 3

When two objects interact, they exert an equal and opposite force on each other and the forces are of the same type

Question 4

If the forces acting on an object are balanced what can you say about its motion?

Answer 4

There is no resultant force so it will continue moving at a constant velocity. It won’t accelerate.

Question 5

What’s wrong with this?

Answer 5

In F=ma, F must be the resultant force!!!

Question 6

How does an object reach terminal velocity?

Answer 6

As it speeds up, air resistance increases, decreasing the resultant force.

Eventually air resistance = driving force, F_res=0 so a=0.

Question 7

What two things are the case for tension?

Answer 7

1. Tension always acts away from the contact points
2. Tension is constant throughout the rope/wire/ cable

Question 8

Why are objects never truly in freefall?

Answer 8

There will always be air resistance opposing the weight

(Apart from when v=0)

Question 9

What is the condition for terminal velocity?

Answer 9

The drag force = driving force (or weight) so F_res=0 and so a=0

Question 10

What factors the drag force on an object?

Answer 10

• Fluid density
• Shape of object
• Cross sectional area of object
• Velocity of object

Question 11

Why does air resistance increase with velocity?

Answer 11

The object is colliding with more air molecules per second

Question 12

What does the velocity time graph of an object reaching terminal velocity look like?

Answer 12

Question 13

What’s wrong with this?

Answer 13

The acceleration is not constant so you cannot use SUVATs

Instead use area under graph

Question 14

How is momentum calculated?

Answer 14

Question 15

What’s wrong with this?

Answer 15

Direction must be taken into account (as momentum is a vector)

Question 16

What two things is impulse equal to?

Answer 16

1. Rate of change of momentum
2. Impact force x impact time

Question 17

What are the units of impulse and momentum?

Answer 17

Question 18

What does the area under a force-time graph represent?

Answer 18

The change of momentum or impulse

Question 19

What is the conservation of momentum?

Answer 19

For a system of interacting objects, the total momentum remains constant…

…provided no external resultant force acts

Question 20

In any interaction, what is conserved?

Answer 20

Total momentum is always conserved

Total energy is always conserved

Kinetic energy is only conserved if collision is elastic

Question 21

What is an elastic collision?

Answer 21

A collision where kinetic energy is conserved

(as well as momentum)

Question 22

What is wrong here?

Answer 22

You have to calculate kinetic energies separately for each object

Question 23

In physics terms what is an explosion?

Answer 23

The total momentum = 0

Question 24

How do you answer flow rate questions? (momentum of a flowing liquid)

Answer 24

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

Question 25

How do you work out the area of a curved graph?

Answer 25

1. Split into boxes
2. Count the boxes (pairing up incomplete boxes)
3. Multiply number of boxes by area of each box