La Force

Question 1

What is Hooke’s Law?

Answer 1

F=kx
where k is force constant, x is extension (or compression) of spring

Question 2

What is force?

Answer 2

• vector qnty
• SI Unit: Newton (N)
• interact n btw objects, ALWAYS come in pairs

Question 3

What is full name of a force?

Answer 3

The (type of force) by (object 1) on (object 2), (direction)

Question 4

Define centre of gravity

Answer 4

of a body is pt where all weight of body may be considered to act

Question 5

Define centre of mass

Answer 5

of a body is pt where all mass of body seems to be concentrated

Question 6

What does weighing scale really measure?

Answer 6

Normal contact force

Question 7

Friction always act in what direction to surface?

Answer 7

parallel

Question 8

Tensional force always points where?

Answer 8

away fr object, along length of string

Question 9

How does force constant add up in these combination of springs:
- two in series?
- two in parallel?
- two in parallel each of two sides of body?

Answer 9

• two in series
  1/k eff = 1/k1 + 1/k2
• two in parallel
  k eff = k1 + k2
• two in parallel each of two sides of body

k eff = k1 + k2 + k3 + k4

(basically opposite behaviour to resistors in parallel and series)

Question 10

What are the formulae for pressure? Unit?

Answer 10

Unit: Pa (or N m^-2)
Normal: P=F/A
Liquid: P=ρgh

Question 11

How to derive P=ρgh?

Answer 11

Step 1. Consider point at depth h below liquid’s surface in container

Step 2: Pa can be determined by finding weight of vertical column of liquid acting above area A at that depth

Step 3:
Weight W of column of liquid is:
W=mg=ρVg=ρAhg

Step 4: Since pressure P=F/A,
P = ρAhg/A = ρgh

Question 12

Absolute pressure at any pt of column of fluid is given by what eqn?

Answer 12

pressure P = (atmos Pa at fluid open surface) +/- ρgh

Question 13

Define upthrust and give eqn

Answer 13

When object is partially or fully submerged in fluid, it experience upward force known as upthrust. Upthrust results fr diff in hydrostatic Pa btw bottom & top surfaces of object

Upthrust U = Vρg

where ρ & V is density & volume of displaced FLUID

Question 14

State Archimedes’ Principle

Answer 14

Upthrust acting on object submerged in fluid is equal in magnitude, opp in direct n to weight of fluid displaced by object

Question 15

State principle of floatation

Answer 15

When object floats in eqm, upthrust acting on object = object’s weight

Question 16

Define moment of a force. Give eqn

Answer 16

moment of force abt pivot is product of force & perpendicular distance of line of action of force fr pivot

Moment of force abt pivot = F x dsinθ
where θ is acute angle btw direction of force and line d

Question 17

Define couple. What is the torque of a couple?

Answer 17

• consists of 2 forces, equal in magnitude, opp in direct n & whose lines of action do not coincide
• torque of couple is pdt of one of forces & perpendicular distance between lines of action of forces

Question 18

What is interesting abt torque of couple?

Answer 18

No matter where pivot is taken (along perpendicular distance btw lines of action of 2 forces), moment is constant

Question 19

State Principle of Moments

Answer 19

For object to b in rotational eqm, sum of clockwise moments abt any axis must equal sum of anti-clockwise moments abt same axis

Question 20

What are 2 conditions needed for object at eqm?

Answer 20

• resultant force in any direct n must be zero
• resultant torque abt any axis must be zero

Question 21

What are other types of forces?

Answer 21

• Lift
• Thrust
• Electric, magnetic, gravitational force
• Stability