4 Forces in Action

Question 1

Describe Newton’s Second Law

Answer 1

Force is directly proportional to the rate of change of momentum IN THE SAME DIRECTION.

Question 2

Describe weight

Answer 2

Weight of an object is the gravitational force acting on an object. F=ma ∴ W=mg

Question 3

What is the centre of mass

Answer 3

The resultant gravitational force on the object, total weight acting through a singular point.

Question 4

Practical - finding the centre of gravity

Answer 4

Freely suspended object comes to rest with centre of mass vertically below point of suspension. Plumb-line - plumb-bob comes to rest, string is vertical. Where these intersect - centre of gravity.

Question 5

6 Forces acting on an object

Answer 5

Weight, friction, drag (air resistance), tension, upthrust, normal contact force

Question 6

Force parallel to the slope?

Answer 6

Fx=mg sinθ
(Wsinθ)

Question 7

Force perpendicular to the slope?

Answer 7

Fy=mg cosθ
(Wcosθ)

Question 8

Define drag force

Answer 8

Drag is a frictional force that opposes the motion of an object.

Question 9

What affects the magnitude of a drag force?

Answer 9

SPEED of an object
cross-sectional AREA
- large area increase drag
- drag force ∝ speed^2

Question 10

Define terminal velocity

Answer 10

Drag force on object is equal and opposite to its weight. Zero acceleration, constant velocity.

Question 11

Define moment

Answer 11

The moment of a force is the turning effect of a force about some axis.
Moment=force X perpendicular distance

Question 11

Moments - how to find perpendicular distance?

Answer 11

x=d cosθ

Question 12

Principle of moments?

Answer 12

For a body in ROTATIONAL equilibrium the sum of the anticlockwise moments is equal to the sum of the clockwise moments

Question 13

Define couple?

Answer 13

Two forces equal and parallel and along different lines

Question 14

Define torque?

Answer 14

Moment of a couple is a torque.
Torque of a couple= one of the forces X perpendicular separation between forces

Question 15

Density equation?

Answer 15

Mass per unit volume ρ=m/v (kg m^-3)

Question 16

Pressure equation and definition?

Answer 16

Normal force exterted per unit cross-sectional area. p=F/A
1Pa= N m^-2

Question 17

Pressure in FLUIDS equation?

Answer 17

p=hρg

Question 18

Derive equation for pressure in FLUIDS

Answer 18

pressure at base=weight/area
W=mass * g (mass=density X volume)
W= ρV * g (volume=Ah)
W= ρ * Ah * g
P =(ρAh*g)/A
P=hρg

Question 19

What is pressure proportional to?

Answer 19

p∝h
- water pressure increases with depth
ρ shows denser liquids will exert greater pressure

Question 20

Define Upthrust

Answer 20

The upward force exerted by a fluid that opposes the weight of a partially or fully immersed object.

Question 21

Equation of upthrust

Answer 21

force at the top: hρgA
force at bottom: (h+𝑥)ρgA -hρgA = 𝑥ρgA
∴ upthrust=A𝑥ρg

Question 22

State Archimedes’ principle

Answer 22

The upthrust exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.