When the lift is moving down with an acceleration
' a ' the weight of the person W is, given by
W = mg = ma = m(g-a)
Fractional change is the weight of the person
= \(\frac { mg-w }{ mg } \)
= \(1-\frac { w }{ mg } -\frac { mg-(mg-ma) }{ mg } \)

 \(=\frac { a }{ g } =\frac { 5.0 }{ 9.8 } =0.51\)
=Percentage change = 0.51\(\times\)100 
= 51 % 

Force F = F₁ = F₂
F₁ = \(\frac { dp }{ { dt }_{ 1 } } ,{ F }_{ 2 }=\frac { dp }{ { dt }_{ 2 } } \)
\({ F }_{ 2 }={ F }_{ 1 }=\frac { { dt }_{ 1 } }{ { dt }_{ 2 } } \)
\(=10\times \frac { 8 }{ 4 } =20N\)

(i) Centrifugal force is a pseudo force, its effects are real.
(ii) When a car takes a turn in a curved road, person inside the car feels an outward force which pushes the person away.
(iii) This outward force is also called centrifugal force.
(iv) If there is sufficient friction between the person and the seat, it will prevent the person from moving-outwards.

(i) If a particle is in uniform circular motion, there must be centripetal acceleration towards the center of the circle.
(ii) If there is acceleration then there must be some force acting on it with respect to an inertial frame. This force is called centripetal force.
(iii) The centripetal acceleration of a particle in the circular motion is given by \(a=\frac { { V }^{ 2 } }{ r } \) and it acts towards center of the circle. According to Newton's second law, the centripetal force is given by
\({ F }_{ Cp }={ ma }_{ cp }=\frac { { MV }^{ 2 } }{ r } \quad a=\frac { { V }^{ 2 } }{ r } \)  
(iv) The word Centripetal force means center seeking force in vector notation \({ F }_{ Cp }=\frac { { MV }^{ 2 } }{ r } \hat { r } \). For Uniform circular Motion  \({ F }_{ Cp }=-m\omega ^{ 2 }r\hat { r } \)

(i) The magnitude of the velocity can be changed without changing the direction of the velocity. In this case the particle will move in the same direction but with acceleration.
Examples:
The particle falling down vertically, bike moving in a straight road with acceleration.
(ii) The direction of motion alone can be changed without changing the magnitude (speed). If this happens continuously then we call it 'uniform circular motion'.
(iii) Both the direction and magnitude (speed) of velocity can be changed. If this happens non-circular motion occurs. For example oscillation of a swing or simple pendulum, elliptical motion of planets around the Sun