Laws of motion 3 with ncert

physics

Object is thrown with velocity v on a rough horizontal surface. Find time and distance after which the particle comes to rest

acc= u g

v=at time= v/a= v/ug

s=v²/2a= v²/2ug

Find acceleration of block

limiting friction= 70

force= 80 so block will accelerate

kinetic friction= 40

acc= 80-40/10= 40/10= 4

Acceleration can be non zero only if there is a net external force on the body.

true false

true

An astronaut accidentally gets separated out of his small spaceship accelerating in inter stellar space at a constant rate of 100 m s–2. What is the acceleration of the astronaut the instant after he is outside the spaceship ?

Since there are no nearby stars to exert gravitational force on him and the small spaceship exerts negligible gravitational attraction on him, the net force acting on the astronaut, once he is out of the spaceship, is zero. By the first law of motion the acceleration of the astronaut is zero.

conservation of momentum is shown by which two laws of motion

second and third

when there is no applied force, there is static friction

true false

false

no applied force, no static friction

static friction opposes impending motion

kinetic friction opposes actual relative motion

true false

true

It is found experimentally that when relative motion has started, the frictional force decreases from the static maximum value (fs)max

true false

true

because max static friction is limiting friction

and kinetic is decreased of limiting

A body like a ring or a sphere rolling without slipping over a horizontal plane will suffer no friction

true false

true

are these statements true regarding second law of motion

(a) The second law is consistent with the First Law (F = 0 implies a = 0)

(b) It is a vector equation

yes

value of static force is always uN

true false

Static friction is a self-adjusting force up to its limit µs N (fs ≤ µs N). Do not put fs = µs N without being sure that the maximum value of static friction is coming into play.

Action and reaction act on same bodies.

true false

false

Action and reaction act on different bodies.

a]velocity

v = 0 + 2 x 10 = 20 ms^-1
The only force acting on the stone is force of gravity which acts vertically downwards.
Its horizontal component is zero. Moreover, air resistance is to be neglected. So, horizontal motion is uniform motion.
.-. vx = v = 20 ms^-1

This is answer for if normal is four times its weight

the answer to this question is B

A