Apparent Weight — Lift problems, weighing machine
Laws of Motion
9
JEE Qs
8%
Hard
45
min
Master drawing accurate Free Body Diagrams for the object within the accelerating lift and apply Newton's Second Law consistently from an inertial frame of reference.
🧮 Key Formulas
✅ Key Points for JEE
- 1Apparent weight is not the true weight (mg); it is the normal force exerted by the supporting surface (or tension in a spring balance) on the object.
- 2Always draw a Free Body Diagram (FBD) for the object *inside* the accelerating lift, identifying all forces acting on it.
- 3Apply Newton's Second Law (F_net = ma) from an inertial frame (ground frame) for the object, where 'a' is the acceleration of the object along with the lift.
- 4The acceleration 'a' in the formulas R = m(g ± a) refers to the acceleration of the lift, relative to the ground.
⚠️ Common Mistakes
- ✕Confusing apparent weight (normal force) with actual weight (mg), leading to incorrect force balances.
- ✕Incorrectly assigning the direction of acceleration 'a' or the signs in the N2L equation (e.g., adding 'a' instead of subtracting when moving down and slowing, or vice-versa).
- ✕Failing to draw an FBD and directly applying formulas, which can lead to errors in complex scenarios or when lift's acceleration is not purely vertical.
📝 Practice Questions
See allQ41.A balloon and its content having mass M is moving up with an acceleration ' a '. The mass that must be released from the content so that the balloon starts moving up with an acceleration ' 3a′ will be (Take ' g ' as acceleration due to gravity) (1) 2Ma (2) 3Ma 3a+g 2a−g (3) 3Ma (4) 2Ma 2a+g 3a−g
Q33.A sand dropper drops sand of mass m(t) on a conveyer belt at a rate proportional to the square root of speed (v) of the belt, i.e. dmdt ∝√v . If P is the power delivered to run the belt at constant speed then which of the following relationship is true? 2025 (29 Jan Shift 2) JEE Main Previous Year Paper (1) P ∝√v (2) P ∝v (3) P2 ∝v5 (4) P2 ∝v3
Q29.A car of mass ' m ' moves on a banked road having radius ' r ' and banking angle θ. To avoid slipping from banked road, the maximum permissible speed of the car is v0 . The coefficient of friction μ between the wheels of the car and the banked road is θ θ tan tan 0−rg (1) μ = v2o+rg (2) μ = v2 θ tan θ rg−v2 tan rg+v2o 0 θ θ tan v2o+rg tan 0−rg (4) μ = (3) μ = v2 θ tan θ rg+v2 tan rg−v2o 0
Q47. A string of length L is fixed at one end and carries a mass of M at the other end. The mass makes ( π3 ) rotations per second about the vertical axis passing through end of the string as shown. The tension in the string is … … ML.
Q5. A wooden block, initially at rest on the ground, is pushed by a force which increases linearly with time t. Which of the following curve best describes acceleration of the block with time: (1) (2) (3) (4)
Q21.Two forces ¯F1 and ¯F2 are acting on a body. One force has magnitude thrice that of the other force and the 1 ) resultant of the two forces is equal to the force of larger magnitude. The angle between →F1 and →F2 is cos−1 ( n . The value of |n| is _____.
NCERT Chapters
- Class 11 Physics Ch 5: Laws of Motion