Advertisements
Advertisements
Question
A man of mass 70 kg stands on a weighing scale in a lift which is moving
- upwards with a uniform speed of 10 m s-1
- downwards with a uniform acceleration of 5 m s–2
- upwards with a uniform acceleration of 5 m s–2. What would be the readings on the scale in each case?
- What would be the reading if the lift mechanism failed and it hurtled down freely under gravity?
Advertisements
Solution
Here, m = 70 kg, g = 10 m/s2
In each instance, the weighing machine measures the reaction R, which represents the apparent weight.
(a) When the lift moves upwards with a uniform speed, acceleration a = 0.
R = mg = 70 x 10 = 700 N
(b) When the lift moves downward with a = 5 ms-2
R = m (g - a) = 70 (10 - 5) = 350 N
(c) When the lift moves upwards with a = 5 ms-2
R = m (g + a) = 70 (10 + 5) = 1050 N
(d) If the lift were to come down freely under gravity, downward acceleration. a = g
∴ R = m (g - a) = m (g - g) = zero.
APPEARS IN
RELATED QUESTIONS
A rocket with a lift-off mass 20,000 kg is blasted upwards with an initial acceleration of 5.0 m s–2. Calculate the initial thrust (force) of the blast.
You are travelling in a car. The driver suddenly applies the brakes and you are pushed forward. Why does this happen?
A car accelerates on a horizontal road due to the force exerted by.
Two objects A and B are thrown upward simultaneously with the same speed. The mass of A is greater than that of B. Suppose the air exerts a constant and equal force of resistance on the two bodies.
A small block B is placed on another block A of mass 5 kg and length 20 cm. Initially, the block B is near the right end of block A (In the following Figure). A constant horizontal force of 10 N is applied to the block A. All the surfaces are assumed frictionless. Find the time that elapses before block B separates from A.
In the following figure shows a uniform rod of length 30 cm and mass 3.0 kg. The strings shown in the figure are pulled by constant forces of 20 N and 32 N. Find the force exerted by the 20 cm part of the rod on the 10 cm part. All the surfaces are smooth and the strings and the pulleys are light.
Define linear momentum and state its S.I. unit.
Show that the rate of change of momentum = mass × acceleration. Under what condition does this relation hold?
Two balls A and B of masses m and 2 m are in motion with velocities 2v and v, respectively. Compare:
(i) Their inertia.
(ii) Their momentum.
(iii) The force needed to stop them in the same time.
State the Newton's second law of motion. What information do you get from it?
Use Newton's second law of motion to explain the following instance :
A cricketer pulls his hands back while catching a fast moving cricket ball .
State the magnitude and direction of the force of gravity acting on the body of mass 5 kg. Take g = 9.8 m s-2.
Calculate the velocity of a body of mass 0.5 kg, when it has a linear momentum of 5 Ns.
An electron of mass 9 × 10−31 kg is moving with a linear velocity of 6 × 107 ms−1. Calculate the linear momentum of electron.
A body of mass 200 g is moving with a velocity of 5 ms−1. If the velocity of the body changes to 17 ms−1, calculate the change in linear momentum of the body.
Define Newton’s second law of motion.
Name the physical entity used for quantifying the motion of a body.
In the previous problem (5.3), the magnitude of the momentum transferred during the hit is ______.
Why does a child feel more pain when she falls down on a hard cement floor, than when she falls on the soft muddy ground in the garden?
How does moving your hand backward while catching a cricket ball reduce pain?
