Answer:
a) 70 N, b) b. Each initially applied a force bigger than static friction to get the box moving and accelerating, then when the desired final speed was achieved they reduced the force to make the net force zero.
Explanation:
a) A constant speed means that magnitude of friction force is equal to the magnitude of the external force. The friction force is directly proportional to the normal force, which is equal to the weight of the box. Therefore, the magnitude of the force is 70 N.
b) Alice used initially a greater force to accelerate the box up to needed speed and later reduced the external force to keep speed constant. The right choice is option b.
Answer:
The height of the image of the candle is 20 cm.
Explanation:
Given that,
Size of the candle, h = 12 cm
Object distance from the candle, u = -6 cm
Focal length of converging lens, f = 15 cm
To find,
The height of the image of the candle.
Solution,
Firstly, we will find the image distance of the candle. Let it is equal to v. Using lens formula to find the image distance.

v is image distance

If h' is the height of the image. Magnification is given by :


So, the height of the image of the candle is 20 cm.
Answer:
0.5 m
Explanation:
Two charges each of magnitude q
Let the third charge is Q is placed at a distance x from the origin so that the charge is in equilibrium.
The force on Q due to q at origin is balanced by the charge on Q due to the charge q placed at x = 1 m.
So,

1 - x = x
1 = 2x
x = 0.5 m
Thus, the third charge is placed at x = 0.5 m .