Answer:
Force, F = 77 N
Explanation:
A child in a wagon seem to fall backward when you give the wagon a sharp pull forward. It is due to Newton's third law of motion. The forward pull on wagon is called action force and the backward force is called reaction force. These two forces are equal in magnitude but they acts in opposite direction.
We need to calculate the force is needed to accelerate a sled. It can be calculated using the formula as :
F = m × a
Where
m = mass = 55 kg
a = acceleration = 1.4 m/s²

F = 77 N
So, the force needed to accelerate a sled is 77 N. Hence, this is the required solution.
This is easily explained saying that the frictional force between the books and the paper isn't big enough to produce a displacement in the books. The displacement in the books doesn't happen because the frictional force between the books and the surface they are standing on is bigger than the paper's one.
The spring constant is 4 N/m
Explanation:
When a spring is stretched/compressed by the application of a force, the relationship between the magnitude of the force applied and the elongation of the spring is given by Hooke's law:

where
F is the magnitude of the spring applied
k is the spring constant
x is the elongation of the spring, relative to its equilibrium position
For the spring in this problem, we have:
F = 0.12 N (force applied)
x = 3 cm = 0.03 m (elongation of the spring)
Therefore, we can solve the formula for k to find the spring constant:

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Answer: 0.2m
Explanation: Firstly only the Rocket's Weight Compress the spring which can be found by

According to Hooks Law

The part b and c of this question is done in the attachment