The answer is: Motion!
Have a great day
Missing question: "how much work is done by the teacher?" (found on internet)
Solution:
The work done to move the desk across the floor is equal to

where F is the horizontal force applied to move the desk, and d is the distance covered by the desk. If we use F=20 N and d=5 m, we find the work done:
Answer
given,
mass of block (m)= 6.4 Kg
spring is stretched to distance, x = 0.28 m
initial velocity = 5.1 m/s
a) computing weight of spring
k x = m g


k = 224 N/m
b) 




c) 


d) 


e)


A = 0.682 m
Force =
=
F = 94.20 N

Explanation:
Natural length of a spring is
. The spring is streched by
. The resultant energy of the spring is
.
The potential energy of an ideal spring with spring constant
and elongation
is given by
.
So, in the current problem, the natural length of the spring is not required to find the spring constant
.

∴ The spring constant of the spring = 
Answer:
option (d) 7.1 kN
Explanation:
Given:
Mass of the car, m = 1600 kg
Acceleration of the car, a = 1.5 m/s²
Coefficient of kinetic friction = 0.3
let the tension be 'T'
Now,
ma = T - f .................(1)
where f is the frictional force
also,
f = 0.3 × mg
where g is the acceleration due to the gravity
thus,
f = 0.3 × 1600 × 9.81 =
therefore,
equation 1 becomes
1600 × 1.5 = T - 4708.8
or
T = 2400 + 4708.8
or
T = 7108.8 N
or
T = 7.108 kN
Hence,
The correct answer is option (d) 7.1 kN