To solve this problem we will apply the concepts related to the conservation of kinetic energy and elastic potential energy. Thus we will have that the kinetic energy is

And the potential energy is

Here,
m = mass
v = Velocity
x = Displacement
k = Spring constant
There is equilibrium, then,
KE = PE

Our values are given as,

Replacing we have that


Therefore the speed of the cart is 2.19m/s
Answer:
Magnitude is 25.8 m/s and direction is 
Explanation:
From the law of conservation of linear momentum
where
and
are masses of bullet and stones respectively,
and
are the initial velocities of bullet and stone respectively,
and
are the final velocities of bullet and stone respectively
Substituting 6g=0.006 Kg for mass of bullet, 0.1Kg for mass of stone, 350 m/s for initial velocity of bullet and 250 m/s as final velocity for stone but initial velocity of stone is zero
Substituting 21 for
and 15 for
To find direction
Therefore, magnitude is 25.8 m/s and direction is
The acceleration of the object if the net force is decreased = 0.13 m/s²
<h3>Further explanation</h3>
Given
A net force of 0.8 N acting on a 1.5-kg mass.
The net force is decreased to 0.2 N
Required
The acceleration of the object if the net force is decreased
Solution
Newton's 2nd law :

The mass used in state 1 and 2 remains the same, at 1.5 kg
ΣF=0.8 N
m=1.5 kg
The acceleration, a:

ΣF=0.2 N
m=1.5 kg
The acceleration, a:
