If the box is moving at constant velocity, net force must be zero, so:
F + fr = 0
fr = -F
<u>fr = -40 N</u>
 
        
                    
             
        
        
        
The letter "B" is used for magnetic field
        
             
        
        
        
Third model shows how a comet's tail changes during its orbit...
mark brainliest
 
        
                    
             
        
        
        
Explanation:
According to Newton's second law of motion, the rate of change of momentum is directly proportional to the applied unbalanced force. The mathematical expression is given by:

Where
F is the applied force
m is the mass of the object
v is the velocity with which it is moving

Momentum of a particle is given by the product of mass and velocity as :

Hence, this is the required solution.
 
        
             
        
        
        
Answer:
<h3> 1.40625m/s²</h3>
Explanation:
Using the equation of motion expressed as v = u+gt where;
v is the  final velocity of the ball
u is the initial velocity
g is the acceleration due to gravity
t is the time taken
Given 
u = 9m/s
v = 0m/s
t = 6.4s
Required
acceleration due to gravity g
Since the rock is thrown up, g will be a negative value.
 v = u+(-g)t
0 = 9-6.4g
-9 = -6.4g
6.4g = 9
divide both sides by 6.4
6.4g/6.4 = 9/6.4
g = 1.40625m/s²
Hence the acceleration due to gravity on the planet is  1.40625m/s²