<u>Answer:</u>
The stopping car has negative acceleration.
<u>Explanation:</u>
We know acceleration is the rate of change of velocity. That is
Acceleration(a) = ( Final velocity - Initial velocity )/ Time taken.
Here Final velocity = 0 km/h( car stops) and initial velocity = 30 km/h
So acceleration = (0-30)/time = -30/ time
Time is always positive , so -30/time is negative, so the stopping car has negative acceleration.
Answer: (b)
Explanation:
Given
mass of Student M=40 kg
mass of box m=3 kg
velocity of box v=8 m/s
suppose u is the velocity of the student after the throw
As there is no external force is applied so momentum is conserved
Initial momentum=final momentum=0
0=Mu+mv
Mu=-mv

Here negative sign indicates the velocity is opposite in direction i.e. opposite to the direction of the box
The solution for the acceleration of gravity is given as
This is further explained below.
<h3>What is the
effective value of g, the acceleration of gravity, at 7900 km above the Earth's surface.?</h3>
Generally,
Mass of earth 
Radius of earth 
Gravitational const. 
height 

In conclusion, acceleration due to gravity at this point will be

for 

Read more about acceleration due to gravity
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The answer is ionic bonds