Answer:
B. a piece of paper being torn
Explanation:
A chemical change is one that cannot be reversed. This means the original properties of the substance or object cannot be restored.
If you cook a raw egg, it would turn into a boiled egg (or a poached egg, however it is being cooked). The reaction is irreversible, so you cannot turn the cooked egg back into a raw egg - it is basically impossible to 'uncook' an already cooked egg. 
When you toast a piece of bread, it turns into toast. You can't 'untoast' it back into bread. The chemical changes have already occurred and cannot be undone. 
If you tear a piece of paper, it is still paper. You are only ripping it, not changing anything about it. You could simply tape the torn bit back to the original bit, or glue it - either way, it is still paper and nothing has occurred to drastically change the physical state of it.
Therefore, B is not a chemical change.
 
        
             
        
        
        
Answer:
The bullet's initial speed is 243.21 m/s.
Explanation:
Given that,
Mass of the bullet, 
Mass of the pendulum, 
The center of mass of the pendulum rises a vertical distance of 10 cm.
We need to find the bullet's initial speed if it is assumed that the bullet remains embedded in the pendulum. Let it is v. In this case, the energy of the system remains conserved. The kinetic energy of the bullet gets converted to potential energy for the whole system. So,
 
 
V is the speed of the bullet and pendulum at the time of collision
Now using conservation of momentum as :
 
 
Put the value of V from equation (1) in above equation as :

So, the bullet's initial speed is 243.21 m/s.
 
        
             
        
        
        
92 grams of Y would combine with 46 grams of X
        
             
        
        
        
Answer:
The frequency of sound heard by the boy is 1181 Hz.
Explanation:
Given that,
Frequency of sound from alarm  
Speed = -8.25 m/s
Negative sign show the boy riding away from the car
Speed of sound = 343
We need to calculate the heard frequency
Using formula of frequency

Where,  = frequency of source
 = frequency of source
 = speed of observer
 = speed of observer
 = speed of source
 = speed of source
 = speed of sound
 = speed of sound
Put the value into the formula

here, source is at rest 


Hence, The frequency of sound heard by the boy is 1181 Hz.
 
        
             
        
        
        
Answer:
Explanation:
Work done by the spring is negative . 
Work done by force F creating displacement  d is given by the following expression .
Work = F x d 
Both force and displacement are vector quantity .
When direction of force and direction of displacement is same , work is positive . When direction of force and direction of displacement is opposite , work is negative . 
When spring is compressed , it exerts a restoring or opposing force in a direction opposite to the direction of displacement of box . Hence here force is opposite to displacement . Restoring force acts opposite to displacement . Hence work done by spring on box is negative .