Answer:
A non-contact force is a force which acts on an object without coming physically in contact with it. 
Explanation:
For example:
<em>Gravitational force.</em>
<em>Magnetic force.</em>
<em>Electrostatics.</em>
<em>Nuclear force.</em>
 
        
                    
             
        
        
        
centripetal acceleration can be determined using the following equation;
a = v² / r
a= centripetal acceleration
v = velocity 
r = radius
substituting the values in the equation,
a = (300 m/s)*(300m/s) / (1.85 *1000) m
   = 48.64 ms-²
        
             
        
        
        
Answer:
By induction method
Explanation:
Induction method involves charging an electrically neutral body by bringing it in contact with an electrically charged body. 
For the electrophus, a charge opposite that on the slab is induced on the side in contact with the slab; driving the opposite charge (this will be the same as that on the slab) to the other end of the elctrophus. Touching the electrophus removes the charge opposite the charge induced on the electrophus by the charged slab either by drawing up charge from the earth or taking the charge to earth (depends on the charge. A negative charge is drawn to earth while a positive charge draws up electrons from the earth) 
 
        
             
        
        
        
As per the question the wavelength of spectral line of the neutral hydrogen atom is 21 cm.
Spectral line of hydrogen atom will be observed when an electron will jump from higher excited to the ground state. If E is the energy of the ground state and E' is the energy of the higher excited state,the energy emitted when electron will jump from higher excited to ground state is E'-E.
As per Planck's quantum theory -
                                                       
           Where f is the frequency of emitted radiation and h is the Planck's constant.Actually this energy emitted is also electromagnetic in nature. We know that all the electromagnetic radiation will move with the same velocity which is equal to the velocity of light.
The velocity of light c= 3×10^8 m/s.Hence the velocity of wave corresponding to this spectral line is 3×10^8 m/s.
We know that velocity of a wave is the product of frequency and wavelength of that corresponding wave. Mathematically we can write it as-
        
[ is the wavelength of the wave ]
 is the wavelength of the wave ]
 It is given that wavelength =21 cm=0.21 m
  The frequency is calculated as -
                                                          
                                                                  
                                                                  
                                                                   
  Hence the antenna will be tuned to a frequency of 14.2857×10^8 Hz
Here Hertz[ Hz] is the unit of frequency.
 
        
             
        
        
        
Answer:
8.86 m
Explanation:
According to the law of conservation of energy, the elastic potential energy initially stored in the spring will be converted into gravitational potential energy of the block when it is at its maximum height:

where
k = 5100 N/m is the spring constant
x = 0.093 m is the spring compression
m = 0.254 kg is the mass of the block
g = 9.8 m/s^2 is the acceleration due to gravity
h is the maximum height of the block
Solving the equation for h, we find
