<u>Answer:</u>
According to newton's first law of motion, friction is required to make an object slow down.
<u>Explanation:</u>
According to the Newton's first law of motion, for an object to change its velocity (either a change in the magnitude or the direction), there must be a cause to it which is defined as a net external force.
For example, an object which is sliding across a table or floor slows down due to the net force of friction that is acting on that object.
Answer:
t = 0.67 [s]
Explanation:
To solve this problem we must use the following kinematics equation.

Vf = final velocity = 20[m/s]
Vi = initial velocity = 10 [m/s]
a = aceleration = 15 [m/s^2]
Now replacing in the equation we have:
20 = 10 + (15*t)
t = (20-10)/15
t = 0.67 [s]
The velocity of the car after it accelerates for 25 seconds would be 820.2 ft/s or 250 m/s
1) Frequency: 
The frequency of electromagnetic radiation is given by:

where
is the speed of light
is the wavelength
In this case, the wavelength of the radiation is

Therefore the frequency is

2) Period: 
The period is equal to the reciprocal of the frequency of the wave:

Using the frequency we found previously,
, we find:

<span>If a book is there on the table, no work is done as no distance is covered. If I hold up a book in my hand and my arm is stretched, if no work is being done, where is my energy going?</span>