The correct choice is
D. 22 Hz and 42 Hz.
In fact, the beat frequency is given by the difference between the frequencies of the two waves:

In this problem, the beat frequency is
, therefore the only pair of frequencies that gives a difference equal to 20 Hz is
D. 22 Hz and 42 Hz.
Answer:
Explanation:
Some common conductors are copper, aluminum, gold, and silver. Some common insulators are glass, air, plastic, rubber, and wood.
Answer:
The most likely items to be used are;
Ultrasound and X-rays
Explanation:
A routine visit to a dentist consists of two areas of activities, including;
a) Dental examination and check up
b) Oral prophylaxis, and dental cleaning
The dental examination may involve the use of X-rays, which allow the detection of cavities between the teeth
The dental cleaning can be carried out with the use of an ultrasound cleaner, which allow the cleaning of sensitive teeth without hurting the patient
Therefore, the items most likely to be used during a routine dental visit are ultrasound and X-rays
Explanation:
It is given that,
Mass of golf club, m₁ = 210 g = 0.21 kg
Initial velocity of golf club, u₁ = 56 m/s
Mass of another golf ball which is at rest, m₂ = 46 g = 0.046 kg
After the collision, the club head travels (in the same direction) at 42 m/s. We need to find the speed of the golf ball just after impact. Let it is v.
Initial momentum of golf ball, 
After the collision, final momentum 
Using the conservation of momentum as :


v = 63.91 m/s
So, the speed of the golf ball just after impact is 63.91 m/s. Hence, this is the required solution.