Work is done when a force acts on an object AND the object moves in the direction of the force.
-- <em>Pulling a trailer up a hill. YES. </em> The trailer is moving in the direction of the pulling force.
-- Carrying a box down a corridor. No. The box is not moving in the direction of gravity (down) OR in the direction of your arm (down the corridor).
-- Suspending a heavy weight with a strong chain, No. There's a force (gravity) acting on the weight, but the weight is not moving at all.
-- Pushing against a locked door. No. There's a force (your muscles) acting on the door, but he door is not moving at all.
Considering the Doppler efect, the frequency heard by the student would change if:
- if the student walked toward the police car.
- if the student walked away from the police car.
- if the police car moved toward the student.
- if the police car moved away from the student.
<h3>Doppler effect</h3>
The Doppler effect is defined as the change in the apparent frequency of a wave produced by the relative motion of the source with respect to its observer. In other words, this effect is the change in the perceived frequency of any wave motion when the sender and receiver, or observer, move relative to each other.
The following expression is considered the general case of the Doppler effect:

Where:
- f', f: Frequency perceived by the receiver and frequency emitted by the transmitter, respectively. Its unit of measurement in the International System (S.I.) is the hertz (Hz), which is the inverse unit of the second (1 Hz = 1 s⁻¹)
- v: Wave propagation speed in the medium. It is constant and depends on the characteristics of the medium. In this case, the speed of sound in air is considered to be 343 m/s.
- vR, vE: Receiver and transmitter speed respectively. Its unit of measure in the S.I. is the m/s
- ±, ∓:
- We will use the + sign:
- In the numerator if the receiver approaches the sender
- In the denominator if the sender moves away from the receiver
- In the numerator if the receiver moves away from the sender
- In the denominator if the sender approaches the receiver
In summary, the Doppler Effect is an alteration of the observed frequency of a sound due to the movement of the source or the observer, that is, they are changes in the frequency and wavelength of a wave due to the relative movement between the wave source and the observer.
<h3>Changes on the frequency </h3>
In this case, considering the Doppler effect, the frequency heard by the student would change if:
- if the student walked toward the police car.
- if the student walked away from the police car.
- if the police car moved toward the student.
- if the police car moved away from the student.
Learn more about Doppler effect:
brainly.com/question/15307081
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brainly.com/question/3841958
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Answer:
Explanation:
magnetic field B = (3 i + 8 x 2 j )x 10⁻³ T
= (3 i + 16 j )x 10⁻³ T
L = - i ( unit length of conductor )
Force F = I ( L x B ) , I is current
= 5 [ - i x ( 3i + 16 j ) 10⁻³]
= 5 ( - 16 k ) x 10⁻³
F = - 80 x 10⁻³ k
Explanation:
because the moon has less mass than earth, the force due to gravity at the lunar surface is only about 1/6 that on earthso,the weight of a body on earth is 6×5N =30N