Question

1.Work is only being done when what happens to an object?

Answer 1

1.

C) it moves in the same direction as the force

D) The object moves in the opposite direction of the force

Work is defined as follows:

$W=Fdcos \theta$

where

F is the magnitude of the force applied on an object

d is the displacement of the object

$\theta$ is the angle between the direction of the force and of the displacement

As we see from the formula, when d=0 (the object does not move), then W=0 (no work done). While we have work done if the object is moving. In particular, we have two situations:

- The object moves in the same direction as the force: $\theta=0^{\circ}$, so $cos \theta= 1$ and the work done is positive

- The object moves in the opposite direction of the force: $\theta=180^{\circ}$, so $cos \theta= -1$ and the work done is negative

2. B) Ben has more power than Jerry.

Power is defined as:

$P=\frac{W}{t}$ (1)

where

W is the work done

t is the time taken

Work is the product of force and displacement. Since Ben and Jerry lift the same mass (250 kg), they apply the same force, and since they lift the barbell the same distance in air the same number of times (10), the displacement is also the same: so, they did the same work.

However, Ben did it in less time (5 s) then Jerry (25 s): looking at eq.(1), we see that less time means more power, so Ben has more power than Jerry.

3. C) UV light energy

The electromagnetic spectrum is the classification of all electromagnetic waves according to their different wavelength.

From shortest to longest wavelength, we have:

Gamma rays

X-rays

UV (ultraviolet)

Visible light

Infrared radiation

Microwaves

Radio waves

So, we see that UV light energy (ultraviolet radiation) is a type of electromagnetic waves.

4. C) If the balloon acquires charge, Sam's hair loses charge

The law of conservation of charge applied to this case states that the total charge on Sam hair and the balloon before must be equal to the total charge after: no charge can be created or destroyed in the process, but only moved from one object to the other.

In this example, Sam rubs the balloon on his head: electrons are transferred from Sam's hair to the balloon's surface. Therefore, we can say that the balloon has acquired negative charge, while Sam's hair has lost negative charge.