Question

bell work 3 - energy try your best

1. an object at rest has a mass of 70 kg. determine the objects kinetic energy
2. describe the energy transformation when ironing your clothes
3. describe the energy transformation when using the swing at the playground.
4. a 20 kg laptop is placed on a desk. how large is the gravitational potential energy of the laptop - earth system relative to the ground? g = 9.
5. a 7 kg phone is placed on a desk. how large is the gravitational potential energy of the phone - earth system relative to the ground? g = 9

Explanation:

Step1: Recall kinetic - energy formula

The formula for kinetic energy is $K = \frac{1}{2}mv^{2}$, where $m$ is mass and $v$ is velocity. Since the object is at rest, $v = 0$.

Step2: Calculate kinetic energy

Substitute $v = 0$ into the formula: $K=\frac{1}{2}\times70\times0^{2}=0$ J.

Step3: Analyze iron - clothes energy transformation

When ironing clothes, electrical energy is converted into thermal energy. The iron's heating element uses electrical energy to generate heat, which is then transferred to the clothes to remove wrinkles.

Step4: Analyze swing energy transformation

When using a swing at the playground, at the highest points, the swing has maximum potential energy (due to its height above the ground). As it swings down, potential energy is converted into kinetic energy (it gains speed). At the lowest point, it has maximum kinetic energy. As it swings back up, kinetic energy is converted back into potential energy.

Step5: Recall gravitational - potential - energy formula

The formula for gravitational potential energy is $U = mgh$, where $m$ is mass, $g$ is the acceleration due to gravity ($g = 9.8$ m/s²), and $h$ is height. For the laptop, assume the height of the desk is $h$ (not given in the problem, let's assume $h = 1$ m for illustration purposes). Then $U_{laptop}=20\times9.8\times1 = 196$ J.

Step6: Calculate phone's gravitational potential energy

Using the same formula $U = mgh$, for the 7 - kg phone, again assuming $h = 1$ m, $U_{phone}=7\times9.8\times1=68.6$ J.

Answer:

1. 0 J
2. Electrical energy is converted into thermal energy.
3. Potential energy and kinetic energy are inter - converted during the swing's motion.
4. Assuming $h = 1$ m, 196 J
5. Assuming $h = 1$ m, 68.6 J