Find the change in internal energy of a system which receives 70 j of heat from its environment and does 20 j of work.
select one:
a. 90 j
b. 50 j
c. zero
d. -70 j

option B

Explanation:

given,

Q = energy input = 70 J

U = increase in internal energy

W is the work done = 20 J

using first law of thermodynamics

The change in internal energy of a system is equal to the heat added to the system minus the work done.

U = Q - W

U = 70 - 20

U = 50 J

The change in internal energy is equal to 50 J

Hence, the correct answer is option B

the closer the planet is to the sun, the larger the gravity

air exerts forces on falling objects near earth’s surface.

explanation:

the term 'free falls' refer to object that are falling towards the earth, in a situation in which gravity is the only force acting on the object.

for an object in free fall, the acceleration is constant (g, the gravitational acceleration), and so the velocity of the object constantly increases.

for objects near earth's surface, there is always another force acting on the object: the resistance due to the air. this force is opposed to the direction of fall of the object, and it is proportional to the velocity of the object, so as the object falls down, this force increases up to a point when it balances the force of gravity, and the object continues its motion at constant velocity (called terminal velocity). since in this case gravity is not the only force acting on the object, we are not in a situation of free fall.