Which describes a force acting on an object? a car moving in a straight line a ball that moves when kicked a billiard ball that rolls at a constant speed

Force is needed to change an objects state of motion and it is always a part of a mutual action that involves another force. The force can be push or pull.
A ball that moves when kicked describes a force acting on an object.
The force is a vector quantity, because it that needs both magnitude and direction in order to be defined.

a ball that moves when kickeda ball that rolls at a constant speed

Explanation:

When a car moves in a straight line

For this statement we don’t know HOW the car is moving, it could be at constant speed, it could be changing its speed, we don’t know. So, this statement is not a very good description of how a force acts.

a ball that moves when kicked

The ball starts at rest, and, after being kicked, is moving. When a force acts in an object, the object is accelerated. This means, the force changes the velocity at which the object moves. That means that the force made by the kick changed its speed. This is the perfect description of how a force works.

a billiard ball that rolls at a constant speed

This one is a little tricky. One could say that this is just like the car, moving at constant speed, but that is not true, cause the car its modeled as an particle. The mechanics of rolling are more complicated. You see, if a ball spins, and there is no friction, the ball spins in its place. For the ball to start rolling, we need to consider the static friction that enables to rolls. So, there is a force acting on the object, the static friction force.

Of course, one could argue that there are friction forces acting on the car, and there is pressure, and so on, and so on, but in physics we model the systems as simple as necessary, and we use to imagine the models, as we say "In the vacuum" even if this is not remotely true.

  sequence  is a continuous and distinctive band of  stars  that appears on plots of stellar  color  versus  brightness. these color-magnitude plots are known as  hertzsprung–russell diagramsafter their co-developers,  ejnar hertzsprungand  henry norris russell. stars on this band are known as  main-sequence stars  or  dwarf stars. these are the most numerous true stars in the universe, and include the earth's  sun.after condensation and ignition of a star, it generates  thermal energy  in its dense core region through  nuclear fusion  of  hydrogen  into  helium. during this stage of the star's lifetime, it is located on the main sequence at a position determined primarily by its mass, but also based upon its chemical composition and age. the cores of main-sequence stars are in  hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward pressure of  gravitational collapse  from the overlying layers. the strong dependence of the rate of energy generation on temperature and pressure to sustain this balance. energy generated at the core makes its way to the surface and is radiated away at the  photosphere. the energy is carried by either  radiation  or  convection, with the latter occurring in regions with steeper temperature gradients, higher opacity or both.the main sequence is sometimes divided into upper and lower parts, based on the dominant process that a star uses to generate energy. stars below about 1.5 times the  mass of the sun  (1.5  m☉) primarily fuse hydrogen atoms together in a series of stages to form helium, a sequence called the  proton–proton chain. above this mass, in the upper main sequence, the nuclear fusion process mainly uses atoms of  carbon,  nitrogen  and  oxygen  as intermediaries in the  cno cycle  that produces helium from hydrogen atoms. main-sequence stars with more than two solar masses undergo convection in their core regions, which acts to stir up the newly created helium and maintain the proportion of fuel needed for fusion to occur. below this mass, stars have cores that are entirely radiative with convective zones near the surface. with decreasing stellar mass, the proportion of the star forming a convective envelope steadily increases. main-sequence stars below 0.4  m☉undergo convection throughout their mass. when core convection does not occur, a helium-rich core develops surrounded by an outer layer of hydrogen.in general, the more massive a star is, the shorter its lifespan on the main sequence. after the hydrogen fuel at the core has been consumed, the star  evolves  away from the main sequence on the hr diagram, into a  supergiant,  red giant, or directly to a  white dwarf.