Every body will stay in a state of rest or uniform motion in a straight line unless that state is changed by forces impressed upon it. Inertia is the property of matter that it resists having its state of motion changed. Change in motion is acceleration Measures how fast the velocity changes. The size of an acceleration is directly proportional to the force applied, and inversely proportional to the mass of the body. Further, the acceleration will take place in the same direction as the applied force.
Forces produce accelerations.
The more mass a body has, the less it will be accelerated by a given force. Application to Planetary Motion Planets are continually changing the speed and direction of their motion as they orbit the Sun. Move along ellipses with the Sun at one focus. Move fastest when at perihelion, slowest when at aphelion. They are thus accelerating in response to a force .