An object will remain at rest or in uniform motion in a straight line unless acted upon by an external force.
e.g. if a car is at rest, then it will remain at rest until a force is exerted on it by either the engine or a person trying to push it.
If a car is moving at a constant velocity, then it will continue to do so until the engine applies more or less force on it, causing it to either accelerate or decelerate.
The rate of change of momentum is directly proportional to the force applied, and this change in momentum takes place in the direction of the applied force.
▵p ∝ F
e.g. If you use the same amount of force to move a bicycle and a car, the bicycle would have more acceleration because it has a smaller mass compared to a car.
If a body A exerts a force on body on body B, then body B will exert an equal and opposite force on body A.
e.g. A standing person of mass 70 kg exerts a force of 700 N on the ground (their weight). The ground will then exert an equal and opposite force on the person (-700 N).