Newtons Laws of Motion
Newtons first law states that an object in motion will stay in motion and an object at rest will stay at rest unless acted on by an unbalanced force. All objects, moving or not, resist changes in motion. Resistance to change in motion is called inertia inertia explains why you move forward in your seat when the car you are in suddenly stops, you keep moving forward because of inertia.
Newtons second law states that the force of an object is equal to its mass times its acceleration. Which is harder to push, a full shopping cart or an empty one? When you increase the force on the cart without changing its mass. The acceleration of the cart will increase too. The cart will accelerate faster if something falls out. That will reduce the mass of the cart, and you keep pushing just as hard.
Newtons third law states that for every action there I'd an equal and opposite reaction. If you were to lean against a wall and it didn't push back on you, you would fall right through. The force exerted by the wall is equal in strength and opposite in direction to the force you exert on the wall.