Newton’s First Law of Motion

What it Says Newton’s First Law of Motion, also called Newton’s Law of Inertia says that an object in motion will remain in motion OR an object at rest will remain at rest unless an unbalanced force acts upon it. In other words, an object that is moving will continue to move at the same speed until something stops it or makes it speed up. Likewise, an object that is sitting still will do so forever until something acts on it and makes it move.

Forces A force is a push or a pull on an object. When a force is great enough to make something move (or to make something stop moving) it is known as an unbalanced force. Not all forces are unbalanced – if you apply a push on a wall, the wall won’t move because you aren’t strong enough to unbalance the other forces (such as weight and the pull of the cement holding the wall) and make it move.

Net Force The total force acting on an object at a given time from all sources is called the Net Force. Net force is very simple to calculate:

Net Force When the forces acting on an object are on the same side, you add. When the forces are acting in opposition, you subtract. In physics, we usually assume a system exists in a vacuum for the purposes of making the math simplified. For the project however, we need to assume that all systems are imperfect, and thus all forces must be accounted for.

Net force When an object has a net force of zero, we say that the object is at equilibrium. An object resting on a table is at equilibrium. All systems in the universe are trying to reach equilibrium. Remember, just because an object is moving does not mean that it has a net force greater than zero. Look at this example: Here, the force of the truck was matched to the force of the cannon shooting the ball out of the back, so the ball seems to hover for a moment as its left/right forces cancel.

Friction Friction is the force of resistance from air or rough surfaces interacting on an object. When an object (like a bullet) travel through the air, friction is an oppositional force that pushes back on the bullet and cause it to lose speed. Friction is almost always an unbalancing force that causes an object to slow to a stop, especially on Earth. What does Newton’s First Law say?

Inertia Inertia is the tendency of an object to keep doing what its currently doing. Inertia can be thought of as an object’s unwillingness to change. Inertia is the same as weight – a very heavy object has a lot of inertia, a very light object has very little inertia. It is easy to make a very light (low inertia) object change position, and hard to make a very heavy (high inertia) object change position.

Inertia When you’re in a car, and someone slams on the brakes, you fly forward. This is because an unbalanced force (the friction from the brakes) acted on the car, but not on the passenger.