You should already know what we mean by an object that has a velocity and what we mean when we say an object has uniform motion. if not, you might want to check out this article first. in this article, we’re going to look at how the velocity of an object depends upon the forces acting on it. in physics, Sir Isaac Newton created three fundamental laws of motion in the 17th century.

Today we look at the first law of motion, sometimes called the law of inertia. Newton’s first law states that an object will remain at rest or move uniformly in a straight line unless an unbalanced force acts on it, but what does that really mean? firstly it means that when an object is not moving it will remain at rest until an unbalanced force acts on it.

The statement also tells us that an object will remain in uniform motion in a straight line unless acted upon by an unbalanced force. take a look at this ball here as the child kicks the ball they exert an unbalanced force on it changing the velocity of the ball and causing it to move but as it rolls along the ground the ball will slow down and will eventually come to a stop because the ball is slowing down we can say that it’s velocity is changing.

By Newton’s first law this means that the ball is experiencing an unbalanced force. in this case, it’s the force of friction that opposes the motion and eventually stops the ball. note that there is no forward force on the ball she is no longer kicking the ball so there are only backward frictional forces that slow the ball to a stop.

If the thrust of the engines is equal to the drag force it will move forward at a constant speed the plane will travel at constant height if its weight is balanced by the lift force from the wings. we have uniform motion. if the plane wants to move faster and increase its velocity then the thrust force pushing the plane forward needs to increase it becomes larger than the force of air resistance and creates an unbalanced net force.

The plane now accelerates no longer moving with uniform motion drag increases with velocity so very soon the plane will have balanced but larger forces again and its velocity remains constant but higher than before.

Quantitatively we can find the net force of any object by considering the size or the magnitude and the direction of each force acting on an object it can help to draw on the forces yourself. take a look at this example here which shows three forces acting on the object. the two vertical forces are equal and opposite therefore canceling each other out however the horizontal force has no opposition and so it remains unbalanced this means that the net force will act to the right and will have the magnitude of the horizontal force.

When working out the net force it’s a good idea to consider the horizontal and vertical directions separately. In the end, these directions can be added up to give you a diagonal force you can think of these forces to be a bit like points on a compass. if we add together north and east assuming the forces are equal we will get a result of the northeast.

The resultant force is obtained by drawing lines parallel to the forces to make a parallelogram or rectangle. well first of all we need to find the resultant horizontal force like this then we need to draw the lines parallel to the forces to construct the resultant force.

The law of inertia is Newton’s first law. objects will keep doing what they are doing when they experience no net force. they will continue to move with uniform motion or remain stationary unless an unbalanced force acts on them. this unbalanced force will cause them to change their velocity speed and/or direction which depends upon the size of the direction of the net force.