# Newton's first law

According to Newton's first law of motion, an object remains in the same state of motion unless an external acts on it. If the resultant force on an object is zero, this means:

• a stationary object stays stationary
• a moving object continues to move at the same (at the same speed and in the same direction)
Newton's first law "An object will remain at rest or in uniform motion in a straight line unless acted upon by an external resultant force."

### Inertia - Higher

Newton's first law can be used to explain the movement of objects travelling with uniform motion (constant velocity). For example, when a car travels at a constant speed, the driving force from the engine is balanced by the resistive forces such as and frictional forces associated with the wheels and tyres. The resultant force on the car is zero.

Other examples include:

• a runner at their top speed experiences the same air resistance as their
• an object falling at experiences the same air resistance as its weight
If the forces acting on an object are balanced, the resultant force is zero

### Examples of objects with non-uniform motion

Newton's first law can also be used to explain the movement of objects travelling with non-uniform motion. This includes situations when the speed changes, the direction changes, or both change. For example, when a car accelerates, the driving force from the engine is greater than the resistive forces. The resultant force is not zero.

Other examples include:

• at the start of their run, a runner experiences less air resistance than their thrust, so they accelerate
• an object that begins to fall, experiences less air resistance than its weight, so it accelerates
If the forces acting on an object are not balanced, the resultant force is not zero

In the examples above the resultant force is in the same direction as the motion. If there is a resultant force in the opposite direction to the motion then the object will decelerate. An example of this would be skydiver opening a parachute.

### Forces on a submarine

Water resistance pulls the submarine back, the engine force pushes the submarine forward, the weight pulls the submarine down and the buoyancy force pulls the submarine up

The submarine above has both vertical forces and horizontal forces acting on it. The horizontal forces will not affect its vertical movement and the vertical forces will not affect its horizontal movement.

The horizontal forces are equal in size and opposite in direction. They are balanced, so the horizontal resultant force is zero. This means that there is no horizontal acceleration just a horizontal constant speed. If a submarine is not moving horizontally, then there are no horizontal forces. The vertical forces are equal in size and opposite in direction. They are balanced, so the vertical resultant force is also zero. This means that there is no resultant vertical acceleration.

The submarine will continue to either remain stationary or move at a constant speed. If the submarine is moving, it is impossible to tell which direction it is moving from the forces alone, only that it will continue in the same direction at the same speed.