Newton's first law
According to Newton's first law of motion, an object remains in the same state of motion unless a The single force that could replace all the forces acting on an object, found by adding these together. If all the forces are balanced, the resultant force is zero. 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 The speed of an object in a particular direction. (at the same speed and in the same direction)
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Examples of objects with uniform motion
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 A force of friction produced when an object moves through the air. and frictional forces in the car's moving parts. The resultant force on the car is zero.
Other examples include:
- a runner at their top speed experiences the same air resistance as their A force used to move a body forwards or up, eg the rocket had a thrust of 10,000 N.
- an object falling at The maximum speed of an object, reached when the forces moving the object are balanced by its frictional forces. experiences the same air resistance as its weight
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 A force used to move a body forwards or up, eg the rocket had a thrust of 10,000 N., so they accelerate
- an object that begins to fall, experiences less air resistance than its weight, so it accelerates
Forces on a submarine
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. 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 with the same motion, either remaining stationary or moving 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.
Motion in a circular orbit - Higher
When an object moves in a circle at a The distance travelled in a fixed time period, usually one second., its direction constantly changes. A change in direction causes a change in velocity. This is because velocity is a A measurement having size (magnitude) and direction, eg a displacement of 4 m North. quantity – it has an associated direction as well as a magnitude. A change in The speed of an object in a particular direction. results in The rate of change in speed (or velocity) is measured in metres per second squared. Acceleration = change of velocity ÷ time taken., so an object moving in a circle is accelerating even though its speed may be constant.
An object will only accelerate if a The single force that could replace all the forces acting on an object, found by adding these together. If all the forces are balanced, the resultant force is zero. acts on it. For an object moving in a circle, this resultant force is the Force, needed for circular motion, which acts towards the centre of a circle. that acts at right angles to the direction of motion, towards the centre of the circle. Gravitational attraction provides the centripetal force needed to keep planets in orbit around the Sun and all types of satellite in orbit.
