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Resultant Forces and Newton's First Law
Teachers Notes
Force is a very important concept in Physics.
Any object with mass you see moving will either have balanced forces acting on it (and will be either stationary or moving at a constant velocity) or have a resultant force acting on it (resulting in the object accelerating i.e. changing velocity) .
But even things that are not moving could have a force acting on it. Even sat typing this, there is the force of weight attracting the typist towards the centre of the earth with an equal and opposite force from the chair that stops them falling through the chair to the floor (where the floor would also have an equal and opposite force stopping the author falling through the floor...and so on).
Newtons First Law simply states that an object remains in the same state of motion (i.e. moving at a constant velocity or not moving at all) unless a resultant force acts on it. If the resultant force on an object is zero, a stationary object stays stationary and an object moving at constant velocity stays at that same constant velocity. In other words, if the forces acting on an object are equal and opposite, an object will not undergo acceleration (or negative acceleration which is more more commonly referred to as deceleration).
Note we do not use the phrase 'change in speed' here; a resultant force will introduce an acceleration but can also introduce a change in direction.
Where this gets interesting is when an object is moving in a circular motion. If you watch the Kahn academy video (below), you will get an understanding of why it is so important to understand the difference between speed and velocity. We love this because it is where the mathematics of Physics and the actual observation of the mathematics actually happen and is best experienced at rides that go round bends very quickly in fun parks. The Detail Videos are very detailed and will probably need to be viewed more than once. This is deliberate because, if students can understand the relationship between Force, velocity and acceleration, then the idea of force becomes much easier to understand as students progress through KS4/GCSE Physics.
The SI Unit for Force is the Newton N
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Overview Videos
Professor Dave gives a great description of the definition at 53 seconds into the video
Detail Videos
This introduces the idea that an object with constant speed moving in a circle experiences acceleration because the velocity is constantly changing. The next on video explains how this occurs and why the direction of the acceleration is towards the centre of the circle (this would be of interest to students are considering Physics at A Level)
This video is worth studying carefully. We would advise having a pencil, paper and rubber and practice what Sal is explaining as you go through the video. This is beautiful Physics as we have all experienced the affects of centripetal force when we go round corners
This video explains centripetal acceleration and, consequently, the concept of centripetal force.
Centripetal force can be a difficult concept as it only comes into existence when an object is moving in a circular motion (or in a non linear direction but you don’t need to be concerned about that in School Physics). When I say exists, an object can only move in a circular motion if a resultant force is present. If an object moves in a circle, the force is described as centripetal but the resultant force is not a new force, it is a recognition that a resultant force has to be present.
As an example. If you spin a stone attached to the end of a piece of rope, then you will ‘feel the force’ through the string of the rope. This force is known as the tension force. But this called also be described as the centripetal force acting on the stone i.e. the Tension force is the centripetal force in this example.
Similarly, a rocket in orbit, is moving in a circular motion. In this example the centripetal force is the force of gravity (and is the example Sal uses in this video)
Revision Videos
Freesciencelessons follows the curriculum and what students need to understand for examination purposes.
This looks into the mathematics as well as explaining resultant forces