Bermuda fitted dinghies racing in Granaway Deep. (Photo: Tom Clarke)
Statics >>
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2.1 - Forces
Objectives:
- To introduce the concept of dynamics and how forces affect the world around us.
- To understand that forces are vectors
- To be able to simplify a problem using a free body diagram (modelling)
- To be able to find the resultant force of two or more forces at an angle to each other.
- To be able to resolve a force into components.
Forces are simply PUSHES and PULLS that can cause something to CHANGE. They are the 'verbs' of science. Without a force being applied to an object, nothing changes - which would be kind of boring..... In our lives, there is never a situation where there are no forces acting on any object as we live in the Earth's gravity. Friction and air resistance (drag) are also prevalent! Forces can change an object's motion or shape. The science of how forces change the motion of an object is known as DYNAMICS.
On a fundamental level there are only four forces in physics:
On a fundamental level there are only four forces in physics:
- Gravity - the weakest and most mysterious of all the forces, which is covered in more detail in Unit 4.
- Weak force - the force associated with nuclear beta-decay. We will not discuss this at all for the AP courses.
- Electrostatic force - the force of attraction or repulsion between two electrical charges. This is touched on in Unit 10, but covered in more detail in AP Physics II. When you push, pull, sit, hit and rub things you are using this force at a fundamental microscopic level. We tend to ignore this and consider more macroscopic forces such as: friction, tension, thrust, normal forces, compression etc.
- Strong force - the strongest of all the forces as it has to hold the nucleus of the atom together against the electrostatic repulsion of the protons. Short ranging and only of interest to nuclear physicists! Discussed briefly in Physics II.
Practical Forces to Learn:
- Weight, \(mg\). This is the force of gravity upon an object. It acts from the centre of mass and ALWAYS acts DOWNWARDS! This is usually the first force than anyone draws.
- Normal or reaction force, \(N\) or \(F_N\). This is the force that a surface exerts on an object to stop it falling through the surface. i.e. the force upwards on an apple that stops it going through the table. It ALWAYS acts PERPENDICULAR (normal...) to the surface.
- Upthrust or buoyancy, \(F_B\). The is an UPWARDS force on an object that is immersed in a fluid. The usual example is the upwards force on a boat that stops it sinking to the bottom of the ocean.
- Friction, \(f\). This is the force that acts between any two surfaces (including fluids). Air resistance and drag are forms of friction. It ALWAYS OPPOSES motion.
- Lift. This is the force on a aero(hydro)foil due to a pressure difference in a fluid flow. More on this in AP Physics II.
- Tension, \(T\) or \(F_T\). This is a pulling force that is usually found in a rope in physics problems. For this topic we consider that the tension in the rope is the same throughout the rope.
Island connections
As Newton's Law's of Motion pretty much underpin everything, whether it stays put or accelerates, they can be found everywhere! Some examples might include
As Newton's Law's of Motion pretty much underpin everything, whether it stays put or accelerates, they can be found everywhere! Some examples might include
- Why bikes need to be as light as possible in order to accelerate quickly.
- Why boats, both power and sail, need to be as light as possible and have clean bottoms to reduce friction.
- The importance of good tyres and a dry road and why bikes deck out when it rains.
- The reason why, no matter how cool you think you are, seat belts, crash helmets and sensible driving are essential.
- Why the fitted dinghy with the largest sails tends to go faster than the one with the smaller sails.
- Why aircraft have to fly at high altitude when going to the US.
- Why the AC45s fly above the water and go so fast!
Forces up = forces down! Archimedes Principle in boats has been eliminated here.... Instead of the downwards weight being counterbalanced by buoyant upthrust it is balanced by the hydrodynamic lift from the hydrofoils. The idea is to reduce the amount of the boat in the water to virtually nothing so as to reduce drag and watch the speed climb. Drag through any fluid is a function of the relative speed through the fluid.
As is always the case - the max speed is achieved when the thrust = drag. The speed of these boats is incredible. The trick is to keep them upright and not to pitch-pole them. (Shortly after I originally wrote this section - Team NZ did exactly that in spectacular style!) |
A free-body diagram of a traffic light. a) original drawing. b) free-body diagram of the light itself - which tells us that \(T_3\) = weight of the light, so we often just ignore this one. c) free-body diagram of the three cables. Notice that the angles have been transferred down for simplicity. If drawing by hand, the reference lines are usually drawn dashed for clarity.
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A more complex free-body diagram of a connected-block system. It is good practice to consider the blocks separately.
1. The masses of the blocks have been labelled with different letters. 2. The weight of the block on the slope has not been broken into components. 3. Don't usually need to show the axes, but it has been done here to emphasize the point that "UP" doesn't always mean up! |
Physics Aviary Game: Non-linear Forces. This is challenging to do in 3 minutes.... Be prepared to have a pencil, paper and calculator handy! A good test in your ability to work out the resultants from non-linear vectors. Remember the work of Mr Pythagoras.
Useful exercise in preparation for the multiple-choice questions. |
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Other Resources
Newton's Laws and Car Crashes - recommended by Morgan Kornarski of Safe Kids
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