A-Level Physics: Forces

Forces are vectors, so we can find a resultant force on an object, no matter how many forces are acting on it. If the resultant force is zero, the forces must be balanced.Balanced forces cause no acceleration (This means that the object will remain stationary or carry on moving at a constant speed.)If the resultant force is not zero the forces are unbalanced. Unbalanced forces cause acceleration in the direction of the resultant force.Every force has a partner force that is the same size but acts on another object and in the opposite direction.Theoretically, as you push yourself forward when you walk the world is being pushed backwards; luckily the Earth is so large that you don't notice. If you try propelling yourself forwards off a boat on the bank of a lake, you should be able to see the boat is pushed away from the bank as you move towards the bank!

Calculating ForceSo the acceleration of an object is related to the resultant force causing it. In fact we can calculate the force needed to cause an acceleration using:\(F = ma\)Where:\(F\) = force (\(N\))\(m\) = mass (\(kg\))\(a\) = acceleration (\(m/s^2\))

Newton's first law of motion"Bodies will continue to move with a uniform velocity (which includes being stationary) unless acted on by a resultant force."This means that if a body changes speed or direction a resultant force must be acting on it.

Newton's second law of motion"Resultant force is equal to the rate of change of momentum."This forms the basis of Newton's equation, F=ma. This equation actually comes from the idea of momentum, which can be found in the Principles of the conservation of momentum Learn-it.The direction of the acceleration is always in the same direction as the unbalanced force.

Newton's third law of motion"Every force has an equal and opposite force acting on another object."

Friction is caused by rubbing. It can be the surfaces between two solids rubbing, a solid surface and a liquid or a gas, etc. Anything! When friction is caused by fluids (liquids or gases) we tend to call it drag or air resistance.A good Physics phrase to use in exams is - "friction dissipates energy." That means energy changes from kinetic energy to heat energy, where it is lost to the surroundings.It is important to remember that friction (or drag) increases as speed increases.

FrictionFriction is caused by rubbing. It can be the surfaces between two solids rubbing, a solid surface and a liquid or a gas, etc. Anything! When friction is caused by fluids (liquids or gases) we tend to call it drag or air resistance.A good Physics phrase to use in exams is - "friction dissipates energy." That means energy changes from kinetic energy to heat energy, where it is lost to the surroundings.It is important to remember that friction (or drag) increases as speed increases.

Terminal VelocityOne particular example of this is that when objects accelerate towards the ground due to gravity they experience an increasing air resistance force up. When the air resistance force up has grown so big that it matches the weight down there is no resultant force and therefore no acceleration. The object will travel at a constant speed. This is called the terminal velocity.It's not just falling objects that have a terminal velocity. You have one when you run! Think about it.

Pressure is caused by forces acting on a surface. The greater the force or the smaller the surface area, the greater the pressure produced.It's the old drawing pin example. You push the flat end with your thumb to spread out the force. The sharp end enters the wall as the smaller area concentrates the force, producing a higher pressure.We can calculate pressure using:\[Pressure = \frac{force}{area}\]Where:\(P\) = pressure (\(N/m^2\) or \(Pa\), Pascals) - Note: 1 \(Pa\) = 1 \(N/m^2\)\(F\) = force (\(N\))\(A\) = surface area \(m^2\)

Transmitting forcesSolids can transmit forces easily. If you push one end of a metal bar the other end will push what ever is near to it in the same direction. Liquids and gases can't do this.Pressure is useful because it can be used to transmit forces from one place to another using liquids and gases.Note: Energy is can't be created from nowhere!

The Basics

Newton's Laws

Friction and Terminal Velocity

Pressure