Forces - 5.1.2 Contact and Non-Contact Forces (GCSE Physics AQA)
Contact and Non-Contact Forces
What is the Difference Between Contact and Non-Contact Forces?
A force is a push or pull which occurs between two objects.
Forces can be either contact or non-contact.
- Contact forces can push apart two objects that were touching. Imagine trying to jump off the ground. When you bend your knees to push down, the ground is simultaneously pushing upwards against you. The forces are equal and opposite, but since the ground can’t move, you get propelled upwards into the air
Contact Forces Examples
For AQA exams, we need to learn 3 examples of contact forces:
- Friction occurs when two surfaces move across each other. A frictional force will oppose motion, preventing two surfaces from sliding over each other.
- Air resistance is a type of frictional force. Air opposes the motion of an object, leading to air resistance. Some call it the ‘friction of air’. The faster the object is moving, the more air resistance it will experience.
- Tension occurs when objects are stretched. Tension usually happens in ropes, cables, strings and chains. When an object is suspended from a string, there are two forces involved. One is a force acting downwards due to the object. The other is a tension force acting upwards in the string to keep everything in place, as shown in Fig 2.
- Normal contact force (or reaction force) occurs when objects are touching at rest. When a plate rests on a table, it will experience a reaction force. This force can be labelled on a diagram, using an arrow going up perpendicular to the table!
Examples of Non-Contact Forces
For AQA exams, we need to learn 3 examples. Below is a list of non-contact forces.
- Gravitational force is an attractive force. Gravitational force can pull planets together in the Solar System, and can also make objects stay on the surface of the Earth. Gravitational force helps us to define the weight of an object, rather than just the mass.
- Electrostatic forces occur between charges. Similar to magnetic forces, electrostatic forces between opposite charges will lead to attraction, whilst forces between similar charges will lead to repulsion.
- Magnetic forces occur due to electromagnetic forces. When a charged particle moves in a magnetic field, it will experience a force called a magnetic force. This is related to the motor effect.
Force Diagrams
Forces as Vectors
- Forces have magnitude and direction. As we discussed previously, forces can be represented as arrows. The length of the arrow is the size of the force, and the direction of the arrow will be the direction of the force (up, down, forwards or backwards).
Drawing Force Diagrams
- More than one force can act on an an object. To find the overall force on an object, you must consider all of the individual forces.
- Label all the forces on a diagram. Labelling all of the forces will help you to see if any of the forces cancel out. This will happen when two arrows are parallel, but point in opposite directions.
Finding the Resultant Force
- You can find the resultant force. The resultant force will be the ‘overall force’, which is the sum of all forces present. When you work out resultant forces, you will need to find the horizontal (forward / backward) force, and the vertical (upwards / downwards) force.
- If all the forces are balanced, then the resultant force will be zero. If forces are equal and in opposite direction, there is a resultant force of zero. All the individual forces will cancel each other out. The object is said to be in equilibrium at this point.
A force is a push or a pull that can change the speed or direction of an object.
The two types of forces are contact forces and non-contact forces.
Contact forces are forces that require direct physical contact between two objects. Examples of contact forces include friction, tension, and air resistance.
Non-contact forces are forces that act on an object without the need for physical contact. Examples of non-contact forces include gravity, magnetism, and electric forces.
Friction works by the rough surfaces of two objects rubbing against each other, causing a force that resists their relative motion. The amount of friction depends on the type of surfaces in contact and the force pressing them together.
Tension is the force exerted by a rope, cable, or string when it is pulled tight. It is a type of contact force.
Air resistance is the friction between an object and the air. It works by the air molecules pushing against an object as it moves through the air, creating a force that opposes its motion.
Gravity is a non-contact force that pulls two objects towards each other. The strength of the gravitational force between two objects depends on their masses and the distance between them.
Magnetism is a non-contact force that can either attract or repel magnetic objects. The strength of the magnetic force depends on the type of magnetic material and the distance between the objects.
Electricity is a flow of electric charges, and it can create a force between charged objects. The strength of the electric force depends on the amount of charge on the objects and the distance between them.
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