=Forces= A force changes the motion of an object. Sometimes this change in motion can cause the object to deform or change its shape. The size of a force is measured in '''Newtons (N)''' or in SI Base units, '''kgms^-2^''' [image:] ==Contact Forces:== '''Push force due to another object''' – E.g. “Push due to Steve”. This force acts away from the object doing the pushing. The push force disappears as soon as the person loses contact. '''Pull force due to another object''' – E.g. “Pull due to Steve”. This is the same kind of force as the push force, but the force acts towards the object doing the pulling. '''Support force due to the ground''' - (or due to whatever surface the object is on): Support force occurs when two solid surfaces are touching each other. The direction of the support force is at right angles to the surface. This means when the ground is horizontal, the support force is directly upwards. '''Friction due to the ground''' – (or due to whatever surface the object is on): Friction always occurs when two solid surfaces are sliding past each other. The correct way to describe friction is that '''FRICTION OPPOSES SLIDING'''. If the two surfaces are sliding past each other the friction is in the opposite direction to the sliding. '''''Note: It is incorrect to say “friction opposes motion” because in many situations it is the friction force that is creating motion, for example when people walk or cycle. When friction does cause motion in these cases the forward friction force can be called “traction”. The friction force increases with greater contact area between surfaces and roughness of surfaces.''''' '''Drag due to a gas or liquid OR resistance force due to a gas or liquid''' – This occurs when an object is moving through the gas or liquid. For example a swimmer has drag due to water, and a runner has drag due to air. The direction of drag is opposite to the motion. '''''Note: This force occurs whenever the object is moving relative to the gas or liquid – for example, if you are standing still on a windy day, you can still say you have drag due to the air.''''' '''Lift force due to the air''' – this force occurs when the shape of the object results in the air being deflected downwards, therefore the air pushes the object upwards. Lift force occurs for planes, birds and gliders because they have wings that have the correct angle to push the air downwards. It does NOT occur for all objects in the air, e.g. balls thrown up into the air. '''Thrust force due to a gas or liquid''' – A thrust force occurs when a gas or liquid is being pushed backwards by the object, which means the gas or liquid pushes the object forwards. Thrust force due to air occurs for planes, since the propellers or jet engines force air backwards. Thrust force due to gases occurs for rockets, since the engines push burning gas out. Thrust force due to water occurs for swimmers and boats, because the object pushes water backwards. '''Buoyancy force due to liquid or gas''' – This is the force that allows things to float in gas or liquid. The direction is upwards. E.g. the upwards force on a boat is “buoyancy force due to the water”. '''Tension Force:''' the pull force associated with the stretching of an object. E.g. a tension force would cause a rope to go from slack to tense. '''Traction force due to the ground''' – This occurs when the object pushes backwards on the ground, therefore the ground creates a forward force on the object. This is the forward force on cars, bikes, walkers, runners, because to get a forward motion, all these objects need to push back on the ground. To increase traction, the object needs to push backwards on the ground more strongly – in a car this means pressing down on the accelerator, on a bike this means pushing on the pedals more strongly. For traction force to occur there needs to be sufficient grip between the object and the ground to prevent the wheel sliding. If the grip is insufficient, then the object would slip on the ground (think about trying to accelerate a car when the wheels are stuck in the mud). Traction is a type of friction force. '''Rolling resistance due to the ground''' – This occurs for rolling objects. The force is opposite to the direction of movement. This force is mainly due to the deformation of the ground and the object (e.g. the tyres getting squashed where they contact the ground, or when the soft ground gets pushed out of place). Tyres that have a deep tread, or are not well inflated, have a large rolling resistance. Surfaces that easily change shape (e.g. sand) will also create larger rolling resistance. '''For rolling objects, it is easiest to combine “rolling resistance due to the ground” and “resistance force due to the air” to create a single force opposite to the motion called “resistance force due to the air and ground”.''' ==Non-Contact Forces:== '''Electrostatic Force:''' The force of attraction or repulsion between two charged objects '''Magnetic Force:''' The force of attraction or repulsion between two magnets or moving charges. '''Weight due to the earth:''' Gravity is a force that attracts all objects with mass together. On Earth, this force is downwards towards the centre of the Earth. Gravity due to the earth acts on all objects on the surface of the Earth, as well as objects anywhere near the Earth, like satellites in orbit. ==Force Diagrams== [image:] The forces acting upon an object are shown using arrows. The size of the arrows can be used to show the relative size of each force. Arrows should be drawn from point at which the force acts. A scale diagram can also be used to construct a force diagram. ==Examples== '''E.g. Book sitting stationary on a desk:''' [image:] '''E.g. An object being pushed, sliding across a horizontal floor:''' [image:] '''E.g. A plane flying:''' [image:] '''E.g. Boat moving with engines on (or when people are paddling):''' [image:] '''E.g. Object that is falling down towards the Earth:''' [image:] '''E.g. Car on a flat road accelerating or at constant speed (labels are the same for bicycles). If the car is accelerating, the traction force is larger than the resistance force. If the car is at constant speed those two forces are equal:''' [image:] The “Resistance force due to air and road” is a combination of rolling resistance and air resistance, The force diagram for a car or a bike is the one that is the most commonly drawn incorrectly. Sometimes people use the word “push” or “engine force” as the forward force – however since there is no person on the outside of the car/bike pushing the car/bike, there is no push force. And since the engine is inside the car, it can’t provide a force on the car. The engine provides the energy, but the ROAD provides the force. For a car which is accelerating or at constant speed it is incorrect to say that friction is backwards. Since traction is actually a friction force, friction is actually forwards. '''E.g. Car coasting – the engine is not engaged, and brakes are not on (labels are the same for bicycles):''' [image:] '''E.g. Car braking (labels are the same for bicycles):''' [image:] '''E.g. Person walking or running at constant speed or accelerating. If the person is accelerating, the traction force is larger than the drag/resistance force. If the person is moving with constant speed those two forces are equal:''' [image:] Note: Similar to cars, it is incorrect to say the friction is backwards for a runner who is at constant speed or accelerating, because the traction force is actually a friction force and points forwards.
Credit: Ben Himme, Tristan O'Hanlon