Stretching, bending or compressing an object requires more than one force. If an object has been elastically distorted it can go back to its original shape and length after the force has been removed. Inelastic distortion means that the changed shape is irreversible and permanent. The elastic limit is the point where an object stops distorting elastically and begins to distort inelastically. This is where a F/x graph begins to curve and a spring is permanently stretched. Work is done when a force stretches or compresses an object and causes energy to be transferred to the EPE store of the object
where F = Force (N), k = Spring Constant (N/m) and x = Extension (m)
Use the equation where EPE = Elastic Potential Energy (J), k = Spring Constant (N/m) and x = Extension (m)
CORE PRACTICAL: Investigating Springs
A – Set up a spring held up by a clamp and stand, with a ruler attached where zero should be level with the bottom of the unstretched spring
B – Measure the length of the spring with no weights hanging on it
C – Hang a 1N weight on the spring and record extension
D – Repeat until 10N are hanging on the spring
E – Plot a graph of extension against force. The linear extension means that the two variables are in direct proportion, after which non-linear extension means the graph is curved
F – Use results to calculate the spring constant for the spring OR calculate it by finding the gradient of the graph. EPE is the area under the graph
Pressure
The atmosphere is a layer of air that surrounds the Earth. Atmospheric pressure is created on a surface by air molecules colliding with the surface. As altitude increases, atmospheric pressure decreases. Atmospheric pressure is affected by the density of atmosphere which decreases with height as there are fewer air molecules. This means that the weight of the air above, which contributes to atmospheric pressure, decreases with altitude
Pressure is the force per unit area. Increases the area decreases the pressure.
where P = Pressure (Pa or N/m2), F = Force (N) and A = Area (m2)
Pressure from a fluid is the pressure caused by collisions of gas or liquid particles. Fluid pressure always exerts a force normal to any surface in contact with the fluid. The pressure in a fluid depends on the area the force is being exerted on and the surrounding atmospheric pressure.
For a given liquid, density is uniform, but the density of a gas can vary. Assuming their particles have the same mass, a fluid which is denser has more particles in a certain space than a less dense one. There are more particles that can collide, so the pressure is higher at a given depth in a denser fluid. As depth increases, the weight of the increasing number of particles above that point adds to the pressure at that point, so fluid pressure increases with depth
Use the equation where P = Pressure (Pa), ρ = Density (kg/m3), g = Gravitational Field Strength (N/kg) and h = Depth (m)
When an object is submerged in a fluid, the pressure of the fluid exerts a force on it in every direction. Pressure increases with depth so the force exerted on the top is less than the force exerted on the bottom, causing a resultant force called upthrust. Upthrust on an object is equal to the weight of the fluid displaced by the object. If upthrust ≥ weight, the object floats but if the weight is greater, the object sinks. An object that is denser than the fluid is unable to displace enough fluid to displace enough fluid to equal its weight so it sinks. An object less dense than water will displace a volume of fluid equal to its weight before submersion, meaning the object floats.