Topic 15 - Forces and matter Flashcards
Explain why stretching, bending or compressing an object requires more than one force to be applied.
If a single force is applied, the object will just move in the force’s direction. If forced inwards from both sides the object (e.g. a spring or other elastic object) will compress. If fixed at one end of the spring and a force is applied at the other, more than one force is still being applied to the spring (a reaction force from the fixed point).
Describe the difference between elastic and inelastic distortion
Distortion or deformation means changing shape.
In elastic distortion the object returns to its original shape when the load has been removed (e.g. an elastic band)).
In inelastic distortion the object does not return to its original shape when the load has been removed (e.g. a spring pulled too far).
What is the equation for linear elastic distortion?
force exerted on a spring (Newton) = spring constant (N/m) x extension (m)
What is the equation used to calculate the work done in stretching a spring?
energy transferred in stretching (joules, J) = 0.5 × spring constant (newton per metre, N/m) × (extension (metre, m))²
Describe linear relationships between force and extension
Linear elastic distortion is showcased by Hooke’s law: force (N) = spring constant (N/m) x extension (m)
Here the extension of a spring is directly proportional to the force applied.
Explain the non-linear line section of Force/extension graphs
The point it stops being linear is the elastic limit. From here on, it is not elastic, does not follow Hooke’s law and plastic deformation occurs.
If this is shallow (less steep) there is lots of extension for not a lot of force and it is easy to stretch.
If the graph is just linear with no non-linear end section the material is brittle so snaps instead of stretches after the elastic limit.
Explain the linear line section of Force/extension graph
The linear line section is elastic and follows hooke’s law, elastic deformation occurs. The gradient is k in Spring constant k = Force required to extend a spring per metre.
Describe the core practical to investigate the extension and work done when applying forces to a spring
-Secure a clamp stand to the bench using a G-clamp or a large mass on the base.
-Use bosses to attach two clamps to the clamp stand.
-Attach the spring to the top clamp and a ruler to the bottom clamp.
-Adjust the ruler so that it is vertical and with its zero level with the top of the spring.
-Measure and record the unloaded length of the spring.
-Hang a 100 g slotted mass carrier - weight 0.98 newtons (N) - from the spring. Measure and record the new length of the spring.
-Add another 100 g slotted mass to the carrier. Measure and record the new length of the spring.
-Continue adding weights until you have added a total of 1,000 g.
Describe how to analyse the results of the core practical
-Calculate extension by extension = length - unloaded length
-Plot a graph with extension on the y-axis and force on the x-axis, draw a curve of best fit
-For the region where force is proportional to extension find the gradient. The spring constant k is the reciprocal of this gradient
-The work done in extending the spring is the area under the line on the graph
-Work done = force x distance moved
-Energy transferred in stretching (joules, J) = 0.5 × spring constant (newton per metre, N/m) × (extension (metre, m))²
Explain why atmospheric pressure varies with height
Atmospheric pressure decreases as the height of a surface above ground level increases. This is because, as altitude increases the atmosphere gets less dense and there is less air and less weight pushing down
What factors affect the pressure of the atmosphere on an object (such as an aeroplane) as it goes higher in the atmosphere?
The height of the atmosphere, the density of the atmosphere and the temperature of the atmosphere.
This is because there is less air above the aeroplane, the air gets thinner and the air gets colder.
Define a fluid
A substance with no fixed shape that yields to external pressure; a gas or liquid
State the cause of pressure in a fluid
The fluid and atmospheric pressure.
What force does pressure in fluids exert?
Pressure in fluids causes a force normal to any surface (acts at 90° to it)
Explain how pressure is related to force and area in a fluid
Pressure (Pa) = force normal to surface(N)/area of that surface (m²)
At a smaller surface area the pressure is greater
Explain how pressure relates to depth in a fluid
As the depth increases, the mass of fluid above it also increases. This means that the force due to the mass increases. Since the force has increased whist the area has remained constant the pressure will increase.
Explain how pressure relates to density in a fluid
As the density of a fluid increases, the number of particles in a given volume increases. Consequently, the weight of the fluid is greater (mass = density x volume) which means that the force from the fluid above is larger. As the force has increased so will the pressure.
What’s the equation used to calculate the magnitude of the pressure in liquids?
pressure due to a column of liquid (Pa) = height of column (m) × density of liquid (kg/m³) × gravitational field strength (N/kg)
How can upthrust be related to weight?
The upthrust force is equal in size to the weight of the fluid displaced by the object.
Describe how weight effects floating and sinking
If the upthrust is larger than the weight of the object, the object will rise.
Sinking only stops when the object reaches the bottom and an extra reaction force is added to the upthrust in order to balance the weight
Explain why an object in a fluid is subject to the upwards force of upthrust
An object partially or completely submerged experiences a greater pressure on its bottom surface than on its top surface. This causes a resultant force upwards called upthrust. An object in a fluid displaces some of the fluid.
Describe how upthrust effects floating and sinking
If the upthrust is less than the weight of the object, the object will sink.
When there is zero resultant force you float.
Describe how the density of a fluid effects floating and sinking
If the density of the object is greater than the density of the fluid, the object can never displace enough fluid to create an upthrust that will hold its weight up so it sinks.
