SP15: Forces and Matter Flashcards
SP15b
1) Recall the equation that links force, extension and the spring constant.
2) What must happen for a spring to stretch?
3) What is the formula relating the energy transferred to the extension of a spring? (This equation is given in the exam.)
1) Force (N) = spring constant (N/m) x extension (m)
F = kx
2) Work has to be done to stretch a spring.
3) Energy transferred in stretching (J) = 0.5 x spring constant (N/m) x extension squared (m squared)
SP15c
1) Use a model of the Earth’s atmosphere to explain why pressure varies with height.
2) What is the pressure in a fluid caused by?
3) Explain how pressure is related to area and a force normal to the surface.
1) The atmosphere is a relatively thin layer of air around the earth. Atmospheric pressure is caused by air molecules colliding with a surface, which exerts a force on the surface. As the altitude increases, the atmosphere gets less dense (there are fewer air molecules in a given volume). This means that there are fewer air molecules to collide with a surface, so the force and pressure exerted decrease. The weight of the air above a surface also contributes to atmospheric pressure. Because there are fewer air molecules as altitude increases, the weight of the air above a surface will also decrease, so the atmospheric pressure decreases too.
2) The pressure in a fluid is due to the depth of the fluid and atmospheric pressure.
3) Pressure in a fluid is due to the depth of the fluid and atmospheric pressure. The deeper you go, the higher the pressure. The pressure in fluids causes a force normal to any surface. Pressure is a measure of the force on a unit of surface area, where the force is normal to the surface.
SP15c
1) Recall the formula relating force, pressure and area.
2) Describe how pressure in fluids increases with depth and density.
1) Pressure (Pa) = force (N) / area of surface (m²)
2) When a fluid has a high density, it has more particles in a given volume. This means there are more collisions of fluid particles on a given surface which means that the fluid exerts a higher pressure. Density increases with depth. The deeper down into the liquid we go, the higher the pressure will be. This is because there is the pressure of the mass of water particles above up pushing down, so the deeper the level, the more particles there are above.
SP15a
1) Give the minimum number of forces that are needed to distort an object.
2) Describe the difference between elastic and inelastic distortion.
3) Describe the relationship between force and extension for a spring.
1) More than one force is required to distort an object.
2) Elastic distortion is when an elastic material changes shape when a force is applied, but it returns to its original when the force is removed.
Inelastic distortion is when a material changes shape when there is a force on it but does not return to its original shape when the force is removed.
3) For a metal spring, there is usually a linear relationship between the force and the length. This means that a graph of force against length will be a straight line. The extension is directly proportional to the force, so the line of a graph will pass through the origin. However, the relationship becomes non-linear if the spring is stretched too far.
SP15a
1) Describe the relationship between force and extension for a rubber band.
2) Compare the force–extension relationship for different objects (for a spring and a rubber band)
1) Rubber bands have a non-linear relationship between the force and extension. This means that the relationship between two variables that does not produce a straight line on a graph.
2) For a spring the increase in extension is proportional to the increase in length (or there is a linear relationship between force and extension), up to a certain force where the relationship becomes non-linear. For a rubber band the relationship is non-linear throughout.
SP15d
1) Explain why the pressure in a liquid depends on density and depth.
2) Use the equation relating pressure in a fluid to height, density and gravitational field strength.
3) Explain how upthrust occurs.
1) The pressure within a liquid increases with depth from its surface. This is because the fluid particles at the bottom of a container have the added weight of the particles on top of them, which creates more pressure on the particles on the bottom.
As the density of the liquid increases, so the pressure also increases.
2) Pressure due to a column of a liquid (Pa) = height of column (m) x density of liquid (kg/m3) x gravitational field strength (N/kg)
3) Objects in a fluid have a force called upthrust acting on them. Upthrust occurs due to the difference in pressure above and below the object.
SP15d
1) Relate the upthrust to the floating or sinking of objects immersed or partially immersed in fluids.
2) Explain the factors that determine whether or not an object will float or sink.
1) An object that is partly or completely submerged experiences a greater pressure on its bottom surface than on its top surface. This causes a resultant force upwards. This force is called upthrust.
An object in a fluid is displacing some of the fluid. The upthrust force is equal in size to the weight of the fluid displaced by the object.
2) Density of the object: if an object is more dense that the fluid displaced, it will sink. If an object is less dense than the fluid displaced, it will float. Upthrust and weight of an object: if the upthrust on an object is less than the weight of the object, it will sink.
SP15b - Core Practical
1) What is the aim of the investigating springs core practical?
2) What is the method for investigating springs core practical?
1) Investigate the extension and work done when applying forces to a spring
2) Method
A. Set up the apparatus as shown in photo B. The zero on the ruler should be level with the bottom of the unstretched spring.
B. Measure the length of the spring with no weights hanging on it and write it down.
C. Hang a 1 newton weight on the spring. Record the extension of the spring (the length shown on the ruler).
D. Repeat step C until you have found the extension of the spring with 10 different masses.
E. Repeat steps A to D for a different spring.
F. Use your results to calculate the spring constant for each spring.
SB15b - Core Practical
1) What are the independent, dependent and control variables for the investigating springs core practical?
2) What are the safety hazards for the investigating springs core practical?
3) How can the accuracy of the experiment be improved
1) The independent variable is the force applied to the free end of the spring. The dependent variable is the extension of the spring. The controlled variable is the spring constant (a measure of the stiffness of the spring), in other words the type of spring used.
2) - The clamp stand should be securely fixed to the bench.
- Do not make the masses bounce up and down on the spring, which could cause the masses to come off, causing harm
- Make sure masses do not fall onto the flood or onto people’s feet
3) If the total extension is 1 cm, the change with each weight added will be very small, and it will be difficult to get an accurate measurement of the extension for each weight. Use heavier weights (1), so the total extension is greater.
SB15b - Core Practical
1) When a force moves an object, the work done in moving the object can be calculated using the equation: work done = force x distance. Explain why this equation cannot be used to calculate the work done in stretching a spring.
2) Give a reason why you should use the values read from a graph to find the spring constant, rather than values read from a table
1) The force needed to stretch a spring increases as the spring is extended, so there is not just one value of force that can be used.
2) Drawing a line of best fit helps to compensate for any random errors, so points taken from the line are more likely to be accurate/give an accurate value for the spring constant.
