Edexcel GCSE Physics Topic 15: Forces and Matter Exam Questions

About Edexcel GCSE Physics Topic 15: Forces and Matter

Topic 15 is the last topic on Paper 2. It covers how forces can stretch, compress or bend objects, the difference between elastic and inelastic distortion, and the linear relationship between force and extension for a spring up to its limit of proportionality. You calculate the energy stored in a stretched spring and describe the core practical investigating extension and work done on springs. The separate physics content extends this to pressure in fluids, how pressure increases with depth, why objects experience upthrust, and why atmospheric pressure decreases with height. It links to Topic 14 on gas pressure. Examiners like force-extension graphs and fluid pressure calculations.

What you need to know

• Deformation More than one force is needed to stretch, compress or bend an object. Elastic distortion means the object returns to its original shape; inelastic distortion means it does not.
• Hooke's law Force exerted on a spring = spring constant x extension (F = k x x), up to the limit of proportionality. Beyond this the force-extension graph is no longer linear.
• Energy in a spring Energy transferred in stretching = 1/2 x k x x^2. This equals the area under a linear force-extension graph.
• Core practical Hang masses on a spring, measure extension with a ruler and pointer at eye level, plot force against extension and find k from the gradient of the linear section.
• Pressure Pressure = force normal to surface / area (P = F / A), measured in pascals (Pa), where 1 Pa = 1 N/m^2.
• Pressure in fluids Pressure due to a column of liquid = height x density x g (P = h x rho x g). Pressure increases with depth and acts in all directions.
• Upthrust and floating Upthrust is caused by greater pressure on the bottom surface than the top. It equals the weight of fluid displaced. An object floats when upthrust equals its weight.
• Atmospheric pressure Caused by air molecules colliding with surfaces. It decreases with height because there is less air above and the air is less dense.

Common mistakes in Topic 15: Forces and Matter

• Using the total length of the spring instead of the extension. Extension = stretched length - original length.
• Using F = k x x beyond the limit of proportionality. The equation only applies to the linear part of the graph.
• Forgetting to convert extension from cm to m before calculating k or energy.
• Saying upthrust depends on the depth of the object once fully submerged. It depends on the volume of fluid displaced.
• Forgetting to square the extension in 1/2 x k x x^2.

Try three Topic 15: Forces and Matter exam-style questions