1. Forces and Matter 2016 EdExcel GCSE Physics Topic 15 W Richards
05/07/2019
Forces and Matter
2016 EdExcel GCSE Physics
Topic 15
W Richards
The Weald School

2. Forces introduction
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How many forces does it take to stretch, bend or compress an object?

3. Force and Extension Consider a mass on a spring:
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Consider a mass on a spring:
What happens when a mass is added?
When a force is applied to this spring it will change shape and extend.

4. Investigating Force and Extension
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Task: Find an expression that relates extension to the amount of weight added.
Weight added (N)
Extension (cm) 1 2 3 4 5 6
Q. What is the spring constant for your spring?
Force = Spring constant x extension
F = kx

5. Force-Extension Graph for a spring
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Force/N
Extension/mm
The “limit of proportionality”.
Force is proportional to extension as long as you don’t go past the “limit of proportionality”. There is a linear relationship up to this point.

6. Questions on Hooke’s Law
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Force = Spring constant x extension
F = ke
A spring has a spring constant of 20N/m. If it extends by 3m how much force was applied?
The force on this spring is now doubled. What is its new extension?
Another spring of spring constant 2N/cm is extended by 5cm. How much force was applied?
Another spring has 10N applied to it and it extends by 50cm. What is its spring constant in N/m?
How much will a spring extend if it has a spring constant of 10N/m and 4N is applied to it?
60N 6m
10N
20N/m
0.4m

7. Elastic and Plastic Deformation
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Force/N
Extension/mm
If you don’t use too much force on the spring you can take the force off and the spring returns to it’s original shape – this is “elastic deformation”.
If you put too much force on the spring it “stretches” – in other words, when you remove the force the spring does not go back to its original length. This is “plastic deformation”.

8. Elastic Potential Energy
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Consider a mass on a spring:
What happens when a mass is added?
When a force is applied to this spring it will change shape and extend. The spring will have “stored elastic potential energy”

9. Elastic Potential Energy
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Task: Calculate how much stored EPE there is in your springs
Stored EPE = ½kx2
Weight added (N)
Extension (m)
Stored EPE (J) 1 2 3 4 5 6

10. Pressure in Gases and Liquids
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Particles in a liquid or a gas (“fluids”) move around randomly, a little like this:
Every time the particles hit the side of the container the particles exert a force at right angles on the container – this is called “pressure”.

11. Atmospheric Pressure (Physics only)
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Why does atmospheric pressure decrease when you go up a mountain?
There are less air molecules up here
… than down here
Less air molecules = fewer collisions = less pressure!

12. Pressure (Physics only)
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Pressure depends on two things:
How much force is applied, and
How big (or small) the area on which this force is applied is.
Pressure can be calculated using the equation:
Pressure (in N/m2) = Force (in N)
Area (in m2) F A P
OR in cm2 and N/cm2

13. Some pressure questions
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Calculate the pressure exerted by a 1000N elephant when standing on the floor if his feet have a total area of 2m2.
A brick is rested on a surface. The brick has an area of 20cm2. Its weight is 10N. Calculate the pressure.
A woman exerts a pressure of 100N/cm2 when standing on the floor. If her weight is 500N what is the area of the floor she is standing on?
(Hard!) The pressure due to the atmosphere is 100,000N/m2. If 10 Newtons are equivalent to 1kg how much mass is pressing down on every square centimetre of our body?
500 Pa
0.5 N/cm2
5cm2
Around 1kg per cm2!

14. Pressure in Fluids (HT only)
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Area A
Consider a column of fluid: h
The pressure at the base of this column would be given by:
Density ρ
Pressure = ρhg
…where ρ = the density of the liquid, h = the height of the container and g = gravitational field strength.

15. Example questions
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Calculate the pressure at the bottom of a 2 litre bottle of water of height 40cm (density of water = 1000kg/m3and g = 10N/kg).
What is the pressure at the bottom of a can of coke if the density of coke is 1000kg/m3 and the can is 15cm tall?
If the density of seawater is 1027kg/m3 what depth would you need to be at to experience a pressure of 50,000Pa?
4000Pa
1500Pa
4.87m

16. Pressure vs. Depth (HT only)
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What does this demonstration tell you?
Pressure increases with ____. This is because the water at the ______ of this container is pushed on by the ______ of the water further up as well as atmospheric pressure, which causes it to be under higher ________.
Words – pressure, bottom, weight, depth

17. Why do objects float?
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Whether or not an object will float depends on its DENSITY. For example:
The metal block will ____ because it is ______ dense than water
The wooden block will ____ because it is ______ dense than water

18. Upthrust = weight of fluid displaced
Upthrust in Fluids
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Archimedes, BC
Any object that is placed in a liquid will experience a force called upthrust. We can use my principle to calculate how big that upthrust is:
Weight
Upthrust
Upthrust = weight of fluid displaced

19. Floating in more detail (Physics only)
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Consider a floating object:
How does the pressure at the bottom of this object compare to the pressure at the top of the object?
This difference in pressure causes the force called “upthrust”.
If weight equals upthrust the object will ____. This is because the object displaces a weight of fluid _____ to its own weight.
If weight is greater than upthrust the object will ____. This is because the object is ______to displace a weight of liquid equal to its own weight.
Words – equal, unable, sink, float