Topic 15 - forces and matter

Question 1

Why does stretching, bending or compressing require more than one force?

Answer 1

Because otherwise, the object would simply move in the direction of the applied force, instead of changing shape

Question 2

What is elastic distortion?

Answer 2

If an object can go back to its original shape/length after the force has been removed

Question 3

What is inelastic distortion?

Answer 3

If an object can’t go back to its original shape/length after the force has been removed

Question 4

When a force stretches/compresses an object, ….. is transferred to the ………. store of the object

Answer 4

1. energy

2. elastic potential

Question 5

What is the equation for linear elastic distortion?

Answer 5

F ( force(N) ) = K ( spring constant(N/m) ) * X (extention(m) )

Question 6

What is the equation to calculate work done in stretching a spring?

Answer 6

e = 1/2 * k * x (squared)

Question 7

What happens when force is directly proportional to extension?

Answer 7

There is a linear relationship between force and extension (straight line on graph)

Question 8

What happens when a force on an object is too great?

Answer 8

Force would no longer be proportional to extension so there would be a non-linear relationship between force and extension. The point at which the force becomes too great is called the limit of proportionality

Question 9

Describe the quick pilot experiment to find out what size masses to use before you do the core practical on investigating elasticity?

Answer 9

1. Using an identical spring to the one you will be testing, load it with masses one at a time and record the force (weight) and extension each time
2. Plot a force-extension graph and check that you get a nice straight line for at least the first 6 points. If the line curves too early, youll need to use smaller masses

Question 10

Describe the core practical on investigating elasticity?

Answer 10

1. Measure natural length of the spring (with no load) with a mm ruler clamped to the stand. Take the reading at eye level and add markers (thin strips of tape) to the top and bottom of the spring for accurate readings
2. Add a mass on spring and record the mass and measure the new length of the spring
3. Repeat until you have enough measurements
4. Plot a force-extension graph of your results. The line would curve when you exceed the limit of proportionality

Question 11

What should your results be from the core practical?

Answer 11

a large force causes a bigger extension

Question 12

For elastic distortions, the gravitational force is equal to …

Answer 12

k * x

Question 13

How can you work out the work done for particular forces using the graph?

Answer 13

By calculating the area under the linear (straight line) section of your graph

Question 14

What is the relationship between altitude and atmospheric?

Answer 14

As altitude increases, atmospheric pressure decreases

Question 15

Why is the line on an atmospheric pressure-altitude/elevation graph curved?

Answer 15

Because atmospheric pressure is affected by the density of the atmosphere, which also varies with height

(atmospheric pressure -altitude/elevation graph is on page 102 of the cgp guide)

Question 16

As altitude increases, the atmosphere becomes (more/less dense?) so there are ….

Answer 16

1. less
2. fewer air molecules that are able to collide with the surface

(book isnt clear on the specific surface, maybe earths surface?)

Question 17

Why does the weight of the air above the earths surface decrease with altitude?

Answer 17

Because there are fewer air molecules, above the surface, as height increases

Question 18

What does fluid pressure depend on?

Answer 18

1. the area the force is being exerted on
2. The properties of a fluid
3. Surrounding atmospheric pressure

Question 19

Define pressure and recall its equation?

Answer 19

The force per unit area as:

pressure = force/area

Question 20

How does pressure increase with density?

Answer 20

There are more particles that are able to collide so the pressure is higher at a given depth in a denser fluid

Question 21

What happens to pressure as depth increases?

Answer 21

As the depth of a fluid increases, the number of partices above that point increases. The weight of these particles adds to the pressure felt at that point, so fluid pressure increases with depth

(‘that’ - not specifically specified in book)

Question 22

What is the equation to calculate the pressure due to the column of liquid above a certain depth?

Answer 22

P = height * density *graviational field strength

Question 23

Why do objects in fluids experience upthrusts?

Answer 23

1. When an object is submerged in a fluid, the pressure of the fluid exerts a force in all directions
2. As pressure increases with depth, the force exerted on the bottom of an object is larger than the force at the top of the object
3. This force is called upthrust

Question 24

What is upthrust equal to?

Answer 24

The weight of fluid that has been displaced by the object

Question 25

What happens if an object is less dense than the fluid it is placed in

Answer 25

• The object will displace a volume of fluid that is equal to its weight before it is completely submerged in water
• Therefore,the objects weight would be equal to the upthrust and the object floats

Question 26

What happens if an object is denser than the fluid it is placed in?

Answer 26

• It is unable to displace enough liquid to equal its weight
• Hence, weight would be larger than upthrust and the object will sink