Topic 15 - Forces and Matter

Question 1

SP15a - What are the three things a force can do to an object?

Answer 1

• Change shape(/size)
• Change direction
• Accelerate (Decelerate)

Question 2

SP15a - What is the name for a force changing the shape of an object and what is required for it to occur?

Answer 2

It is called deformation and requires two forces rather than just one.

Question 3

SP15a - What does it mean if an object elastically deforms?

Answer 3

The forces change the object’s shape but it returns back to its original shape when forces are removed. (e.g spring, diving board, and archer’s bow.)

Question 4

SP15a - What does it mean if an object inelastically deforms?

Answer 4

The forces change the object’s shape and it doesn’t return back to its original shape when forces are removed. (e.g Clay, Blu-tac, and spoons.)

Question 5

SP15a - How may some objects behave elastically and inelastically?

Answer 5

Some objects (like springs for example) will behave elastically. However, when the forces exceed a certain amount, they will become permanently deformed, behaving inelastically and keeping its new shape.

[Life Lesson #44: Don’t overextend yourself]

Question 6

SP15a - What is the extension of a spring?

Answer 6

The change in length of a spring when a force is applied

New length - old length = extension

Question 7

SP15a - How would you describe the relationship between force and length of a spring

Answer 7

It is a linear relationship (straight line) as long as the spring behaves elastically.

Once it is permanently deformed, the relationship is non-linear (curved).

Question 8

SP15a - What is the relationship between extension and force of a spring and rubber band?

Answer 8

• Spring: They are directly proportional as it would be a straight line through the origin.
• Elastic band: It behaves completely inelastically with a non-linear relationship

Question 9

SP15b - What is a spring constant?

Answer 9

The force (N) required to produce and extension of 1 metre

Question 10

SP15b - What is the equation linking force and spring constant?

Answer 10

F = k × x

Force (N) = Spring constant (N/m) × Extension (m)

Question 11

SP15b - What is the relationship between force and extension?

Answer 11

F ∝ x (Directly proportional)

Question 12

SP15b - What does the gradient of a Force/extension graph show you?

Answer 12

Sign gradient is Δy÷Δx it would be force/extension which will give you the spring constant of a spring

Question 13

SP15b - How may you be able to tell that a spring has a higher spring constant by holding it?

Answer 13

Stiffer springs have a higher spring constant (As they require more force to extend)

Question 14

SP15b - How do you calculate the work done when stretching a spring?

Answer 14

E = 1/2 × k × x²

Energy transferred by stretching (J) = 1/2 × Spring constant × Extension²

Question 15

SP15b CP - Describe how to find the spring constants of different springs.

Answer 15

• Set up your equipment as shown
• Measure the length of the spring without any weights at eye-level
• Add 1 newton (100g) weights one at a time, measuring the new length at eye level each time
• Take away the original length from all your values to get the extension
• Repeat a few times and take the average of your results for reliability
• Plot a graph of force/extension and calculate the gradient of the line. This will be the spring constant
• If you want to find the work done for a particular extension, square that extension, divide by 2 and then multiply by the spring constant

Question 16

SP15c - What is the formula for pressure?

Answer 16

P = F ÷ A

Pressure (Pa) =

Force [normal to surface] (N) ÷ Area of that surface (m²)

Question 17

SP15c - What are the units for pressure?

Answer 17

Pa - Pascals, which is equivalent to N/m²

(however the m in N/m² can be changed to any other metric length value but won’t be equal to 1 Pa)

Question 18

SP15c - What are fluids?

Answer 18

Liquids or gases

(Because they f̶l̶o̶o̶o̶d̶ flow)

Question 19

SP15c - What is atmospheric pressure?

Answer 19

The pressure exerted perpendicular to any surface by the air at all times. It is highest at sea level around 100,000 Pa (1 atm)

Question 20

SP15c - What two things affect the pressure exerted by a fluid?

Answer 20

• Depth of the fluid: The more particles above you, the higher pressure you will experience as more particles can produce a higher force. This is why pressure is highest at sea level and reduces as you increase altitude
• Density of fluid: The denser the fluid you are in, the more particles there are in a fixed volume, the more particles can produce a force. This is why water pressure is higher than air pressure

Question 21

SP15c - When calculating total pressure when underwater, what must you do?

Answer 21

Find the pressure exerted by the water and add 100,000 Pa of air pressure

Question 22

SP15c - Why does a bag of crisps sealed at sea level expand when at higher altitudes?

Answer 22

• When sealed at sea level, the air inside the bag will around 100,000 Pa
• When taken to a higher altitude, the pressure outside the bag decreases
• As the pressure inside the bag (acting outwards) is greater than outside the bag (acting inwards), the bag expands

Question 23

SP15d - What formula calculates the pressure due to a column of liquid?

Answer 23

P = h × ρ × g

Pressure [due to column] (Pa) =

height of column (m) × density of fluid (kg/m) × gravitational field strength (N/kg)

Question 24

SP15d - What is upthrust?

Answer 24

The upward force exerted on the bottom of an object in a liquid.

Question 25

SP15d - How do you calculate upthrust?

Answer 25

Difference in pressure between top and bottom of the object multiplied by the surface area of the bottom of the object

[Upthrust = Δh × ρ × g × surface area (m²)]

OR

Upthrust is the weight (in Newtons) of the liquid displaced when an object is placed in the liquid

Question 26

SP15d - If an object is floating what does this mean?

Answer 26

The density of the object is less than the liquid it is in. The lower the density, the better it floats (more will be above the liquid surface)