Topic 15 - Forces And Matter

Question 1

Why do you need more than one force to stretch, bend or compress an elastic object?

Answer 1

When you apply a force to an object you may cause it to stretch, bend or compress. To do this you need more than one force acting on the object otherwise the object will simply move in the direction of the applied force, instead of changing shape.

Question 2

What is the difference between elastic and inelastic distortion?

Answer 2

Elastic distortion is when an object can go back to its original shape and length after the force has been removed. These objects are called elastic objects. Inelastic distortion is when an object doesn’t return to its original shape of length after the force has been removed.

Question 3

What is the elastic limit?

Answer 3

The point where an object stops distorting elastically and begins to distort inelastically.

Question 4

How do you work out the force exerted on a spring?

Answer 4

force exerted on a spring (N) = spring constant (N/m) × extension (m)

F = k × x

A stiffer spring has a greater spring constant

Question 5

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

Answer 5

energy transferred in stretching (J) = 0.5 × spring constant (N/m) × (extension (m))2

E = 1/2 x k x (x)2

Question 6

Why is there a linear relationship between force and extension?

Answer 6

The extension of the stretched elastic object is directly proportional to the force applied.

Question 7

When does the linear relationship between force and extension turn into a non-linear relationship?

Answer 7

After a certain amount of force, the object stretches more for each unit of force. This point is the limit of proportionality - the point beyond which Hooke’s law is no longer true when stretching a material. Then a certain amount of force after the limit of proportionality, the object reaches its elastic limit where the object is permanently stretched after this point.

Question 8

Describe how to investigate relationships between force, and extension for a spring, and the work done in extending the spring

Answer 8

1. Secure a clamp stand to the bench using a G-clamp or a large mass on the base.
2. Use bosses to attach two clamps to the clamp stand.
3. Attach the spring to the top clamp and a ruler to the bottom clamp.
4. Adjust the ruler so that it is vertical and with its zero level with the top of the spring.
5. Measure and record the unloaded length of the spring.
6. Hang a 100 g slotted mass carrier - weight 0.98 newtons (N) - from the spring. Measure and record the new length of the spring.
7. Add a 100 g slotted mass to the carrier. Measure and record the new length of the spring.
8. Repeat step 7 until you have added a total of 1,000 g.

Question 9

How is atmospheric pressure created?

Answer 9

By air molecules colliding with the surface of the Earth

Question 10

How does altitude vary with atmospheric pressure? (5)

Answer 10

As the altitude (height above Earth) increases, atmospheric pressure decreases. As the altitude increases, the atmosphere gets less dense, so there are fewer air molecules that are able to collide with the surface. There are also fewer air molecules above a surface as the height increases. This means that the weight of the air above it, which contributes to atmospheric pressure, decreases with altitude.

Question 11

Describe how pressure in liquids increases with depth and densityv

Answer 11

The pressure in a liquid is different at different depths. Pressure increases as the depth increases. For example, the pressure acting on a dam at the bottom of a reservoir is greater than the pressure acting near the top. This is why dam walls are usually wedge-shaped. The pressure in a liquid is due to the weight of the column of water above. Since the particles in a liquid are tightly packed, this pressure acts in all directions.

Question 12

How do you work out pressure in fluids (gases and liquid)?

Answer 12

P = F ÷ A

Pressure (Pa) = force normal to surface (N) ÷ area of surface (m2)

Question 13

Describe pressure in fluids (gases and liquids)

Answer 13

Liquids and gases are fluids. An object in a fluid experiences a pressure caused by both the fluid itself and the atmospheric pressure.

The pressure in fluids causes a force normal to a surface. A force that is normal to a surface acts at right angles (90°) to it.

Question 14

What equation is used yo calculate the magnitude of the pressure in liquids and calculate the difference in pressures at different depths in a liquid?

Answer 14

pressure due to a column of liquid (Pa) = height of column (m) × density of liquid (kg/m3) × gravitational field strength (N/kg)

P = h x p x g

Question 15

What is upthrust?

Answer 15

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.

Question 16

When does an object sink?

Answer 16

If the upthrust is less than the weight of the object, the object will sink.

If the density of the object is greater than the density of the fluid, the object can never displace enough fluid to create an upthrust that will hold its weight up so it sinks.

Question 17

When does an object stop sinking?

Answer 17

The sinking only stops when the object reaches the bottom and an extra reaction force is added to the upthrust in order to balance the weight.

Question 18

When does an object float?

Answer 18

If the upthrust is larger than the weight of the object, the object will rise.

This happens when you jump into a swimming pool and then rise back up to the surface. Under the surface, the weight of water you displace - the upthrust - is more than your weight, so there is a resultant upwards force.

When your head sticks up out of the water, the upthrust reduces as your head is no longer displacing water, and there is zero resultant force, so you float at that level.