Forces and Shape

Question 1

What can a force do?

Answer 1

• stop or start
• speed up of slow down (change in speed)
• change the direction
• rotate
• change the shape

Question 2

What are the different types of forces?

Answer 2

• push or pull
• friction
• buoyancy (upthrust)
• magnetic force
• electrostatic force
• gravitational force (weight) - NOT GRAVITY (which is an acceleration)

Question 3

What is the strength of Earth’s gravitational field?

Answer 3

10 N/Kg (9.8)

Question 4

What are forces measured in?

Answer 4

Newtons (N)
It is a vector
A force of 1N will make a mass of 1Kg accelerate at 1m/s

Question 5

How do you work out weight?

Answer 5

weight = mass x g
N = Kg x N/Kg

Question 6

What is normal reaction force?

Answer 6

The force that is acting in an upwards direction on the car (opposes weight)

Question 7

What happens if the forces acting in opposite directions are the same size?

Answer 7

The unbalanced force is zero, and the two forces are balanced
The rope will remains stationary

Question 8

What happens if the forces acting in opposite directions are unbalanced?

Answer 8

The rope will start to move in the direction of the greater force
The unbalanced forces acting on it cause the rope to change the way it is moving and to accelerate

Question 9

What apparatus would you use to measure weight?

Answer 9

A spring balance or a newton balance

Question 10

Describe Friction

Answer 10

1. Friction is a force that opposes motion
2. Generated when one surface tries to move past another
3. Lubrication e.g oil reduces friction
4. The frictional force between two surfaces is proportional to the force pressing them together, but does not depend on the area of the surfaces
5. Friction occurs:
   - Between solid surfaces which are gripping
   - Between solid surfaces which are sliding past each other and you can reduce these types of friction by putting a lubricant like oil or grease between these surfaces, friction between solids can often cause wear of the two surfaces in contact
   - Resistance or “Drag” from fluids (liquids or gases e.g. air)

Question 11

How do you investigate friction?

Answer 11

1. Add a weight onto the test block
2. Change the surfaces (to change friction)
3. Add weights to the pulley until the test block begins to slide over the test surface

Question 12

Extensions: Helical Steel Spring

Answer 12

1. A-B: The spring will begin by stretching uniformly
2. B: When the elastic limit is reached, the stretch is no longer uniform, so as the force increases the stretches become much larger
3. C-D: Since the elastic limit has been reached when the force on the spring is gradually reduced, it follows the dotted line
4. A-E: The spring has now had a permanent stretch

Question 13

Extensions: Copper Wire

Answer 13

1. A-B: The wire will stretch uniformly (elastically) and so if load is removed the wire will return to its original length
2. B: The wire begins to yield
3. B-C: The wire yields and plastic flow, extension becomes a lot larger for a given increase of force (not uniform)
4. C: The wire breaks

Question 14

Extension: Rubber band

Answer 14

1. At the start the rubber band is very easy to stretch, there is a large extension for a small increase in force
2. At the middle it becomes more difficult to stretch, as the force is increased the band does not get much longer
3. Does not obey Hooke’s Law
4. There is no permanent extension but it does not follow the loading curve

Question 15

Who is Hooke?

Answer 15

17th Century British Scientist Robert Hooke

Question 16

What is Hooke’s Law?

Answer 16

-Extension is directly proportional to force
The extension of the spring is proportional to the force
Force = spring constant x extension
F = K (stiffness of the spring) x x
-The initial linear region of a force-extension graph is associated with Hooke’s law

Question 17

What is elastic behaviour?

Answer 17

• The spring (if it does not reach its elastic limit) and the wire (if it does not reach its yield point) because they return to their original length when the force acting on them is removed
• Elastic behaviour is the ability of material to recover its original shape after the forces causing deformation have been removed

Question 18

What is not elastic?

Answer 18

The rubber band because the loading line does not follow the unloading line (the extension is not directing proportional to the force causing it
There is energy loss with the elastic bad stretch but not when the spring is stretched.

Question 19

What happens when two forces interact?

Answer 19

They exert forces on each other which are equal in size and act in opposite directions

Question 20

How do you investigate Hooke’s law with a spring?

Answer 20

1. Make sure you have plenty of extra masses and measure the weight of each (with a balance)
2. Measure the length of the spring (with an accurate mm ruler) when NO LOAD is applied
   -Ensure the ruler is vertical (e.g. With a set square)
   -Measure the spring at eye level (This is the spring’s natural length)
3. Add one mass at a time and allow the spring to come to rest, then measure the new length of the spring. The extension is the change in length from the original length. Adding a marker to the top and bottom of the spring
   might make measuring the lengths easier
4. Repeat this process until you have enough measurements (no fewer than 6)
5. Once you are done, repeat the experiment and calculate an average value for the length of the spring for each applied weight. This will make your results more reliable.

Question 21

What does elastic behaviour mean?

Answer 21

1. If a material returns to its original shape once the forces are removed, it displays elastic behaviour
2. Metals display elastic behaviour as long as Hooke’s law is obeyed

Question 22

Describe Drag

Answer 22

• Type of friction from fluids
• Shape affects drag as reducing drag more streamline
• For a given thrust, the higher the drag the lower the top speed of the car
• Roof boxes on a cars spoil their streamlined shape and so slow them down
• In a fluid, Friction always increases as the speed increases