Flashcards in Forces and Shape Deck (22):
What can a force do?
-stop or start
-speed up of slow down (change in speed)
-change the direction
-change the shape
What are the different types of forces?
-push or pull
-gravitational force (weight) - NOT GRAVITY (which is an acceleration)
What is the strength of Earth's gravitational field?
10 N/Kg (9.8)
What are forces measured in?
It is a vector
A force of 1N will make a mass of 1Kg accelerate at 1m/s
How do you work out weight?
weight = mass x g
N = Kg x N/Kg
What is normal reaction force?
The force that is acting in an upwards direction on the car (opposes weight)
What happens if the forces acting in opposite directions are the same size?
The unbalanced force is zero, and the two forces are balanced
The rope will remains stationary
What happens if the forces acting in opposite directions are unbalanced?
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
What apparatus would you use to measure weight?
A spring balance or a newton balance
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)
How do you investigate friction?
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
Extensions: Helical Steel Spring
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
Extensions: Copper Wire
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
Extension: Rubber band
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
Who is Hooke?
17th Century British Scientist Robert Hooke
What is Hooke's Law?
-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
What is elastic behaviour?
-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
What is not elastic?
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.
What happens when two forces interact?
They exert forces on each other which are equal in size and act in opposite directions
How do you investigate Hooke's law with a spring?
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.
What does elastic behaviour mean?
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