1.2.1 - 1.2.9 - Forces, Movement and Changing Shape Flashcards
How can forces affect different bodies?
Changes in speed: forces can cause bodies to speed up or slow down
Changes in direction: forces can cause bodies to change their direction of travel
Changes in shape: forces can cause bodies to stretch, compress, or deform
What are the different type of forces?
Gravitational (or weight) - the force between any two objects with mass (like the Earth and the Moon)
Electrostatic - the force between any two objects with charge (like a proton and an electron)
Thrust - the force pushing a vehicle (like the push from rocket engines on the shuttle)
Upthrust - the upward force on any object in a fluid (like a boat on the surface of a river)
Air resistance (or drag) - the force of friction between objects falling through the air (like a skydiver in freefall)
Compression - forces that squeeze an object (like squeezing a spring)
Tension - forces that stretch an object (like two teams in a tug-of-war)
Reaction force - the force between any two objects in contact (like the upwards force from a table on a book)
What are the two quantities of forces?
All quantities can be one of two types:
A scalar
A vector
What is a scalar?
Scalars are quantities that have only a magnitude
For example, mass is a scalar quantity since it is a quantity that has no direction to it
What is a vector?
Vectors have both a magnitude and a direction
Velocity is a vector quantity since it is described with both a magnitude and a direction
What type of quantity are distance and displacement?
Distance is a value describing only how long an object is or how far it is between two points - this means it is a scalar quantity
Displacement on the other hand also describes the direction in which the distance is measured - this means it is a vector quantity
What are the different scalars and vectors quantities?
What quantity is force?
Force is a vector quantity because it describes both magnitude and direction
The length of the arrow represents the magnitude of the force
The direction of the arrow indicates the direction of the force
What is a resultant force?
A resultant force is a single force that describes all of the forces operating on a body
When many forces are applied to an object they can be combined (added) to produce one final force which describes the combined action of all of the forces
What does a resultant force determines?
The direction in which the object will move as a result of all of the forces
The magnitude of the final force experienced by the object
The resultant force is sometimes called the net force
How can the resultant force be calculated?
Resultant forces can be calculated by adding or subtracting all of the forces acting on the object
Forces working in opposite directions are subtracted from each other
Forces working in the same direction are added together
What is friction?
The force which opposes the motion of an object
Frictional forces always act in the opposite direction to the object’s motion
Friction emerges when two (or more) surfaces rub against each other
What is a balanced force?
Balanced forces mean that the forces have combined in such a way that they cancel each other out and no resultant force acts on the body
What is an unbalanced force?
Unbalanced forces mean that the forces have combined in such a way that they do not cancel out completely and there is a resultant force on the object
What is the link between unbalanced forces, mass and acceleration?
When forces combine on an object in such a way that they do not cancel out, there is a resultant force on the object
This resultant force causes the object to accelerate
The object might speed up
The object might slow down
The object might change direction
What is eq. for Force?
Force = Mass x acceleration
What is weight?
The force acting on an object due to gravitational attraction
Planets have strong gravitational fields
Hence, they attract nearby masses with a strong gravitational force
What effects does weight have?
Objects stay firmly on the ground
Objects will always fall to the ground
Satellites are kept in orbit
What is the eq. for weight?
Weight = mass x gravitational field strength
What does the weight of an object depend on?
The object’s mass
The mass of the planet attracting it
Mass is related to the amount of matter in an object
Weight is the force of gravity on a mass
The weight of an object and the mass of an object are directly proportional
The size of this force depends on the gravitational field strength
What is stopping distance?
The total distance travelled during the time it takes for a car to stop in response to some emergency
What is the eq. for stopping distance?
Stopping distance = Thinking distance + Braking distance
What is Thinking distance, Braking distance and stopping distance?
Thinking distance = the distance travelled in the time it takes the driver to react (reaction time) in metres (m)
Braking distance = the distance travelled under the braking force in metres (m)
Stopping distance = the sum of the thinking distance and braking distance, in metres (m)
For a given braking force, the greater the speed of the vehicle, the greater the stopping distance
What are the factors affecting stopping distance?
Vehicle speed - the greater the speed, the greater the vehicle’s braking distance will be (because the brakes will need to do more work to bring the vehicle to a stop)
Vehicle mass - a heavy vehicle, such as a lorry, takes longer to stop
Road conditions - wet or icy roads make it harder to decelerate
Driver reaction time - a driver’s thinking distance depends on their reaction time. Being tired, or intoxicated (i.e. alcohol, or drugs) can increase reaction time
What are the 2 forces on falling objects?
Weight (due to gravity)
Air resistance (due to friction)
What is terminal velocity?
Initially, the upwards air resistance is very small because the skydiver isn’t falling very quickly
Therefore, there are unbalanced forces on the skydiver initially
As the skydiver speeds up, air resistance increases, eventually growing large enough to balance the downwards weight force
Once air resistance equals weight, the forces are balanced
This means there is no longer any resultant force
Therefore, the skydiver’s acceleration is zero - they now travel at a constant speed
This speed is called their terminal velocity
What is the method for core practical 2: Investigating force and extension?
Set up the apparatus so the wire is taut. No masses should be attached just yet
Measure the original length of the wire using a metre ruler and mark a reference point with tape preferably near the beginning of the scale eg. at 1 cm
Record the initial reading on the ruler of the reference point
Add a 100 g mass onto the mass hanger
Read and record the new reading of the tape marker from the meter ruler now that the metal wire has extended
Repeat this process until all masses have been added
Remove the masses and repeat the entire process again, until it has been carried out a total of three times, and an average length (for each mass attached) is calculated
What is the analysis of this experiment?
The force, F added to the spring / rubber band / metal wire is the weight of the mass
Weight calc using W = M X G
Therefore, multiply each mass by gravitational field strength, g, to calculate the force, F
The force can be calculated by multiplying the mass (in kg) by 10 N/kg
The extension e of the spring / rubber band is calculated using the equation:
e = average length – original length
The final length is the length of the spring / rubber band recorded from the ruler after the masses were added
The extension e of the metal wire is calculated using the equation:
e = new marker reading − reference point reading
The original length is the length of the spring / rubber band / metal wire when there were no masses attached
What is the evaluation for this experiment?
Systematic Errors:
Make sure the measurements on the ruler are taken at eye level to avoid parallax error
Random Errors:
The accuracy of such an experiment is improved with the use of a pointer (a fiducial marker)
Wait a few seconds for the spring / rubber band / metal wire to fully extend when a mass is added, before taking the reading for its new length
Make sure to check whether the spring has not gone past its limit of proportionality otherwise, it has been stretched too far
What is Hooke’s law?
The relationship between the extension of an elastic object and the applied force is defined by Hooke’s Law
The extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality
What is the link between extension of an elastic object and force applied?
Directly proportional means that as more force is applied, the greater the extension (and vice versa)
The limit of proportionality is where if more force is added, the object may extend but will not return to its original shape when the force is removed (it will be inelastically deformed)
This limit varies according to the material
What is a force extension graph?
Hooke’s law is the linear relationship between force and extension
This is represented by a straight line on a force-extension graph
Any material beyond its limit of proportionality will have a non-linear relationship between force and extension
What is elastic behaviour?
When some objects, such as springs or rubber bands, are stretched they will return to their original shape and length once the forces are removed
Other materials, such as plastic, remain permanently deformed (stretched)
A change of shape is called a deformation and can either be:
Elastic
Inelastic
What is elastic deformation?
When objects return to their original shape when the stretching force is removed
Examples of materials that undergo elastic deformation are:
Rubber bands
Fabrics
Steel springs
What is inelastic deformation?
When objects remain stretched and do not return completely to their original shape even when the stretching force is removed
Examples of materials that undergo inelastic deformation are:
Plastic
Clay
Glass
