1.2.1 - 1.2.9 - Forces, Movement and Changing Shape

Question 1

How can forces affect different bodies?

Answer 1

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

Question 2

What are the different type of forces?

Answer 2

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)

Question 3

What are the two quantities of forces?

Answer 3

All quantities can be one of two types:
A scalar
A vector

Question 4

What is a scalar?

Answer 4

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

Question 5

What is a vector?

Answer 5

Vectors have both a magnitude and a direction
Velocity is a vector quantity since it is described with both a magnitude and a direction

Question 6

What type of quantity are distance and displacement?

Answer 6

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

Question 7

What are the different scalars and vectors quantities?

Answer 7

Question 8

What quantity is force?

Answer 8

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

Question 9

What is a resultant force?

Answer 9

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

Question 10

What does a resultant force determines?

Answer 10

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

Question 11

How can the resultant force be calculated?

Answer 11

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

Question 12

What is friction?

Answer 12

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

Question 13

What is a balanced force?

Answer 13

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

Question 14

What is an unbalanced force?

Answer 14

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

Question 15

What is the link between unbalanced forces, mass and acceleration?

Answer 15

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

Question 16

What is eq. for Force?

Answer 16

Force = Mass x acceleration

Question 17

What is weight?

Answer 17

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

Question 18

What effects does weight have?

Answer 18

Objects stay firmly on the ground
Objects will always fall to the ground
Satellites are kept in orbit

Question 19

What is the eq. for weight?

Answer 19

Weight = mass x gravitational field strength

Question 20

What does the weight of an object depend on?

Answer 20

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

Question 21

What is stopping distance?

Answer 21

The total distance travelled during the time it takes for a car to stop in response to some emergency

Question 22

What is the eq. for stopping distance?

Answer 22

Stopping distance = Thinking distance + Braking distance

Question 23

What is Thinking distance, Braking distance and stopping distance?

Answer 23

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

Question 24

What are the factors affecting stopping distance?

Answer 24

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

Question 25

What are the 2 forces on falling objects?

Answer 25

Weight (due to gravity)
Air resistance (due to friction)

Question 26

What is terminal velocity?

Answer 26

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

Question 27

What is the method for core practical 2: Investigating force and extension?

Answer 27

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

Question 28

What is the analysis of this experiment?

Answer 28

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

Question 29

What is the evaluation for this experiment?

Answer 29

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

Question 30

What is Hooke’s law?

Answer 30

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

Question 31

What is the link between extension of an elastic object and force applied?

Answer 31

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

Question 32

What is a force extension graph?

Answer 32

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

Question 33

What is elastic behaviour?

Answer 33

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

Question 34

What is elastic deformation?

Answer 34

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

Question 35

What is inelastic deformation?

Answer 35

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