Forces and Motion Flashcards

Question

What is up thrust ?

Answer 1

When an object is fully or partially submerged in a fluid, the fluid exerts an upward-acting push force on the object A boat floats on a lake due to the upthrust exerted by the water on the boat A ball held underwater will shoot upwards when released due to the upthrust exerted by the water pushing it back to the surface

Answer 2

Electrostatic force There is an electrostatic force between two objects with charge Like charges repel one another, and opposite charges attract one another When an electron gets close to a positively charged ion, the ion exerts a pull force on the electron (attraction) When an electron gets close to another electron, the electrons experience a push force from one another (repulsion)

Answer 3

Magnetic force There is a magnetic force between objects with magnetic poles Like poles repel one another, opposite poles attract one another When a north pole gets close to a south pole, they experience a pull force from one another (attraction) When a north pole gets close to a north pole, they experience a push force from one another (repulsion)

Answer 4

Tension occurs in an object (like a rope or spring) that is stretched When a pull force is exerted on each end of an object, tension acts across the length of the object When two people pull a rope in opposite directions, tension acts along the rope and pulls back on each person Exam Tip The force of gravitational attraction on an object is called its weight. Remember not to refer to this force as simply 'gravity', as this term can mean several different things, and examiners will probably mark it as wrong. Similarly, when referring to air resistance, avoid using terms like 'wind resistance' (there is no such thing!) or 'air pressure', which is a different concept. Drag is an acceptable alternative to the force of air resistance because air resistance is a special type of drag.

Answer 5

When a force acts on an object, the force can affect the object in a variety of ways The object could: change speed change direction change shape The effects of forces on an object often depend on the type of force acting The push force (thrust) of an engine can cause a car to speed up, whilst the force exerted by the brakes (friction) can cause it to slow down The gravitational pull of the Sun on a comet causes the comet to change direction When two opposing forces push on each end of a spring, the spring changes shape (it compresses)

Answer 6

Scalars are quantities that have magnitude but not direction For example, mass is a scalar quantity because it has magnitude but no direction

Answer 7

Vectors are quantities that have both magnitude and direction For example, weight is a vector quantity because it is a force and has both magnitude and direction

Answer 8

Distance is a measure of how far an object has travelled, regardless of direction Distance is the total length of the path taken Distance, therefore, has a magnitude but no direction So, distance is a scalar quantity Displacement is a measure of how far it is between two points in space, including the direction Displacement is the length and direction of a straight line drawn from the starting point to the finishing point Displacement, therefore, has a magnitude and a direction So, displacement is a vector quantity

Answer 9

Speed is a measure of the distance travelled by an object per unit time, regardless of the direction The speed of an object describes how fast it is moving, but not the direction it is travelling in Speed, therefore, has magnitude but no direction So, speed is a scalar quantity Velocity is a measure of the displacement of an object per unit time, including the direction The velocity of an object describes how fast it is moving and which direction it is travelling in An object can have a constant speed but a changing velocity if the object is changing direction Velocity, therefore, has magnitude and direction So, velocity is a vector quantity

Answer 10

Vector quantities can be represented by arrows The length of the arrow represents the magnitude The direction of the arrow indicates the direction Force is a vector quantity because force has magnitude and direction When using arrows to represent forces: the length of the arrow represents the magnitude of the force the direction of the arrow indicates the direction of the force the scale of the arrows should be proportional to the relative magnitudes of the forces an arrow for a 4 N force should be twice as long as an arrow for a 2 N force the arrows should be labelled with the names of the forces, or a description of the forces for example, weight, W, or the gravitational pull of the Earth on the object Not all forces are directed perfectly horizontally or vertically and so need to have an angle described for the direction It is useful to describe an angle with respect to the vertical or the horizontal

Answer 11

A resultant force is a single force that describes all of the forces operating on a body When multiple forces act on one object, the forces can be combined to produce one net force that describes the combined action of all of the forces This single resultant force determines: The direction in which the object will move as a result of all of the forces The magnitude of the net force experienced by the object.

Answer 12

Resultant forces can be calculated by adding 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 If the forces acting in opposite directions are equal in size, then there will be no resultant force – the forces are said to be balanced Imagine the forces on the boxes as two people pushing on either side In the first scenario, the two people are evenly matched - the box doesn't move In the second scenario, the two people are pushing on the same side of the box, it moves to the right with their combined strength In the third scenario, the two people are pushing against each other and are not evenly matched, so there is a resultant force to the left

Answer 13

When multiple forces act on a single object, the forces can be added together to produce a resultant force When the forces acting on an object completely cancel out the forces are balanced the resultant force is zero When the forces acting on an object do not completely cancel out the forces are unbalanced there is a resultant force.

Answer 14

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 For example, the weight of a book on a desk is balanced by the normal force of the desk As a result, no resultant force is experienced by the book, the book and the table are equal and balanced

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 (i.e. change its velocity) The object might speed up The object might slow down The object might change direction The relationship between resultant force, mass and acceleration is given by the equation: F = m × a Where: F = resultant force, measured in newtons (N) m = mass, measured in kilograms (kg) a = acceleration, measured in metres per second squared (m/s2) This equation is also known as Newton's second law of motion

Answer 16

The force experienced by an object with mass when placed in a gravitational field

Answer 17

Planets have strong gravitational fields Hence, they attract nearby masses with a strong gravitational force Different planets have different gravitational field strengths This depends on the mass of the planet More massive planets have stronger gravitational field

Answer 18

Weight and mass are different in physics Mass is a measure of how much matter there is in an object Mass has magnitude but not direction Therefore, mass is a scalar quantity Weight is a force Forces have magnitude and direction Therefore, weight is a vector quantity

Answer 19

Weight, mass and gravitational field strength are related using the equation: W = m x g Where: W = weight, measured in newtons (N) m = mass, measured in kilograms (kg) g = gravitational field strength, measured in newtons per kilogram (N/kg) The gravitational field strength on Earth is 10 N/kg g is also used to describe the acceleration of an object in freefall in a gravitational field The acceleration of freefall on Earth is 10 m/s2 These quantities are two ways of describing the thing The weight that an object experiences depends on: The object's mass The mass of the planet attracting it The mass of an object and the weight acting on it are directly proportional If one doubles, the other also doubles If one is halved, the other is also halved The magnitude of the weight force depends on the gravitational field strength

Answer 20

The stopping distance of a car is defined as: The stopping distance of a car is the total distance travelled during the time it takes to stop in an emergency The stopping distance is the sum of the distance travelled as the driver makes the decision to stop plus the distance travelled as the driver applies the brakes

Answer 21

The stopping distance is calculated using the following formula: Stopping distance = Thinking distance + Braking distance

Answer 22

Thinking distance is defined as Thinking distance is the distance travelled in the time it takes the driver to react to an emergency and prepare to stop The main factors affecting thinking distance are: The speed of the car The reaction time of the driver The reaction time is defined as: A measure of how much time passes between seeing something and reacting to it The average reaction time of a human is 0.25 s Reaction time is increased by: Tiredness Distractions (e.g. using a mobile phone) Intoxication (i.e. consumption of alcohol or drugs)

Answer 23

Braking distance is defined as the distance travelled under the braking force in metres (m) For a given braking force, the greater the speed of the vehicle, the greater the stopping distance

Answer 24

For a given braking force, the speed of a vehicle determines the size of the stopping distance The greater the speed of the vehicle, the larger the stopping distance

Answer 25

Terminal velocity is the fastest speed that an object can reach when falling Terminal velocity is reached when the upward and downward acting forces are balanced The resultant force on the object reaches zero The object no longer accelerates and a constant terminal velocity is reached.

Answer 26

Falling objects experience two forces: Weight Air resistance The force of air resistance increases as the object's speed increases This is because the object collides with air particles as it moves through the air The faster the object is travelling, the more collisions it has with the air particles The weight of the object does not change This is because W =mx g The mass, m, of the object does not change The acceleration of freefall, g, does not change

Answer 27

At the instant the skydiver steps out of the plane, the support force of the plane is no longer acting on the skydiver, but they are not yet falling, so the only force exerted them is the weight force There is a downward acting resultant force on the skydiver The resultant force is equal to the weight force The skydiver accelerates downward at maximum acceleration As the skydiver begins to fall, the force of air resistance is very small because the skydiver's speed is small There is a downward acting resultant force on the skydiver The resultant force is equal to the weight force minus the force of air resistance The skydiver accelerates downward but the acceleration decreases As the skydiver accelerates, their speed increases, so the force of air resistance increases There is a downward acting resultant force on the skydiver The resultant force is equal to the weight force minus the force of air resistance The skydiver accelerates downward but the acceleration continues to decrease As the skydiver's acceleration decreases, their speed increases at a slower and slower rate Eventually, the skydiver reaches a speed at which the force of air resistance is equal to the force of weight The forces are balanced, so the resultant force is zero The skydiver no longer accelerates and a constant velocity is reached This is terminal velocity

Answer 28

Experiment 1: investigating force and extension for springs and rubber bands The aim of this experiment is to investigate the relationship between force and extension for a spring and a rubber band: Variables Independent variable = Force, F Dependent variable = Extension, e Equipment

Answer 29

1. Align the marker to a value on the ruler with no mass added, and record this initial length of the spring / rubber band 2. Add the 100 g mass hanger onto the spring / rubber band 3. Record the mass (in kg) and position (in cm) from the ruler now that the spring / rubber band has extended 4. Add another 100 g to the mass hanger 5. Record the new mass and position from the ruler now that the spring / rubber band has extended further 6. Repeat this process until all masses have been added 7. 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

Answer 30

he aim of this experiment is to investigate the relationship between force and extension for a metal wire Variables Independent variable = Force, F Dependent variable = Extension, e

Answer 31

1. Set up the apparatus so the wire is taut with no masses added 2. 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 3. Record the initial length of the wire to the marker 4. Add a 100 g mass onto the mass hanger 5. Read and record the new reading of the tape marker from the meter ruler now that the metal wire has extended 6. Repeat this process until all masses have been added 7. 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

Answer 32

Analysis of results The force, F added to the spring / rubber band / metal wire is the weight of the mass The weight is calculated using the equation: W = m × g Where: W = weight in newtons (N) m = mass in kilograms (kg) g = gravitational field strength on Earth in newtons per kg (N/kg) 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 1. Plot a graph of the force against extension for the spring / rubber band / metal wire 2. Draw a line or curve of best fit 3. If the graph has a linear region (is a straight line), then the force is proportional to the extension Evaluating the experiments 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 Safety considerations Wear goggles during this experiment in case the spring, rubber band or wire snaps Stand up while carrying out the experiment, making sure no feet are directly under the masses Place a mat or a soft material below the masses to prevent any damage in case they fall Use a G clamp to secure the clamp stand to the desk so that the clamp and masses do not fall over As well as this, place each mass carefully on the hanger and do not pull the spring so hard that it breaks or pulls the apparatus over Exam Tip Remember - for the spring and rubber band, the extension measures how much the object has stretched by and can be found by subtracting the original length from each of the subsequent lengths. For the metal wire, each extension is measured by finding the difference between the new marker point and the original reference point. A common mistake is to calculate the increase in length by each time instead of the total extension. If each of your extensions is roughly the same, then you might have made this mistake!

Answer 33

The relationship between the extension of an elastic object and the applied force is defined by Hooke's Law Hooke's Law states that: The extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality Directly proportional means that as the force is increased, the extension increases If the force is doubled, then the extension will double If the force is halved, then the extension will also halve The limit of proportionality is the point beyond which the relationship between force and extension is no longer directly proportional This limit varies according to the material

Answer 34

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

Answer 35

Elastic behaviour is the ability of a material to recover its original shape after the forces causing the deformation have been removed Deformation is a change in the original shape of an object Deformation can be either: elastic inelastic

Answer 36

Elastic deformation is when the object does return to its original shape after the deforming forces are removed Elastic deformation results in a change in the object's shape that is not permanent Examples of materials that undergo elastic deformation are: Rubber bands Fabrics Steel springs

Answer 37

Inelastic deformation is when the object does not return to its original shape after the deforming forces are removed Inelastic deformation results in a change in the object's shape that is permanent Examples of materials that undergo inelastic deformation are: Plastic Clay Glass

Answer 38

Momentum is a property that all moving objects have An object with mass, m, moving at a velocity, v, will have a momentum, p

Answer 39

Momentum is the product of an objects mass and velocity space p space equals space m v Where: p = momentum in kilogram metre per second (kg m/s) m = mass in kilograms (kg) v = velocity in metres per second (m/s) This means that an object at rest (i.e. v = 0) has no momentum Momentum keeps an object moving in the same direction It is difficult to change the direction of an object with a large momentum Velocity is a vector with both magnitude and direction Therefore, the momentum of an object depends on its direction of travel This means momentum can be either positive or negative If an object travelling to the right has positive momentum, then an object travelling in the opposite direction (to the left) will have negative momentum This isn't a solid rule, but in questions, usually the positive direction is to the right and negative to the left

Answer 40

The momentum of an object will change if: it's velocity increases or decreases (acceleration) it's direction changes (acceleration) it's mass changes Exam Tip Remember the units of momentum as kg m/s which is the product of the units of mass (kg) and velocity (m/s). Which direction is taken as positive is completely up to you in the exam. In general, forwards, to the right, and upwards are taken as positive, and backward, to the left, or down are taken as negative.

Answer 41

The principle of conservation of momentum states that: The total momentum before an interaction is equal to the total momentum after an interaction, if no external forces are acting on the objects In this context, an interaction can be either: A collision i.e. where two objects collide with each other An explosion i.e. where a stationary object explodes into two (or more) parts

Answer 42

For a collision between two objects: The total momentum before a collision = the total momentum after a collision In the example below: Before the collision: The momentum is only generated by mass m because it is the only moving object If the right is taken as the positive direction, the total momentum of the system is m × u After the collision: Mass M also now has momentum The velocity of m is now -v (since it is now travelling to the left) and the velocity of M is V The total momentum is now the momentum of M + the momentum of m This is (M × V) + (m × -v) or (M × V) – (m × v) written more simply Since momentum is a vector quantity, a system of objects moving in opposite directions (e.g. towards each other) at the same speed will have an overall momentum of 0 since they will cancel out Momentum is always conserved over time

Answer 43

When a force acts on an object that is moving, or able to move, the object will accelerate (or decelerate) This causes a change in momentum

Answer 44

The resultant force acting on an object is defined by the equation: F = ma Momentum is calculated using the equation: P= mv Change in momentum is given as: triangle p=mv-mu Combining these equations gives: force = change in momentum divided by time f = (mv-mu) divided by t Where: F = resultant force, measured in newtons (N) a = acceleration, measured in metres per second squared (m/s2) m = mass, measured in kilograms (kg) ∆p = change in momentum, measured in kilogram metres per second (kg m/s) v = final velocity, measured in metres per second (m/s) u = initial velocity, measured in metres per second (m/s) t = time, measured in seconds (s) Remember to consider the direction of object's motion If you take the initial direction as positive then the reverse direction is negative Force can also be described as the rate of change of momentum on a body The rate of change describes how a variable changes with respect to time The shorter the time over which momentum changes, the bigger the force So, force and time are inversely proportional to each other Exam Tip When two quantities are inversely proportional, it means that as one increases, the other decreases by a proportional amount If one is doubled, the other is halved If one is decreased by a factor of 4, the other is increased by a factor of 4

Answer 45

Newton's third law of motion can be defined as follows: Whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction Newton's third law explains the forces that enable someone to walk The foot exerts a push force on the ground The ground exerts a push force force on the foot The forces are equal in magnitude and opposite in direction

Answer 46

Force diagrams can be used to represent Newton's third law Use the following three rules to help you identify a third law pair: 1. The two forces in a third law pair act on different objects 2. The two forces in a third law pair always are equal in size but act in opposite directions 3. The two forces are always the same type: weight, reaction force, etc.

Answer 47

According to Newton's Third Law: When two objects collide, both objects will react, generally causing one object to speed up (gain momentum) and the other object to slow down (lose momentum) Consider the collision between two trolleys, A and B: When trolley A exerts a force on trolley B, trolley B will exert an equal force on trolley A in the opposite direction In this case: FB–A = –FA–B While the forces are equal in magnitude and opposite in direction, the accelerations of the objects are not necessarily equal in magnitude From F = ma, acceleration depends upon both force and mass, this means: For objects of equal mass, they will have equal accelerations For objects of unequal mass, they will have unequal accelerations

Answer 48

Since force is equal to the rate of change in momentum, the force of an impact in a vehicle collision can be decreased by increasing the contact time over which the collision occurs The contact time is the time in which the person is in contact with what they have collided with Therefore, safety features are created to reduce the impact of a force, such as in: Vehicles Playgrounds Bicycle helmets Gymnasium crashmats

Answer 49

Vehicle safety features are designed to absorb energy upon an impact by changing shape The main vehicle safety features are crumple zones, seat belts and airbags For a given force upon impact, these absorb the energy from the impact and increase the time over which the force takes place This, in turn, increases the time taken for the change in momentum of the passenger and the vehicle to come to rest The increased time reduces the force and risk of injury on a passenger The usefulness of safety features depends on two main factors: mass and velocity If the impact is from a large mass, for example, a truck travelling very fast and colliding with a wall, the momentum will be very large The change in momentum (ie. from a high speed to rest) will also be very large This means that a very long contact time is needed to reduce the force of impact . Seat belts These are designed to stop a passenger from colliding with the interior of a vehicle by keeping them fixed to their seat in an abrupt stop They are designed to stretch slightly to increase the time for the passenger’s momentum to reach zero and reduce the force on them in a collision Airbags These are deployed at the front on the dashboard and steering wheel when a collision occurs They act as a soft cushion to prevent injury on the passenger when they are thrown forward upon impact Crumple zones These are designed into the exterior of vehicles They are at the front and back and are designed to crush or crumple in a controlled way in a collision This is why vehicles after a collision look more heavily damaged than expected, even for relatively small collisions The crumple zones increase the time over which the vehicle comes to rest, lowering the impact force on the passengers

Answer 50

Crash mats used in gymnasiums help reduce the risk of injury for falls in gymnastics and climbing They are thick and soft to offer shock absorption of the force created by the person landing on the mat When a person lands on a crash mat with a large force, for example, after jumping, the soft landing means their body is in contact with the mat for a longer period of time than if it were otherwise not there This increases the contact time over which their momentum is reduced, creating a smaller impact force and a lower chance of injury In a similar way, playgrounds utilise cushioned surfaces as children will often fall onto these with a large force The cushioned surface reduces the risk of a severe injury by increasing their contact time with the ground Meanwhile, a child in a gymnasium can use a thinner crash mat than an adult due to having a lower mass This is the same for activities where a person/adult will fall with a low velocity such as falling from lower heights Therefore, thin crash mats are suitable for low-impact activities Safety features are intended to reduce the chance of serious injury but do not completely prevent it in all cases

Answer 51

The moment of a force is the turning effect produced when a force is exerted on an object Examples of the rotation caused by the moment of a force are: A child on a see-saw Turning the handle of a spanner A door opening and closing Using a crane to move building supplies Using a screwdriver to open a tin of paint Turning a tap on and off Picking up a wheelbarrow Using scissors Forces can cause the rotation of an object about a fixed pivot This rotation can be clockwise or anticlockwise

Answer 52

The turning effect of a force about a pivot. The size of a moment is defined by the equation: M = F × d Where: M = moment in newton metres (Nm) F = force in newtons (N) d = perpendicular distance of the force to the pivot in metres (m) The forces should be perpendicular to the distance from the pivot For example, on a horizontal beam, the forces which will cause a moment are those directed upwards or downwards Exam Tip The moment of a force is measured in newton metres (N m), but can also be newton centimetres (N cm) if the distance is measured in cm instead. If your IGCSE exam question doesn't ask for a specific unit, always convert the distance into metres

Answer 53

The principle of moments states that: If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot For a balanced object, the moments on both sides of the pivot are equal clockwise moment = anticlockwise moment Clockwise and anticlockwise moments

Answer 54

A light beam is one that can be treated as though it has no mass The supports, therefore, must exert upward forces that balance the downward acting weight of any object placed on the beam

Answer 55

The centre of gravity of an object is defined as: The point through which the weight of an object acts For a symmetrical object of uniform density, the centre of gravity is located at the point of symmetry For example, the centre of gravity of a sphere is at the centre Finding the centre of gravity of symmetrical objects

Answer 56

The centre of gravity of an irregular object can be found using suspension The irregular shape is suspended from a pivot and allowed to settle A plumb line (weighted thread) is then held next to the pivot and a pencil is used to draw a vertical line from the pivot (the centre of mass must be somewhere on this line) The process is then repeated, suspending the shape from two additional points The centre of mass is located at the point where all three lines cross Exam Tip Since the centre of gravity is a hypothetical point, it can lie inside or outside of a body. The centre of gravity will constantly shift depending on the shape of a body. For example, a human body’s centre of gravity is lower when learning forward than when standing upright However, when you are drawing force diagrams, always draw the weight force as if it were acting from the centre of gravity of the object!

Forces and Motion Flashcards

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