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Flashcards in Physics 4: Explaining Motion Deck (54)
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What is the equation for speed

Speed (m/s) = distance travelled (m)/ Time taken (s)


Average speed

Distance travelled divided by the time taken


What is speed measured in

Metres per second (m/s) or in kilometres per hour (km/h)


Instantaneous speed

The speed at a particular moment in time


What is the displacement of an object

1. the displacement of an object is the distance from its start point in a straight line. When you run once all the way around a running track, the distance you run is 400m, but your displacement is zero
2. The displacement is expressed as a distance with a direction
3. Displacement is a vector quantity - it has a size (magnitude) and a direction


What is the velocity of an object

The velocity of an object is its speed in a certain direction


How doe we calculate the average velocity

Average velocity (m/s) = displacement (m) / time taken (s)


How do distance time graphs show a journey

1. A distance- time graph can be used to visualise a journey
2. The time for the journey is plotted on the x-axis (horizontal)
3. The distance travelled is plotted on the y-axis(vertical)


What does the speed of an object tell us

How far it will travel in a certain time


How does the gradient of the line relate to the speed

1. The gradient if the line is equal to the speed
2. The steeper the gradient of a line, the faster it is moving


What does a curved line mean

1. A curved line means that the speed is changing
2. If the curve is getting steeper it means that the vehicles speed is increasing


Why do displacement-time graphs show

1. Return journeys can be visualised on a displacement- time graph
2. When the vehicle has returned to its displacement will be zero
3. The displacement would be negative of the vehicle travelled behind its starting point



The rate at which the velocity of an object changes


What is acceleration measures in



What is the equation for acceleration

Acceleration (m/s²) = change in speed (m/s) /time taken (s)


What is deceleration or retardation

1. When an object slows down it has a negative acceleration, sometimes called deceleration or retardation
2. There has to be a net force acting on an object to cause acceleration (or declaration)
3. When the net or overall force is zero, the acceleration is zero


What does a speed-time graph show

1. A speed-time graph is used to show the changes in speed during a journey
2. A horizontal line means that the speed of an object is constant- it is at a steady speed
3. The steeper the line, the greater the size of acceleration
4. The instantaneous speed of a vehicle in a certain direction is its instantaneous velocity


What does a velocity- time graph show

1. A velocity- time graph shows the direction in which an object is travelling
2. A positive velocity means that the object travels in a certain direction and a negative velocity means that the object is travelling in the opposite direction



The push or pull tags acts between two objects. They always act in pairs


Repulsive force and attractive force

A repulsive force pushes objects apart; an attractive force pulls objects towards each other


What at ethene properties of force

1. The size of a force is always the same on both objects
2. The force on each object acts in the opposite direction to each other e.g. when a ship floats on water, the up thrust (buoyancy) pushes up on the ship and the ship pushes done on the water. This provides a repulsive force pair
3. When you stand you push downwards on the floor. This force is equal to your weight


What is the reaction force

1. At the same time, the floor exerts an upwards force on you- called the reaction force (or the reaction of the surface). Is stops gravity from pulling you through the floor
2. The reaction force is equal and opposite to your weight, so you are not accelerating up or down. The reaction force balances your weight.
3. If you jump upwards, you need to push harder on the floor - the reaction force increases. Now the forces are unbalanced and you will accelerate upwards


Forces are vector quantities

1. Forces are vector quantities, i.e. they have a magnitude (size) and a direction
2. A force is represented by an arrow on a diagram
3. On a scale diagram, the length of the arrow represents the magnitude and the direction of the arrow shows which way the force acts


What is friction

Friction is a force which acts between two surfaces. As the two surfaces slide over one another m, friction acts to oppose the motion


How does size affect friction

The size of the friction force depends on:
1. The roughness of the surfaces (rougher surfaces give more friction)
2. How hard the surfaces are pushed together (the heavier the object the more friction)


What happens as you try to move an object along a surface

1. as you try to move an object along a surface, friction will be equal to the applied force and acts in the opposite direction, so the object will not move
2. As you increase the applied force, the friction


Limiting friction

Eventually the friction teaches a maximum value and the object will start to move


How does heat energy effect kinetic energy

1. The kinetic energy of the moving object is transferred to heat energy in both surfaces
2. Lubrication (oil) is used to reduce the friction between moving parts of machinery to stop them getting too hot wearing out


How does friction enable you to move

1. When you walk your feet push against the friction on the floor, so the floor pushes your foot forwards
2. The resultant force (overall force) acting between your foot and the floor is a combination of friction and the reaction of the surface
3. These forces combine to push the foot in a diagonal line upwards direction


What happens when the resultant force is zero

1. when the resultant force is zero, the foxes are balanced and the acceleration is zero.
2. The object will carry on moving in a straight line at a constant speed


What happens in a frictionless space

In a frictionless space, once an object starts moving it should keep moving at the same speed


What happens when the forces are unbalanced

When the forces are unbalanced, there is a net force on the object and it will speed up, slow down or change direction


The earth's terminal velocity

1. Objects falling accelerate towards the ground at 9.8 m/s², due to gravity
2. The force of gravity always acts towards the centre of the earth
3. The atmosphere created an upwards force that slows down falling objects. This is known as air resistance or drag


How does drag affect a falling object

1. Drag acts in the opposite direction to the speed (or velocity) of the object
2. Drag force increases as the speed of the object increases
3. The larger the surface area of the object, larger the drag force


Terminal velocity

The maximum speed achieved by any object falling through a gas or liquid


Give an example of terminal velocity

With vehicles, with friction and drag acting in the opposite direction to the driving force


Describe the forces exerted during a collision

Large forces are exerted during collisions. This size of the force depends on:
1. The mass of the object (the heavier the object, the larger the force)
2. The speed (or velocity) of the object (the faster the object, the larger the force)
3. The duration of the impact (the longer the time to stop, the lower the force)


How do cars reduce collision

Most car safety devices, e.g. air bags, crumple zones, seat belts, crash helmets, are designed to increase impact time, thus reducing the force in a collision


Momentum equation

Momentum (kg m/s) = mass (kg) x velocity (m/s)


How does the reluctant force affect the momentum

A resultant force will change an objects momentum. The larger the force exerted, the larger the change in momentum


How do we calculate the force

Force is equal to the rate in change in momentum

Force (N) = change in momentum (kg m/s) / time taken (s)



The energy used by the movement of a force us known as the. They are both measure in joules (J)


How do we calculate work done

Work done (J) = force (N) x distance moved in the direction of the force (m)


How does our bodies transfer work energy

1. All forms of energy have potential to do work
2. Energy from food is transferred in our bodies so we can do exercise
3. Not all energy is transferred as work- some is always dissipated as heat


Gravitational potential potential energy

The energy an object gains due to its height


How does the gravitational potential energy relate to its height

As the object is raised, it's gravitational potential increases. As an object falls, it's gravitational potential energy decreases


Describe potential energy

1. When you lift an object, you'd do woke against gravity
2. 1 joule of work will lift a weight of 1 Newton a distance of 1 metre
3. The work is transferred to gravitational potential energy (GPE) of the object.


How do we calculate the change in gravitational potential energy

Change in gravitational potential energy (J) = weight (N) x vertical height difference (m)


How do we find the gain in potential energy

gain in potential energy = Weight x height


Kinetic energy

The energy an object has due to motion


How do we find me kinetic energy

Kinetic energy (J) = ½ × mass (kg) × [velocity]² ([m/s]²


Describe the energy transfer of a roller coaster

1. As a roller coaster travels round it's track, going up and down, it's energy changes from kinetic energy to gravitational potential energy
2. It's total energy at any time is the sum of its Kinetic energy and its potential gravitational energy


Conservation of energy

When energy cannot be created or destroyed. When there there should no resistant forces the total energy remains constant. This is known as the principle of conservation of energy


How is energy transferred in general

1. Energy can be neither created nor destroyed; it can only transfer between object or change its form
2. Usually some energy is used up doing work against friction and air resistance - this means that some energy is dissipated as heat
3. Ignoring energy transferred due to friction and air resistance: loss in gravitational potential energy = gain in kinetic energy