# Physics OCR, P4

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How do you calculate the average speed?
speed (m/s) = distance travelled (m) / time taken (s)
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What is instantaneous speed?
the speed of an object at a particular point in time
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What is the displacement of an object at a given moment?
it's net distance from its starting point together with an indication of direction
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What does a steeper gradient on a distance-time graph mean?
a higher (increasing) speed
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What is acceleration?
the change in speed of an object (increasing speed) in a given time interval
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What is the instantaneous velocity of an object?
it's instantaneous speed together with an indication of the direction
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When is the velocity of an object positive and negative?
velocity of an object moving in a straight line is positive if it is moving in one direction and negative if it is moving in the opposite direction
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What is the equation for acceleration?
acceleration (m/s²) = change in velocity (m/s) / time taken (s)
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When does a force arise?
from an interaction between two objects
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What happens when two objects interact?
both experience a force and the two forces from an interaction pair
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What are the two forces in an interaction pair and what do they do?
the two forces are equal in size and opposite in direction and they act on different objects
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What happens in the interaction between two surfaces?
each surface experiences a force in the direction that prevents relative movement. This is FRICTION
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Describe the interaction between an object and a horizontal surface it is resting on.
the object pushes down on the surface, the surface pushes up on the object with an equal force. This is the reaction of the surface
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When does friction and reaction of a surface arise?
in response to the action of an applied force, their size matches the applied force up to a limit
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When does friction occur when a person is walking?
there is a frictional force experienced by the pavement and a frictional force experienced by the sole of the shoe. A person applies a force to the ground and an opposite force is generated, pushing back on the person.
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How do rocket and jet engines produce a driving force?
the engine pushes gas backwards (action) and the gas pushes the rocket forwards (reaction)
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What is the resultant force of an object?
the overall effect of adding or subtracting the individual forces acting on the object, taking their directions into account
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What happens if a resultant force acts on an object?
it causes a change of momentum in the direction of the force
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what is the equation for momentum?
momentum (kg m/s) = mass (kg) x velocity (m/s)
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What is the size of the change of momentum of an object proportional to?
it's proportional to the size of the resultant force acting on the object and to the time for which it acts
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What is the equation for change of momentum?
change of momentum (kg m/s) = resultant force (N) x time for which it acts (s)
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How can the horizontal motion of objects (like cars) be analysed?
they can be analysed in terms of a driving force (produced by engine) and a counter force (due to friction and air resistance)
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What happens when different counter forces are applied to an object moving in a straight line?
if the driving force is: greater than the counter force- vehicle speeds up, equal to counter force- vehicle moves at a constant speed in straight line and smaller than counter force- vehicle will slow down
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What happens in situations involving a change in momentum, such as a collision?
the longer the duration of the impact (more time it takes), the smaller the resultant force
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How can the force of the impact on the human body be reduced during a collision?
by increasing the time of the impact, which is the purpose of road safety devices
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What road safety measures are there?
seat belts, crumple zones, air bags, cycle and motorcycle helmets
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How can the vertical motion of objects (thrown upwards or falling down) be analysed?
they can be analysed in terms of the forces acting (gravity, air resistance)
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What happens if the resultant force on an object is zero?
its momentum does not change. If it is stationary, it stays at rest; if it is already moving, it continues at a constant velocity
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What is the energy of a moving object called?
its kinetic energy
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When does an object's gravitational potential energy increase/decrease?
as an object is raised, its gravitational potential energy increases, and as it falls, its gravitational potential energy decreases
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What is the equation for work done by a force?
work done by a force (J) = force (N) x distance moved in the direction of the force (m)
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What happens when work is done ON an object?
energy is transferred to the object
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What happens when work is done BY an object?
energy is transferred from the object to something else
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What happens when an object is lifted to a higher position above the ground?
work is done by the lifting force, this increases the gravitational potential energy
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What is the equation for change in gravitational potential energy?
change in GPE (J) = weight (N) x vertical height difference (m)
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What happens when a force acting on an object makes its velocity increase?
the force does work on the object and results in an increase in its kinetic energy
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When increases an object's kinetic energy?
the increase in mass of the object and the faster it is moving
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What is the equation for kinetic energy?
kinetic energy (J) = 1/2 x mass (kg) x [velocity]² (m/s)²
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If you ignore drag and friction, what will the increase in kinetic energy be equal to?
the work done by the force
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What will happen to an object's kinetic energy if drag and friction are not ignored?
the object's kinetic energy will be less than the work done by its applied force because some energy is dissipated (lost) through heating
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## Other cards in this set

### Card 2

#### Front

What is instantaneous speed?

#### Back

the speed of an object at a particular point in time

### Card 3

#### Front

What is the displacement of an object at a given moment?

### Card 4

#### Front

What does a steeper gradient on a distance-time graph mean?

### Card 5

#### Front

What is acceleration?