Forces and Motion Flashcards
what is speed?
the distance travelled per unit time
what is it called if the speed of something is changing?
acceleration or deceleration
is the acceleration of free fall near to the Earth constant?
yes
what is distance measured in?
metres (m)
what is time measured in?
seconds (s)
what is speed/velocity measured in?
m/s
what is acceleration measured in?
m/s2
in a displacement-time graph, how do you work out the velocity?
the gradient
in a displacement-time graph, what does negative gradient show?
negative gradient is returning back to the starting point
in a displacement-time graph, what does a horizontal line mean?
it is stationary
in a displacement-time graph, if the displacement is zero what does this show?
it is back at the starting point
in a displacement-time graph, what does a curved line show?
the velocity is changing - it is accelerating/decelerating
in a velocity-time graph, how do you work out acceleration?
the gradient
in a velocity-time graph, what does negative gradient show?
deceleration
in a velocity-time graph, if the speed is zero what does this show?
it is at rest
in a velocity-time graph, what does a horizontal line mean?
it is at constant speed
in a velocity-time graph, how do you work out distance travelled?
the area under the line
in a velocity-time graph, what does a curved line mean?
the acceleration is changing
what is a vector?
a quantity with magnitude and direction
what is a scalar?
a quantity with just a magnitude
what are examples of scalars?
- distance
- speed
- time
- energy
what are examples of vectors?
- displacement
- velocity
- acceleration
- force
what can force change?
the speed, shape or direction of a body
what is force measured in?
Newtons (N)
what is friction?
a force between two surfaces which impedes motion and results in heating e.g. air resistance
how do you find the resultant of two or more forces acting along the same line?
they should be added together if in the same direction and subtracted if in the opposite direction
what does Newton’s first law state?
an object has constant velocity unless acted on by a resultant force
what does Newton’s second law state?
force = mass x acceleration
what does Newton’s third law state?
every action force has an equal and opposite reaction force - for example, the force of the Earth’s gravity on an object is equal and opposite to the force of the object’s gravity on the Earth
what is mass?
a measure of how much matter is in an object
what is mass measured in?
kilograms (kg)
what is weight?
gravitational force (the effect of a gravitational field on a mass)
what is the gravitational field strength on Earth?
10N/Kg
where does the weight of an object act?
through its centre of gravity
what are the steps of motion that happens to a body falling in a uniform gravitational field?
- initially, there is no air resistance and the only force acting on it is weight
- as it falls, it accelerates which increases its speed and hence air resistance
- this causes the resultant force downwards to decrease
- therefore, the acceleration decreases, so it is not speeding up as quickly
- eventually they are equal and opposite and balance so there is no resultant force
- so, there is no acceleration and terminal velocity is reached
what is thinking distance?
the distance travelled in the time between the driver realising they need to brake and actually pressing the brakes
what are factors which increase thinking distance?
- greater speed
- slower reaction time due to alcohol, tiredness or distractions
how does caffeine affect reaction time?
reduces thinking distance
what is the braking distance?
the distance travelled in the time between pressing the brakes and the vehicle coming to a stop
what are factors which increase the stopping distance
- greater speed or mass
- poor road conditions e.g. icy or wet
- car conditions e.g. worn tires, worn brake pads
what is the stopping distance?
the sum of the thinking distance and breaking distance
what is elastic deformation?
when the object returns to its original shape when the load has been removed
what does Hooke’s law state
for a spring, force applied to the spring equals the spring constant times the extension
what does a linear force-extension graph show?
- elastic deformation following Hooke’s law
- the point it stops being linear is called the limit of proportionality
- from then on it does not obey Hooke’s law
- the gradient is the spring constant
what does a non-linear force-extension graph show?
- deformation not following Hooke’s law
- after this region, it will fracture
what is the moment of a force?
a measure of its turning effect
what is moment of a force measured in?
Newton metres (Nm)
when is an object in equilibrium?
when the sum of clockwise moments equals the sum of anticlockwise moments (the principle of moments) and there is no resultant force
how does the force change at the supports underneath a horizontal beam?
- for a horizontal beam supported at its ends, the upwards forces at the supports change with the position of a heavy object placed on the beam
- the nearer the heavy object to a given support, the greater the force at that support
what is the momentum of an object?
the product of its mass and velocity
what is momentum measured in?
kilogram metres per second (kgm/s)
what is the force exerted on an object equal to?
its change in momentum over time
how do safety features in cars work?
by increasing the time taken for the people in the car to come to rest (there is the same change in momentum in a longer time, so the force is reduced) for example, a seatbelt achieves this by stretching
what is the principle of the conservation of momentum?
in a collision, the total momentum before is equal to the total momentum afterwards, known as the principle of the conservation of momentum
