Physics - Forces Part 2 Flashcards
Forces Part 2
What are typical values for the
speed of walking, running and
cycling?
Walking 1.5m/s
Running 3 m/s
Cycling 6 m/s
What is a typical value for the
speed of sound in air?
330m/s
What is non-uniform motion?
Non uniform motion is motion in which the speed is not constant.
How do we calculate average
speed for non-uniform motion?
Average Speed can be calculated using the formula, average speed = total distance / total time
What is the difference between
speed and velocity?
Speed is a scalar quantity so only has a magnitude, velocity is a vector quantity so has a magnitude and
direction.
What is the difference between
distance and displacement?
Distance is the length of the total journey and the displacement is the straight
line distance between the start and end points of a journey.
Distance is a scalar but displacement is a vector so must be quoted with a direction, usually given as an angle to the horizontal or vertical directions.
How do you calculate average
velocity?
Average velocity = displacement/time
v = s/t
Velocity (meters per second, m/s), Displacement (metres, m), Time (seconds, s)
What is acceleration?
Acceleration is the rate at which an object’s velocity changes.
The unit of acceleration is m/s2
(metres per second per second, metres per second squared).
How do you calculate
acceleration?
Acceleration = change in velocity/time
a = (v-u)/t
Acceleration (meters per second squared, m/s2), Change in velocity (metres per second, m/s), Time (seconds, s)
How can something be moving
at a constant speed but
changing velocity at the same
time?
An object moving around a circular path (e.g. the Earth moving around the sun) can be moving at a steady speed (i.e. covering the same distance each second) but will not have constant velocity because velocity is a vector quantity and the direction of the journey is constantly changing. If velocity changes then the object is also accelerating since acceleration = change in velocity / time.
What does a distance time
graph tell us?
A straight line tells us the object has a constant speed. The gradient of a line on a distance time graph tells us the speed. The steeper the line is on a distance time graph the faster an object is moving. A horizontal line on a distance time graph tells us the object is stationary. A line with an increasing gradient tells us the object is accelerating, and a line with a decreasing gradient tells us it is decelerating.
How do you determine the
speed of an object undergoing
uniform motion over a period of time?
Find the gradient of the line in the period of time you are interested in.
How do you determine the
speed of an object undergoing
non-uniform motion at a
particular point in time?
Draw a tangent to the curve at the time you are interested in then determine the gradient of the tangent.
What does a velocity time graph
tell us?
A straight sloping line tells us that the object is moving with constant acceleration. The steeper the line on a speed time graph the larger the object’s acceleration. A horizontal line on a velocity-time graph tells us the
object is moving at a constant speed. Calculating the gradient of a line tells us the acceleration.
What if the acceleration is
constant?
If the acceleration is constant (straight line), then v2 - u2 = 2as (where s is the displacement) can be used.
How do you determine distance
travelled from a speed time
graph?
Distance travelled can be obtained by determining the area under a speed- time graph.
If the line is not a straight one then the area can be determined by counting squares.
What is Newton’s first law?
Newton’s first law tells us that if no resultant force acts on an object then that object’s motion will not change; it will either remain stationary, or if it is already moving, then it will carry on moving at a steady speed. If there is a resultant force acting on an object then the object’s motion will change: it will accelerate.
What is Newton’s second law?
The acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to the mass.
Force = mass x acceleration
F = ma
Force (Newtons, N), Mass (kilograms, kg), Acceleration (metres per second squared, m/s2)
What is inertial mass?
Inertial mass is a measure of how difficult it is to change an object’s velocity. It is defined as the resultant force acting on an object divided by the acceleration that force generates.
How can Newton’s second law
be verified experimentally?
What happens to the forces and
acceleration of a falling object?
An object falling under only the influence of gravity is said to be in freefall. The object will accelerate at g = 9.8 m/s2 (the acceleration due to gravity on Earth)
In reality, as an object falls through the air (a fluid) air resistance it experiences increases (because it is
colliding with more fluid particles in a certain period of time). As a result, the resultant force on the object decreases and therefore its acceleration decreases. Its speed continues to increase, but by smaller amounts until eventually the resultant force on the object is
zero and the object stops accelerating. It will be travelling at a constant velocity called the terminal velocity.
What is stopping distance?
Stopping distance is the total distance required to stop a moving vehicle.
Stopping distance = thinking distance + braking distance
Thinking distance: the distance travelled whilst reacting to a hazard.
Stopping distance: the distance travelled whilst the brake pedal is pressed.
What factors affect thinking distance?
Speed (the faster you are going the further you will travel whilst thinking about braking)
Drugs/Alcohol (these increase your reaction time and you travel further whilst reacting)
Tiredness/illness (these increase your reaction time and you travel further)
Distractions e.g. mobile phones (these increase your reaction time and you travel further)
What factors affect braking distance?
Speed (the faster you are going the further you will travel whilst braking because the brakes need to do more
work to reduce the greater amount of energy in the kinetic store to zero)
Condition of the road surface e.g. ice/rain (less friction means it will take the brakes longer to do the work,
increasing the braking distance)
Condition of brakes/tyres (brakes/tyres in poor condition will apply less friction so work will be done over a
longer distance)
