T5 forces and motion Flashcards
does a distance quantity require a specific direction
no specific direction required so it is a scalar quantity
typical value for the speed of sound
330 m/s
typical value for human walking speed
1.5 m/s
typical value for human running speed
3 m/s
typical value for human cycling speed
6 m/s
equation linking distance, speed and time
distance = speed x time
distance (m), speed (m/s), time (s)
why can an object travelling at a constant speed in a circle not have a constant velocity
- speed is a scalar quantity
- velocity is a vector meaning, it can only be constant if the direction is constant
- in a circular motion, the direction is continuously changing
how can speed be calculated from a distance-time graph
speed is equal to the gradient of the graph
what must be done to calculate speed a given time from a distance-time graph for an accelerating object
- drawing a tangent to the curve at the required time
- calculating the gradient of the tangent
equation for the average acceleration of an object
acceleration = (change in velocity) / (time taken)
Acceleration (m/s²), Velocity (m/s), Time (s)
how can the distance travelled by an object be calculated from a velocity-time graph
equal to the area under the graph
give an approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface
9.8 m/s²
what can said about the resultant force acting on an object when it is falling at terminal velocity
- resultant force is zero
- when at terminal velocity, the object is moving at a constant speed and so isn’t accelerating
state Newton’s first law for a stationary object
if the resultant force on a stationary object is zero, the object will remain at rest
state Newton’s first law for a moving object
if the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction)
what can be said about the braking forces and driving forces when a car is travelling at constant velocity
the braking forces are equal to the driving forces
if an object changes direction but remains at a constant speed, is there a resultant force
since there is a change in direction, there is a change in velocity and so there must be a resultant force
what is inertia
tendency of an object to continue in its state of rest or uniform motion
state the defining equation for Newton’s second law
resultant force = mass x acceleration
F = ma
state Newton’s second law in words
an object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass
what is inertial mass
- measure of how difficult it is to change a given object’s velocity
- ratio of force over acceleration
State Newton’s third law
whenever two objects interact, the forces that they exert on each other are always equal and opposite
what is the stopping distance of a vehicle equal to
the sum of thinking distance and braking distance
for a given braking distance, if the vehicle’s speed is increased, what can be said about its stopping distance
the stopping distance is increased with an increase in speed
give a typical range of values for human reaction time
0.2 - 0.9 s
3 factors which can affect a driver’s reaction time
- tiredness
- drugs
- alcohol
2 factors which may affect braking distance
- adverse road conditions
2. poor tyre/brake conditions
describe the energy transfers that take place when a car applies its brakes
- work is done by the friction force between the brakes and wheel
- kinetic energy of the wheel is converted to heat and is dissipated to the surroundings through the brake discs
to stop a car in a given distance, if its velocity is increased, what must happen to the braking force applied
the braking force must also be increased
2 consequences of a vehicle undergoing very large develerations
- KE converted to heat is very high causing brakes to overheat
- loss of control of the vehicle
