Topic 2 - Motion And Forces Flashcards
What is a scalar quantity?
A quantity that only has magnitude but no direction.
What is a vector quantity?
A quantity that has both a magnitude and an associated direction
Three examples of vector quantities
Velocity
Displacement
Force
Give examples of scalar quantities
Temperature Mass Time Speed Distance Energy
What is velocity
The speed of an object in a specific direction.
Equation relating average speed, distance and time
Average speed = distance / time
On distance/time graph, what value does the gradient of the line represent?
The speed
On a displacement/time graph, what value does the gradient of the line represent?
The velocity
What must be done to calculate speed at a given time from a distance-time graph for an accelerating object?
Draw a tangent to the curve at the required time
Calculate the gradient of the tangent
Equation for the average acceleration of an object
Acceleration = (change in velocity)/(Time taken)
A = (v-u)/t
Equation final velocity with initial velocity, distance and acceleration
V^2 - U^2 = 2 x a x x
How can the distance travelled by object be calculated from a velocity-time graph?
It is equal to the area under the graph
On a velocity/time graph what does the gradient of the graph represent?
The acceleration
Typical value of the speed of sound
330m/s
Typical value for human walking speed
1.5m/s
What is a typical value human running speed?
3m/s
What is a typical value for human cycling?
6m/s
What is weight?
The force that acts on an object due to gravity and the objects mass.
Weight unit
N (newton)
Gfs unit
N/kg
When can an object travelling at a constant speed not have a constant velocity?
When the object is changing direction; for example, moving in a circle
Why does an object travelling at a constant speed in a circle not have a constant velocity?
Speed is a scalar quantity
Velocity is a vector quantity which means it can only be constant if the direction is constant.
In circular motion, the direction is continuously changing
Velocity is constant changing
What is the resultant force that acts on an object moving in a circle called, and in which direction does it act?
It is called the centripetal force and acts towards the center of the circle
What is inertial mass?
A measure how difficult it is to change a given object’s velocity
The 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.
Equation to calculate object s momentum
Momentum = mass x velocity P = m x v
Unit used for momentum
Kgm/s
Kilogram meters per seconds
In a closed system, what can be said about the momentum before and after a collision?
The total momentum before and after are equal
Equation linking change in momentum, force and time
Force = change in momentum/time (Mv-mu)/t = F
How can you measure human reaction times?
Using the ruler drop test
Person A and B hold each end of a ruler with 0cm mark at the bottom
Person A drops the ruler without telling person B
Person B catches it
The distance travelled corresponds to their reaction time.
Why is it impossible that the 0cm mark is at the bottom?
So you can obtain the distance directly without having to calculate it; otherwise a zero error would need to be accounted for.
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
Typical range of values for human reaction time
0.2 - 0.9 seconds
Give three factors which can affect a drivers reaction time
Tiredness
Drugs
Alcohol
Give two factors which may affect braking distance
Adverse (wet/icy) road conditions
Poor tyre/brake conditions
Describe energy transfers that take place when a car applies its brakes
Work is done by 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 given distance, if it’s velocity is increased, what must happen to braking force applied?
The braking force must also be increased
State two consequence of a vehicle undergoing very large decelerations
KE converted to heat is very high causing brakes to overheat
Loss of control of the vehicle
