Topic 2 - Motion And Forces Flashcards
What is a scalar quantity?
- A quantity that only has a magnitude
- A quantity that isn’t direction dependent
What is a vector quantity ?
A quantity that has both a magnitude and an associated direction.
How can a vector quantity be represented ?
Using vector arrows
How do vector arrows represent vectors?
- The length of the arrow represents the magnitude
- The arrow points in the associated direction
Give 3 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
Give an equation linking speed , distance and time
Average speed = distance / time
On a distance time graph , what value does the gradient of the line represent ?
The speed
On a displacement / time graph, what does the gradient of the line represent ?
The velocity
How do you 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
State the equation for the average acceleration of an object. Give appropriate units
Acceleration = change in velocity / time taken
a = m/s^2. V = m/s. t = s
Give an equation linking final velocity with initial velocity , displacement and acceleration
V^2 = u^2 + 2as
How can the distance travelled by an 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.
State a typical value for the speed of sound
330 m/s
What is a typical value for human walking speed?
1.5 m/s
What is a typical value for human running speed?
3m/s
What is a typical value for human cycling speed?
6m/s
Give an approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface
10 m/s ^2
What is the name given to the single force that is equivalent to all the other forces acting on a given object?
The resultant force
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. )
State the defining equation for Newton’s second law
Resultant force = Mass x Acceleration
F=ma
State Newton’s second law
An object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass
What is weight ?
The force that acts on an object due to gravity and the object’s mass
What quantities does weight depend on ?
Weight = mass x gravitational field strength
- the object’s mass
- the gravitational field strength at the given position in the field
What is the unit used for weight?
Newton N
What is the unit used for gravitational field strength ?
N/kg
What piece of equipment can be used to measure an object’s weight?
A calibrated spring balance
A newton metre
When can an object travelling at a constant speed not have a constant velocity?
When the object is changing direction
E.g 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 constantly changing
What is the resultant force that acts on an object moving in a circle called, and I’m which direction does it act?
It is called the centripetal force and acts towards the centre of the circle
What is inertial mass ?
- a measure of how difficult it is to change a given object’s velocity
- the ratio of force over acceleration
State Newton’s third law
Whenever 2 objects interact, the forces that they exert on each other are always equal and opposite
Equation for momentum
Momentum = Mass x velocity
What unit is used for momentum ?
kgm / s
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 x time = change in momentum
How can you measure human reaction times?
Ruler drop test
- person A and B hold a ruler with the 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
What is the stopping distance of a vehicle equal to?
Thinking distance + braking distance
For a given braking distance, if the vehicles 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 times
0.2-0.9 s
3 factors which can affect reaction time
Tiredness
Drugs
Alcohol
2 factors which may affect braking distance
Adverse ( wet/icy) road conditions
Poor tyre or 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 it’s velocity is increased, what must happen to the braking force applied?
The braking force must also be increased
Two consequences of a vehicle undergoing very large decelerations
- kinetic energy converted to heat is very high causing brakes to overheat
- loss of control of vehicle
