P9. Motion Flashcards
Does a distance quantity require a specific direction? i.e. Is it a scalar or vector quantity
No specific direction is required so it is a scalar quantity
If an object moves 3 metres to the left and then 3 metres back to its initial position, what is the object’s total displacement?
The object has zero displacement
Displacement is a vector quantity so it also involves direction
The object starts and ends at the same point
State a typical value for the speed of the sound
330m/s
State the equation linking distance, speed and time
Distance = Speed × time
Why can an object travelling at a constant speed in a circle do not have a constant velocity
Speed is a scalar quantity
Velocity is a vector quantity which means it can only ne constant if the direction is constant
In circular motion, the direction is continuously charging
How can speed be calculated from a distance-time graph?
The speed is equal to the gradient of the graph
What must be done to calculate speed at 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
State the equation for the average acceleration of an object. Give appropriate units
Acceleration = Change in velocity/time taken
How can the distance travelled by an object be calculated from a velocity-time graph?
It is equal to the area under the graph
Give an approximate value for the acceleration of an object in free fall under the gravity near the Earth’s surface
9.8m/s²
What can be said about the resultant force acting on an object it is falling at terminal velocity?
The 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?
The 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 × 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?
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 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 seconds - 0.9 seconds
Give three factors which can affect a driver’s reaction time
- Tiredness
- Drugs
- Alcohol
Give two factors which may affect braking distance
- Adverse (wet/icy) road conditions
- 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
State 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 the vehicle
State the equation used to calculate an object’s momentum
Momentum = Mass × Velocity
What is the unit used for momentum?
kg m/s
In a closed system, what can be said about the momentum before and after a collision?
The total momentum before is equal to the total momentum afterwards
State an equation linking change in momentum, force and time?
Force × Time = Change in Momentum
What quantity is equal to the force experienced in a collision?
The rate of change of momentum
If an object’s change of momentum is fixed, what is the only way tk reduce the force that the object experiences?
Increase the length of time over which the change of momentum occurs
Explain a seatbelt improves a passenger’s safety during a collision
Passenger must decelerate from the vehicle’s velocity at impact to zero, meaning they undergo a fixed change of momentum
The force they experience is equal to the rate of change of momentum
Seatbelts increase the time over which force is applied, reducing the rate experienced
