5.6 Forces and motion Flashcards
Does a distance quantity require a specific direction?
No specific direction is required as it is a scalar quantity
If an object moves 3 meters to the left, and then 3 meters back to its initial position, what is the object’s total displacement?
The object has zero displacement. It 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 sound
330 m/s
What is a typical value for a human walking speed?
1.5 m/s
What is a typical value for human running speed?
3 m/s
What is a typical value for human cycling speed?
6 m/s
State the equation linking distance, speed and time. Give appropriate units.
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? (Higher)
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 constantly changing.
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 on a distance-time graph for an accelerating object? (Higher)
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
Acceleration (m/s^2), Velocity (m/s), Time (s)
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 gravity near the Earth’s surface
9.8 m/s^2
What can be said about the resultant force acting on an object when it is falling at terminal velocity?
The resultant force is zero. When at terminal velocity, the object is moving at constant speed - it’s not 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 on a moving object is zero, the object will remain at constant velocity (same speed in the 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? (Higher)
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 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 is inversely proportional to its mass
What is inertial mass? (Higher)
A measure of how difficult it is to change a given object’s velocity. The ratio of force over acceleration.
What is the symbol used to represent an approximate value?
~
State Newton’s third law
Whenever two objects interact, the forces that they exert on each other are equal and opposite.
What is the stopping distance of a vehicle equal to?
The sum of thinking distance and braking distance.
For a given 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 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. The kinetic energy of the wheel is converted to heat and dissipated to the surroundings through the brake discs.
To stop a car in any given distance, if it’s 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
