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 drawn?
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 examples of vector quantities
- Velocity
- Displacement
- Force
- Acceleration
- Weight
- Momentum
Give examples of scalar quantities
- Temperature
- Time
- Mass
- Speed
- Distance
- Energy
What is velocity?
The speed of an object in a specific direction.
Give an equation relating average 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 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.
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 line represent?
The acceleration.
State a typical value for the speed of sound
340 m/s
What is a typical value for human walking speed?
1.4 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
Give an appropriate value for the acceleration of an object in free fall under gravity near the Earth’s surface.
10 m/s²
What is displacement?
A vector quantity which measures the distance and the direction in a straight line from an object’s starting point to its finishing point.
What is acceleration?
The change in velocity in a certain amount of time.
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.
What is Newton’s First Law for a stationary object?
If the resultant force on a stationary object is zero, the object doesn’t move.
What is 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
What is 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 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 or newton-meter.
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.
- Therefore velocity is constantly 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 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 two objects interact, the forces that they exert on each other are always equal and opposite.
State the equation used to calculate an object’s momentum.
Momentum = mass x velocity
What is the unit used for momentum?
kgm/s
kilogram metres per second
What is conservation of momentum?
In a closed system, the total momentum before and after are equal.
How can you measure human reaction times?
- Using the ruler drop test.
- Person A and B hold each end of 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.
In the ruler drop test, why is it important 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.
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
- Distractions (such as noise)
Factors which may increase braking distance.
- Faster vehicle speed
- Poor, wet, or icy road surface conditions
- Damaged or worn brakes or tyres conditions.
Factors which may increase thinking distance.
- Faster vehicle speed
- Long driver reaction times
What is thinking distance?
How far a vehicle moves during driver’s reaction time.
What is braking distance?
The distance taken to stop whilst brakes are applied.
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, causing it to skid.
