Topic 2 - Forces And Motion Flashcards
Scalar vs Vector
- A scalar quantity only has a magnitude
- A vector quantity has both a magnitude and a direction
How do vector arrows represent vectors?
- The length of the arrow represents magnitude
- The arrow points in the associated direction
Examples of vector quantities
- Velocity
- Displacement
- Force
6 examples of scalar quantities
- Temperature
- Time
- Mass
- Speed
- Distance
- Energy
Definition of velocity
The speed of an object in a specific direction
What does the gradient represent on a distance-time graph?
The speed.
On a displacement-time graph, what does the gradient represent?
The velocity.
How to calculate the speed at a given time for 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?
- Calculate the area under the graph
On a velocity-time graph, what does the gradient represent?
The acceleration.
Speed of sound
330 m/s
Human walking speed
1.5 m/s
Human running speed
3 m/s
Human cycling speed
6 m/s
Gravity strength on Earth
10 m/s^2
Define resultant force:
- The name given to the single force that is equivalent to all the other forces acting on a given object
Newton’s First Law for a stationary object:
If the resultant force on a stationary object is zero, the object will remain at rest.
Newton’s first law for a moving object
- If the resultant force on a moving object is zero, the object will remain at a constant velocity (same speed in the same direction).
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.
Unit for gravitational field strength
N/kg
What piece of equipment can measure an object’s weight?
- A calibrated spring-balance
- A newton meter
When can an object 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 scalar
- 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
- The velocity is therefore constantly changing
The resultant force that acts on an object moving in a circle and its direction
- Centripetal force
- 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.
Unit 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 and after are equal.
How to measure human reaction times:
- Using a ruler drop test, where:
- 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.
Why does the 0cm mark need to be at the bottom in the ruler drop test?
- 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 the thinking distance and the 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 increases
Range of values of human reaction time
0.2 to 0.9 s
Factors affecting a driver’s reaction time
- Tiredness
- Drugs
- Alcohol
Factors affecting braking distance
- Adverse (wet/icy) road conditions
- Poor tyre/brake conditions
Describe the energy transfers that take place when a car applies its brakes (4 marks)
- Work is done by the friction force between the brakes and the wheel.
- Kinetic energy of the wheel is:
- Converted to heat
- Dissipated to the surroundings through the brake discs
To stop a car in a given distance, if its velocity is increased, what happens to the braking force applied?
It also increases.
Two consequences of a vehicle undergoing very large decelerations:
- The amount of kinetic energy converted to heat is very high causing breakers to overheat.
- Loss of control of the vehicle
