Topic 5d - Momentum and Car Safety Flashcards
What is momentum?
Momentum = mass x velocity
What is the equation for momentum?
p = mv
What is the unit for momentum?
kg m/s
OR
Ns
What kind of quantity is momentum?
A vector quantity => direction is important. (usually left and right direction).
What is impulse?
The change in momentum of an object.
What is the unit for impulse / change in momentum?
kg m/s
OR
Ns
What is the equation for impulse?
change in p = m (v-u)
What must you ensure to do when calculating an impulse problem?
State the directions you are choosing to use (which way you put the + and -)
What is a force in relation to momentum?
A force is the change in momentum per unit time/ the rate of change of momentum.
What is the equation for Force in relation to momentum?
F = change in p / t
OR
change in P = F t
What kind of force is the force used in F = change in p / t?
An average force.
What is the principle of conservation of momentum?
Total final momentum of the system (after) = Total initial momentum of the system (before)
Provided there are no external forces acting.
In what 2 situations is the principle of conservation of momentum applicable?
- Collisions (when objects join)
- Explosions (when objects move apart)
How can propulsion be explained using the example of a balloon? (4)
- Air filled balloon = before. The total momentum of the air + balloon = 0.
- When clamp released, air is ejected at high speed from the neck of the balloon.
- Air at high speed has momentum to the left, so the balloon has the same momentum to the right.
- Releasing the air makes the balloon move at speed in opposite direction to the air due to the principle of conservation of momentum.
What is true about all collisions?
They conserve momentum.
What do the types of collisions depend on?
The total kinetic energy.
What does it mean if a collision is elastic?
Total kinetic energy of the system before = Total kinetic energy of the system after.
What does it mean if a collision is inelastic?
Total KE before =/ (NOT) Total KE after
What happens to objects in inelastic collisions?
One or more of the objects get deformed.
What is the definition of stopping distance?
The total distance travelled during the time it takes for a car to stop in response to some emergency.
What is the equation for stopping distance?
Stopping distance = Thinking distance + Braking distance
What is an emergency stop?
Where maximum force is applied by the brakes to stop car in shortest possible distance.
What is thinking distance?
The distance travelled by a car from when the driver realises they need to break, to when they apply the brakes.
What is braking distance?
The distance travelled under the braking force.
How would you describe the relation / proportionality between braking force, speed and stopping distance?
For a given braking force, the greater the speed of the vehicle, the greater the stopping distance.
What is the definition of reaction time?
A measure of how much time passes between seeing something and reacting to it.
What is the human reaction time for an alert person?
Usually in range of 0.2 - 0.9 seconds
What are the 5 steps to the reaction time test with a ruler?
- Person A holds a 30cm ruler vertically, such that the bottom end of the ruler hovers over the top of the hand of Person B.
- Person A should release the ruler unexpectedly.
- As soon as Person B sees the ruler move, they should close their hand, catching it.
- The ruler is marked at the point which it was caught by Person B => in line with top of hand.
- This gives a measurement of the distance the ruler feel => greater distance, longer reaction time.
What is the equation for reaction distance?
Reaction distance = Speed of the car x Driver’s reaction time
What 2 things affect the thinking distance?
- Speed
- Reaction time
What 5 factors is braking distance affected by?
- Speed
- Condition of brakes
- Condition of tyres => minimum tread of depth 1.6 mm
- How good the grip is
- Load of the vehicle (mass/ weight)
How do tread patterns help tyres in wet conditions?
In wet conditions tread patterns help to stop water getting trapped between the tyres and the road => provides a channel through which the water can ‘escape’.
What will happen if a vehicle has too little tread?
Tyres may lose contact with the ground, causing vehicle to slide.
What are 4 factors that affect reaction time?
- Distractions
- State of mind => intoxication
- Tiredness
- Age (not best one)
How does a car slow down when brakes are applied? (4)
- When brakes applied, there is a frictional force between brakes and wheels of car.
- This frictional force does work on brakes => transfers energy from car to brakes.
- So KE of car decreases, and thermal energy of brakes increases.
- Means car decelerates.
What could a large deceleration lead to?
To the brakes overheating and / or loss of control of the vehicle.
What is the equation for work done by the brakes?
Braking force x Breaking distance = 1/2 x Mass x Velocity squared
