P10,P11 Flashcards
Explain what happens to a falling object in terms of speed and friction
- To start with, the force of gravity is more than the frictional force so it accelerates
- As the speed increases, the friction also increases gradually, reducing the acceleration until the frictional force is equal to the accelerating force so the resultant force is zero
What does terminal velocity depend on in terms of the type of object?
Shape and area
What is the accelerating force acting on all objects and what would happen without it?
- Gravity
- On the Moon, where there is no air resistance, bricks and feathers dropped simultaneously will hit the ground together. This is because there is no frictional force acting on the objects.
What is required to make something start moving, speed up or slow down?
A resultant force
What will a non-zero resultant force produce?
Acceleration or deceleration in the direction of the force
What is the unit used for momentum?
kg m/s
kilogram metres per second
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.
What quantity is equal to the force experienced in a collision?
The rate of change of momentum
Explain how a seatbelt improves a passenger’s safety during a collision
- Passengers 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 the force is applied, reducing the rate of change of momentum and thereforce reducing the force applied, resulting in fewer injuries
If a car travelling at a velocity of +10m/s suddenly turns around and continues at the same speed, what is its new velocity?
-10m/s
What is the name for the pulling force that a string or cable exerts when something or someone pulls on it?
Tension
What does a curved line show on a speed-time graph?
- a curved line shows that an object is accelerating or decelerating at a rate that is not constant
Unit for acceleration
m/s2
When will an object topple?
When its centre of mass is located outside its base
What is the resultant force on a stationary object?
0
At least how many forces must be acting on an object to stretch, bend or compress it?
2
What property of a spring does the spring constant represent?
Stiffness
In context of springs, what is extension?
The change in length of a spring when a force is applied
What does the area under the force-extension graph represent?
Elastic potential energy
Which symbol is used to denote the acceleration due to gravity?
g
On the speed-time graph for an object in free fall, what is equal to g?
The gradient/slope of the line
What is the acceleration of an object at terminal velocity?
Zero, the object is moving at a constant speed
What is the name of the distance travelled between when the driver realises they need to brake and when they apply the brakes?
Thinking distance
3 factors that affect the thinking distance of a driver?
- Alcohol
- Tiredness
- Distractions
What does the gradient on a velocity-time graph show?
Acceleration
What is gravitational field strength?
It is a way of measuring how much gravity there is
What can extension be replaced with in the equation for spring force?
Compression
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
What can be said about the resultant force on an object when it is falling at terminal velocity?
The resultant force is zero
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
Typical value for human reaction time
0.2 to 0.9 seconds
Give 2 factors which may affect braking distance
- Icy/wet conditions
- Poor tyres/brakes
What is the relationship between acceleration and mass?
Inversely proportional
How to work out the velocity after a collision of multiple objects?
- work the total momentum before the event
- p = mv
- work the total momentum after the event
- work total mass after collision
- work out new velocity
What force keeps an object afloat?
Upthrust
What are changes in momentum called?
Impulses
3 Safety Features used in cars
- air bags
- seat belts
- crumple zone
Which event applies the greatest force to car passengers?
Car crash
What is the atmosphere?
The atmosphere is a thin layer (relative to the size of the Earth) of air round the Earth
If the resultant force acting on an object is zero and the object is moving, what happens?
The object continues to move at the same speed and in the same direction. So the object continues to move at the same velocity.
What is inertia?
The tendency of objects to continue in their state of rest or of uniform motion
Explain why you don’t move when you lean on a wall, even though you are exerting a force
- You exert a force on the wall and so does the wall back on you (Newton’s 3rd Law)
- These forces are equal and opposite
- The resultant force is 0, so you remain stationary
What does Newton’s Third Law state?
For every force that acts on an object, there will be an equal and opposite force acting on another object.
What does Newton’s 2nd Law state?
The more mass an object has, the harder it is to accelerate.
What does Newton’s 1st Law state?
An object remains in the same state of motion unless a resultant force acts on it.
If the resultant force on an object is zero, this means:
Dangers of large decelerations
- brakes overheating
- loss of control
Formula for stopping distance
Stopping distance = Braking distance + Thinking distance
What is weight the same as?
Force
What happens to the total momentum if the cars are travelling in opposite directions?
Total momentum = Momentum A - Momentum B
Is force scalar or vector?
Vector
What is thinking distance?
Distance a vehicle travels during driver’s reaction time
