P10 Flashcards
What is the formula for resultant force?
Resultant force(N)= mass(kg) x acceleration(m/s2)
Newton’s second law of motion
The acceleration of an object is:
Proportional to the resultant force on the object
Inversely proportional to the mass of the object
Resultant force = mass x acceleration
Inertia
The tendency of an object to stay at rest or continue in uniform motion(constant velocity)
What direction is the acceleration in?
Acceleration in same direction as velocity
Velocity increases if resultant force is in the same direction as the velocity(positive acceleration)
Velocity decreases if resultant force is in opposite direction to velocity(negative acceleration)
How does mass affect acceleration?
The greater the mass of an object, the smaller its acceleration for a given force
What is weight?
The force acting on the object due to gravity - is why an object falls
What happens when no forces act on an object?
Resultant force is weight - object has constant acceleration of 9.8m/s2, acceleration due to gravity
How do mass and weight change?
Mass stays the same always, at the equator you weigh less than at the poles since gravity is weaker at the equator
Mass
Quantity of matter in an object
Gravitational field strength
Gravitational force on a 1kg object at a place where the object is
Earth’s gravitational field strength at the surface
9.8N/kg
Equation for weight
Weight = mass x gravitational field strength(N/kg)
Terminal velocity
Constant velocity when frictional force on object is equal and opposite to its weight(maximum velocity on an object)
Resultant force on object
Weight minus frictional force
What happens to an object when it falls
Acceleration decreases, frictional force increases as it speeds up,resultant force decreases
How can you ensure an object in the air can reach constant velocity?
An object can fall further through the air to reach a constant velocity as the frictional force when an object moves through the air is air resistance
When an object falls in the fluid
Fluid drags on object due to friction between fluid and surface of moving object, frictional force increases with speed. Object reaches a constant velocity when frictional force is equal and opposite to its weight
Object falling in liquid
Two forces are drag force and weight
Forces on a car
For any car travelling at a constant velocity, the resultant force is zero, driving force of engine balanced by resistive forces caused by air resistance
What affects the braking force
Speed of vehicle when brakes are first applied - greater the speed, greater deceleration to stop vehicle in given distance
Mass - greater the mass, greater the braking force for a given deceleration
Thinking distance
Distance travelled by vehicle in time it takes driver to react, directly proportional to speed, thinking distance = speed x reaction time
To do with the brain:
Level of alertness
Drugs/alcohol
Distractions
Speed of vehicle
Mist and fog don’t affect the thinking distance as they don’t affect the brain.
Braking distance
Distance travelled by the vehicle during the time the braking force acts
Anything to do with friction between brakes/tyre/road:
Mass of vehicle
Speed of vehicle
State of vehicles’ brakes
Weather
State of the vehicles’ tyres
State of the road(gravel/ice/snow/rain/general condition)
Curve of best fit on graph due to kinetic energy, proportional to v2(speed2)
If you increase the speed, the braking distance increases by speed2:
Doubling the speed quadruples the braking distance
Tripling the speed increases the braking distance by 9 times
Stopping distance
Thinking distance + braking distance, shortest distance a vehicle can stop in, how far a vehicle will travel between the moment it brakes and the moment it comes to a complete stoo
Factors affecting stopping distances
Tiredness, alcohol, drugs and mobile phones(distractions) increase reaction time and thinking distance, so increase stopping distance
Thinking distance and braking distance increase with increased speed
In adverse road conditions, drivers brake with less force to avoid skidding, stopping distances greater in poor weather + road conditions
Poorly maintained vehicles with ineffective brakes and tyres take longer to stop
Deceleration
a = -u2/2s
where u= initial speed
s = distance travelled
Deceleration to avoid skidding on a dry flat road
No more than 6m/s2
Forces needed to accelerate a road vehicle
6KN-250KN
Momentum
Momentum(kg m/s)= mass x velocity, a vector quantity
Conservation of momentum
In a closed system, momentum same before and after an event as long as there are no external forces
Elastic objects
If they return to their original shape after the forces deforming it are removed(elastically deformed objects), squash ball, rubber band
Extension
Length at stage - original length
Hooke’s Law
The extension of a spring is directly proportional to the force applied as long as the limit of proportionality isn’t exceeded(when spring stretches more than predicred)
Rubber and polythene have a non-linear relationship between their extension and force(low limit of proportionality)
Steel spring has higher limit of priportionality
Force applied = spring constant(N/m) x extension
Applies to a compressed object, change of length is a compression, not an extension
Acceleration on moon
Lower acceleration due to no air resistance and no atmosphere
Being streamlined
Streamlined(aerodynamic, smaller surface area), the more streamlined you are, the faster the terminal velocity
Parachute
1.Hanging from the leg of a helicopter - Weight is equal to tension, no acceleration, balanced forces, terminal velocity
2. Person falling from plane - Weight stays the same, air resistance increases as you fall, big acceleration due to difference between weight + air resistance
3) Air resistance and weight balanced forces, no acceleration
4)Pulling the shoot-
Increasing air resistance due to larger surface area, same weight, unbalanced forces so big deceleration
5)After pulling the shoot - Terminal velocity, air resistance = weight, falling out at a constant speed, balanced forces, slows down so air resistance reduced
Is gravity a force?
NO!
Gravity
Acceleration of an object due to the Earth’s mass, the weakest of the fundamental forces
What happens to two objects of equal mass
They hit the floor at the same time due to equal amounts of graviy acting on them and equal amounts of air resistance acting in them
Acceleration in a vacuum
In a vacuum, there is no air, so the object continues to accelerate
How does gravitational force affect size
Planets with a similar gravitational force have a similar size
How does mass affect the gravitational force?
Objects without mass - no gravitational force
Objects with mass - gravitational force
Forces on orbiting objects
There are unbalanced forces on orbiting objects, with only the gravitational force acting, since the object accelerates. If you are orbiting, you are weightless as you feel you’re always falling, there is little gravity
Forces on unorbiting objects
If you’re next to something like the sun without orbiting, you have weight as you’re pulled by the gravitational force of the sun
What is an object’s weight in deep space?
An object is weightless in deep space, but its mass doesn’t change
Does mass affect the acceleration when an object falls?
No, because the force could be the weight: acceleration = gravity
What affects how far a car goes before it stops
Momentum(mass + velocity of a car)
Friction in tyres
Quality of road
Drugs
Tiredness
Brake pads
Type of brakes
How does mass affect momentum?
The bigger the mass, the more momentum + force needed to stop an object
Momentum experiment
You could change the height of a slope when trying to test momentum with one block at the end, measure how much block moves.
Measure speed near the end of a ramp.
