1. Forces and their effects Flashcards
What is the conservation of momentum?
The idea that in a closed system, the total momentum before an event = total momentum after the event
How do safety features like air bags, seat belts and cushioned surfaces work?
Increase the impact time
p = m x v
Momentum (kgm/s) = Mass (kg) x velocity (m/s)
What are scalar quantities?
Quantities that have magnitude only
What are vector quantities?
Quantities that have magnitude in a given direction
What is velocity?
Speed in a given direction
Distance time-graph
The gradient of it is the speed, if accelerating use a tangent to find the gradient
Velocity-time graph
The gradient gives the acceleration
Area under the graph equals the distance travelled
v = s/t
Velocity(m/s) = Displacement (m) / Time (s)
a = (vā u)/t
Acceleration (m/s/s) = (Final velocity(m/s) - Initial Velocity (m/s)) / Time (s)
What is referred to as āElastic Behaviourā?
When a force acts on an object it may cause a change in shape, the object behaves Elastically if it returns to its original shape when the force is removed
What happens when you apply force to an elastic object?
The object stretches and stores elastic potential energy
For an object behaving elastically, what is the extension is directly proportional to?
The force applied
F = k x e
Force (N) = Spring constant x Extension (cm)
The faster an object moves through a fluid the greater what is?
The greater the frictional force that acts upon it
Terminal velocity
An object falling through a fluid will initially accelerate due to gravity
Eventually the friction will be the same as the downwards forces
The resultant force will equal zero and the object will move at its terminal velocity (greatest steady speed)
Whenever two objects interactā¦
The forces they exert are equal and opposite
Resultant forces
A resultant force as a single force obtained by combining a system of forces on a single body
If the resultant force acting upon a stationary object is:
Zero, the object will remain stationary
Not zero, the object will accelerate in the direction of the resultant force
If the resultant force acting on a moving object is:
Zero, the object will continue to move at the same speed and direction
Not zero, the object will accelerate in the direction of the resultant force
What are the resistive forces when driving?
Air resistance and friction
When a car travels at a steady speed the resistive forces balance the driving forces
The greater the speed the greater the braking force needed to stop it in a certain distance
For a given braking force, the greater the speed the greater the stopping distance
What is the stopping distance?
The thinking distance + The braking distance
What affects the drivers reaction time?
Tiredness, drugs, alcohol, (distractions)
What happens when the brakes are applied?
Work done by friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increases
What can affect the vehicles braking distance?
Poor road, weather (icy & wet) & vehicle (brakes and tyres) condition
F = āp / T
Force (N) = Change in momentum (Kgm/s) / Time (s)
F = m x a
Force (N) = Mass (kg) x Acceleration (m/s/s)
W = m x g
Weight(N) = Mass(kg) x Gravitational field strength
W = F x d
Work done (J) = Force (N) x Distance moved in Direction of force (m)
P = W / t
Power (Watts) = Work done (J) / Time (s)
Ep = m x g x h
Change in gravitational potential energy = Mass x Gravitational field strength x Change in height
Ek = 1/2 x m x v^2
Kinetic Energy = 1/2 x Mass(kg) x Velocity^2
Work is done when a force causes an object to move through a distance
Energy is transferred when work is done
What does Work Done against frictional forces cause?
Energy transfer by Heating
What happens when an obstacle is lifted vertically up?
Work is done against gravity and the object gains gravitational potential energy
What will happen to the Kinetic energy if an object with double the mass of another object travels at the same speed?
The Kinetic Energy is doubled
What will happen to the Kinetic energy if an object the same mass as another object travels at double the speed?
The Kinetic Energy will be four times greater
What is the centre of mass?
The point at which the mass of the object is thought to be concentrated
Where is the centre of mass of symmetrical objects?
Along the axis of symmetry
Where will the centre of mass be of an object freely suspended ?
Directly below the point of suspension
What does the time period of a pendulum depend on?
The length of the pendulum
Applications of pendulums
Simple fairground and playground rides
T = 1 / f
Time period = 1 / frequency
What is a moment?
A moment is a turning effect of a force around a fixed point called a pivot
What does a moment depend on?
The size of the force applied
The perpendicular distance from the pivot to the line of action of the force
What can simple levers be used as?
Force multipliers
What happens if the line of action of the weight of an object lies outside the base of the object?
There will be a resultant moment and the body will topple
Useful uses of Moments?
Cranes, tools such as hammers and balancing toys
M = F x D
Moment (Newton-Metres) = Force (Newtons) X Perpendicular Distance (Metres)
Hydraulics
Liquids are cutely incompressible and the pressure in a liquid is transferred equally in all directions
How do Hydraulics work as force multipliers?
P = F/A
the pressure in a liquid remains constant so, indoor to keep the same pressure, if the cross sectional surface area increases the force must increase with it
P = F / A
Pressure (Pascals, Pa) = Force (Newtons) / Cross-sectional area (m^2)
What happens with an object moves in a circle?
It is continuously accelerating towards the centre of the circle, as its velocity is changing not its speed
What is centripetal force?
The resultant force causing the acceleration seen in circular motion
It always acts directly towards to centre
Centripetal force needed to make an object perform circular motion include:
The mass of the object increasing
The speed of the object increasing
The radius of the circle decreasing
