P5 forces Flashcards
A force
A push it pull that is applied by one object on another
Measured in newtons (N)
Contact forces
The objects are physically touching
Such as a parachute in contact with air
Non contact forces
The objects are physically separated
Eg. Attraction/repulsion of charged objects
Scalar quantity
Has magnitude
Size only
Vector quantity
Has magnitude and direction
Displacement
Distance moved and the direction
Speed
How fast something is moving
Distance time graphs
Gradient=to speed of the object
Speed changes graph is a curve so draw tangent
Acceleration
When the velocity of a car is increasing it is accelerating
When the velocity of a car is decreasing it is decelerating
Motion in a circle
Car goes round a roundabout
At constant speed direction of movement is changing
Velocity is changing as it is a vector, it is accelerating
Velocity time graphs
Shows job the velocity of a moving object changes over time
Will show when accelerating and decelerating
Gradient= rate of chang of velocity or acceleration
Displacement= area under a velocity time graph (distance travelled)
Uniform motion
Acceleration is constant
Symbols for describing motion
s= displacement in m u= initial velocity in m/s v= final velocity in m/s a= acceleration in m/s2
Applying equation in vertical motion
If no air resistance, gravity give acceleration of appox. 9.8 m/s2 downwards. If ball thrown up it decelerates -9.8 m/s2
When ball falls back it accelerates at +9.8m/s2
Only used when object travels with constant uniform acceleration in a straight line
Mass
Amount of substance that is present in an object
Kg
Weight
The force acting on that mass,mid it is in a gravitational field
N
Newton’s first law
If the resultant force acting on an object is zero it will:
If stationary remain stationary
If moving keep moving at a steady speed in a straight line
The resultant force
The combined force
Force by one team - force of other team= combined force
Free body diagram
Shows magnitude and direction of the forces acting on an object
Don’t have to draw image of object instead just a circle
Vector diagram
Resultant is from point a to point b (gradient)
Newton’s second law
Where F is the resultant force in N, m is the mass in kg and a is the acceleration in m/s2
The resultant force is a single force that has the same effect as all the original forces acting together
Inertia
Massive objects that are hard to stop have an inbuilt reluctance to stop or start moving
Natural tendency of objects to resist changes in their velocity
Intertial mass
Measure of how difficult it is to change the velocity of an object
Defined by the ratio of force over acceleration
Required practical- investigating the acceleration of an object
Looking at how force affects acceleration
The greater the force the greater the acceleration
Force pairs
When 2 objects interact
A exerts a force on B, there is always another force B exerts a force on A
Forces are equal in size but act in opposite directions an on different objects
Newton’s third law
When 2 objects interact, the forces they exert on each other are equal and opposite
Forces in the pair
Same size
Act in opposite directions
Act on different objects
Same type of force
Momentum
A moving object has momentum
Amount depends on mass and velocity
Vector
Changes in momentum
Eg. A car stops suddenly. Momentum becomes 0
The larger the force applied to the car, the quicker its momentum becomes zero
Calculate by using F= m(v-u)/t
Crumple zones
Increase the time between first impact and the car stopping
The rate of change of momentum is smaller, reducing the force on the cars occupant
Conservation of momentum
The total momentum before a collision is equal to the total momentum after the collision
m1u2 + m2u2= (m1+m2)v
Reaction time
The time it takes for the driver to react when o a stimulus
Thinking distance
The distance travelled during the reaction
It is the distance travelled between the driver seeing a danger and taking action to avoid it, such as braking
Braking distance
Is the distance travelled before a car stops after the brakes have been applied
It increases as the speed of the car increases
Stopping distance
Thinking distance + braking distance
Reaction time and thinking distance increase when
Tired
Under the influence of drugs or alcohol
Distracted or lacks concentration
Braking distance may increase when
Road is wet or icy
Car has poor brakes or bald tyres
The speed of the car is greater
To estimate the force needed to stop a vehicle
Calculate deceleration and find force from F=ma
Calculate how much kinetic energy must be transferred away from the car and equate this to the work done by the braking force
Moment
Turning effect Newton metre (Nm) The point of moment acts around is called the pivot
Centre of mass
The weight of the object can be considered to act at a single point
Lever
Used to moment of a force.
If a force pushing down in greater than the load it turns in a clockwise direction
Gears
are used for transmitting the rotational effect of a force from one part of a machine to another.
If the gears are the same size they will rotate at the same speed
Pressure in fluids
Causes a force normal to (at right angles to) any surface
Acts nine all directions
Pressure at different depths in a liquid
The deeper you are the greater the pressure. There is a greater weight of water above you
Sink or float
If the density of the solid material is greater than the density of the liquid it will sink. As weight is greater than up thrust.
Atmospheric pressure
The air exerts pressure. Force acts normal to any surface. The higher you go the less pressure As there is is less air above you.
The greater the density of the air, the moe weight of air above a surface means more collisions in a given space= pressure
Elastic deformation
Occurs when spring can returns back to original length when forces are removed
Inelastic deformation
A spring that is permanently altered
Elastic potential energy
Amount of work done =elastic potential energy
Required practical: investigate the relationship between force and the extension of a spring
Force and extension are directly proportional until spring becomes deformed and graph starts to curve
