Physics force and motion

distance

the length of the path covered by someone or something

displacement

the length of the shortest route between the start and end point

speed

distance travelled per unit time- scalar

velocity

displacement per unit time= vector/ speed in a given direction

Gradient of a distance time graph

represents the speed, the steeper the gradient, the faster the speed

average speed

total distance(m)/ total time(s)

acceleration

change in velocity per second, change in velocity/time taken

Upwards straight line on velocity time graph

constant acceleration

Straight line(180) on velocity time graph

constant velocity- still moving but at a constant speed

Downwards straight line on a velocity-time graph

deaccelerating constantly

Area under a velocity time graph

represents the distance travelled- split into shapes to work out the area

S (suvat)

distance travelled/ displacement (m)

U (suvat)

initial velocity (m/s)

V (suvat)

final velocity (m/s)

A (suvat)

acceleration

T (suvat)

time (s)

SUVAT equation

equations of motion- only need to know equation 4 and will be on equation sheet

inertia

describes the tenancy of objects to remain in their state of rest or uniform motion unless a resultant for acts on it OR objects don’t move unless a resultant force is acting on it

Effects of air resistance/drag

1. No drag- all falling objects are equally affected by gravity and accelerate at the same rate.

2. Drag will increase with speed

Terminal velocity

When a free-falling object reaches a stage where forces are equal so there is no speed increase/ reaches a constant speed

When/how to use tangents

When there is a non-linear acceleration, draw a tangent and find the gradient. Acceleration is the gradient of a tangent.

Thinking distance

distance travelled whilst the driver reacts to a situation

thinking distance equation

reaction time (around 0.1s) x speed

factors affecting thinking distance

• distractions- children music conversations

• tiredness

• age/medical

• drugs/alcohol

braking distance

distance a car travels while the car is stopped by the brakes (assume constant braking force)

Factors affecting braking distance

• worn tires

• road surface

• car mass

• worn brakes

• speed

• ice/rain

braking distance equation

final velocity squared= initial velocity squared + 2(acceleration x distance travelled)

stopping distance

thinking distance + braking distance

newtons 3rd law

when object a exerts a force on object b, object b exerts an equal force on object a in the opposite direction.

These pairs of forces are:

• equal in magnitude

• opposite in direction

• of the same type of force

• acting on different objects- do not cancel each other

Circular motion

a movement of an object along the circumference of a circle or rotation of an arc

centripetal force

a force which acts towards the centre of a circle. It causes acceleration towards the middle of a circle.

momentum

product of mass and velocity, vector quantity,

momentum equation

p=mv

momentum= mass and velocity

closed system

no external objects acting on the object in the system

conservation of momentum

in a closed system, the total momentum before an event is equal to the total momentum after the event

two objects going in opposite directions

one object must have a positive velocity, direction and momentum

the other object must have a negative velocity, direction and momentum

effects of force on momentum

when a force is applied to an object the object’s velocity changes. Therefore the momentum must also change

force and momentum equation

force = change in momentum/ time

Reducing force in car crashes

modern car safety features work by increasing the amount of time taken for a person to deaccelerate in a collision e.g seatbelts, airbags. crumple zones.

Longer deacceleration means that change in momentum occurs over a longer time- therefore a smaller force acting on the person

elastic collisions

momentum is conserved, kinetic energy is also conserved

inelastic collisions

momentum is conserved, kinetic energy is not conserved

newtons 2nd law

acceleration is directly proportional to the resultant force

acceleration is inversely proportional to mass