Physics Forces and Motion

practical: investigate how extension varies with applied force for helical springs, metal wires and rubber bands

1. set up the apparatus
2. measure the length of the spring without any hanging masses
3. hang a mass of 100g on the spring
4. measure the new length of the spring
5. calculate the extension of the spring (the increased length)
6. repeat and increase the mass in increments of 100g

Hooke’s law

force is directly proportional to extension

Hooke’s law region

if the graph has a linear region (straight line), then it obeys Hooke’s law

elastic behaviour

the ability of a material to recover its original shape after the forces causing deformation have been removed

momentum =

mass x velocity

p (momentum) =

m x v

how to reduce the force experienced by a passenger in an accident

* you need to increase the time taken for a passenger to stop in a collision
* this is because force = change in momentum/time

conservation of momentum

momentum before the collision = momentum after the collision

force =

change in momentum/time taken

f =

mv-mu / t

Newton’s third law

whenever two bodies interact, the forces they exert on each other are equal and opposite

all forces arise in pairs

if object A exerts a force on object B, then object B exerts an equal and opposite force on object A

force pairs are the same …

type and magnitude

moment =

force x perpendicular distance from the pivot

the weight of a body acts

through its centre of gravity

principle of moments

when the clockwise moments are equal to the anti-clockwise moments a body will be in equillibrium

principle of moments diagram

memorise

f2 x d2 =

f1 x d1 + f3 x d3

how do the upward forces on a lightbeam vary with the position of a heavy object

* Clockwise moments = anticlockwise moments
* Moment = force x perpendicular distance from pivot
* If the the distance from the pivot is less on the left hand side it means that the force must be greater to compensate for the larger distance on the right hand side.

work done =

force x distance moved

w =

f x d

work done is equal to

energy transferred

gravitational potential energy =

mass x gravitational field strength x height

GPE =

m x g x h

kinetic energy =

1/2 x mass x speed ^2

KE =

1/2 x m x v^2

how does conservation of energy produce a link between GPE, KE and work done

because energy cannot be created or destroyed, the decrease in GPE = increase in KE for a falling object if no energy is lost to the surroundings.

power is

the rate of transfer of energy or the rate of doing work

power =

work done / time taken

p (power) =

w / t

average speed =

distance moved / time taken

acceleration =

change in velocity/time taken

how to find the acceleration from a velocity-time graph

measure the gradeint

how to determine the distance traveled

area underneath the graph

v² =

u² + 2 a x s

final speed =

initial speed + 2 x acceleration x distance moved

difference between an electrostatic force and a gravitational force

electrostatic force is a force between objects that have an electric charge, gravitational force is a force between objects that have mass

electrostatic force can either be attractive or repulsive and is much greater than gravitational force

electrostatic force depends on the medium whereas gravitational force is independent of the medium

what is the difference between a vector quantity and a scalar quantity?

Vector quantities have magnitude and direction, like velocity. Scalar quantities have only magnitude, like speed.

what is friction?

a force that opposes motion

f = (relationship between unbalanced force, mass and acceleration)

m x a

weight =

mass x gravitational field strength

stopping distance =

thinking distance + braking distance

factors that affect thinking distance

speed, alcohol, tiredness

factors that affect braking distance

icy road, worn tires, speed

how do falling objects reach terminal velocity?

As an object falls, air resistance increases until it equals gravity. At this point, the object stops accelerating and reaches a constant speed called terminal velocity.

As an object falls, air resistance increases until it equals gravity. at this point, the object stops accelerating and reaches a constant speed called terminal velocity