Topic 9- Forces and their effects

force

a push or pull that acts on an object due to the interaction with another object

can cause an object to change direction, speed or shape

non-contact forces are caused by

fields

examples of non-contact forces

gravitational force

• caused by gravitational attraction between two masses

• increased by increasing mass

electrostatic force

• caused by charged objects repelling or attracting each other

• increased by increasing strength of electrical field, bringing charges closer together

magnetic force

• caused by a force exerted on a magnetic material in a magnetic field, two magnetic poles

• increased by increasing strength of field, bringing objects closer together

contact forces

friction

• opposes motion

• occurs when objects rub against each other

air resistance

• type of friction

• occurs when air particles hit and push against an object travelling in air

tension

• pulls two objects connected by a length (e.g. string)

• occurs when a force is applied to both ends of a length

normal contact/reaction force

• perpendicular to the surface of contact due to the push of one object on another (pushes touching objects apart)

• occurs when objects are supported by a surface

force pairs (why and characteristics)

when there is an interaction between two objects, force is exerted on both objects (different objects)

• same type of force

• acting on different bodies

• in opposite directions

• in a line

Newton’s third law

when two forces interact, they exert equal and opposite forces on each other

how to find the resultant force of non-aligned forces (scale drawings)

connect the arrows tip to tail

in free body force diagrams, should the arrows be to scale? what should happen if they are balanced?

yes; they should form a closed triangle when put tip-to-tail

when can forces cause an object to rotate? (2 things)

• a force on one side of a fixed pivot

• two forces act on an object but DON’T pass through the same point (even if they are balanced)

moment

turning effect of a force about a pivot

moment equation

m = F x d (perpendicular distance from pivot)

principle of moments

if an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot

levers and gears can be used to

transmit and amplify the rotational effects of forces

how do levers transmit and amplify the rotational effect of a force?

smaller force applied to end of lever (further away from the pivot than where the force is needed)

greater distance increases moment, therefore the force exerted will be greater at the end with the smaller distance from pivot

how do gears transmit and amplify the rotational effect of a force?

turning force applied to one wheel

interlocking teeth turn adjacent wheel in opposite direction

moment = turning force x wheel radius

a larger gear can turn a smaller gear faster with a smaller moment

a smaller gear can turn a larger gear slower but with a greater moment

what is the relationship between the ratio of teeth, moments, and radii between 2 gears?

ratio of moments = ratio of teeth no. = ratio of radii