12.1 forces

What is a force? A force is a push or pull acting on an object that can cause it to move or accelerate (change speed or direction); wind is an example of a force.

Measuring force Weight is a type of force that can be measured with a spring scale like the ones you see in the grocery store.

Units of force The SI (metric system) unit for force is the newton (N) where 1 N is the force that causes a 1 kg mass to accelerate at a rate of 1 m/s/s N = kg * m/s/s

Representing force Arrows can be used to show the direction and strength of a force

Combining forces Net force is the overall force acting on an object after all forces are combined; forces in the same direction add together forces in the opposite direction subtract from one another

Balanced forces When forces are balanced (equal and in opposite directions) net force = zero and the object that the forces are acting on doesn’t move (e.g. tug-o-war).

Unbalanced forces When the net force acting on an object is not = zero the force in unbalanced and the object moves.

Friction Friction is a force that opposes the motion of objects as they move past each other and it acts where objects touch no matter what state they are in (solid, liquid or gas); there are four main types of friction: static, sliding, rolling and fluid friction.

Static friction Static friction is friction between two solid objects that are not moving relative to each other. e.g. pushing a potted plant, walking.

Sliding friction Sliding friction is a smaller force than static friction and opposes the direction of motion once an object is moving (like pushing a potted plant) so less force is needed to keep the object moving.

Rolling friction Rolling friction, the force acting on rolling objects, is caused by a change in shape at the point of contact and is much less than either static or sliding friction.

Fluid friction Fluid friction is the force opposing objects that are moving through fluids (water, air) and it increases with the speed of the object moving through the fluid E.g. The faster you stir a batter the greater the friction is. A shark is streamlined so it can move through water easily

Gravity Gravity is an attractive force that acts between any 2 masses to pull them together and it can act over large distances (Earth orbiting the sun). Gravity is a force that attracts objects together. On earth this force attracts everything to Earth

Falling objects Falling objects are subjected to 2 forces: gravity and air resistance, the farther an object falls the greater its speed and the greater the air resistance becomes; when the force of gravity is equal to the force of air resistance it is called terminal velocity where acceleration is constant.

Projectile motion When an object is thrown it forms a curved path due to the combination of initial velocity and the downward force of gravity acting on the object.