Forces and motion Force and mass Gravity and weight Action and reaction Vectors and scalars Circular motion

Forces and their effects You cannot see a force but you can see what it does. You can also feel effects of a force on your body

What can a force do?

Make an object move Make a moving object stop Change the speed of a moving object Change the direction of a moving object Change the shape of an object deformed

Forces in balance A force is a push or a pull, exerted by one object on another. It has direction as well as magnitude (size), so it is a vector. The SI unit of force is the newton (N). Example:

Motion without force Newton’s first law of motion- law of inertia. If no external force is acting on it, an object will - if stationary, remain stationary - if moving, keep moving at a steady speed in a straight line. On Earth, unpowered vehicles soon come to rest because of friction. With no friction, gravity, or other external force on it, a moving object will keep moving for ever- at a steady speed in a straight line.

Balanced forces An object may have several forces on it. If the forces are in balance, they cancel each other out. The object behaves as if there is no force on it at all. With balanced forces on it, an object is either at rest, or moving at a steady velocity.

Terminal velocity

When a skydiver falls from a hovering helicopter, as her speed increases, the air resistance on her also increases. Eventually, it is enough to balance her weight, and she gains no more speed. She is at her terminal velocity At a steady velocity, the forces must be in balance. That follows from Newton’s first law.

Linking force, mass, and acceleration Resultant force=mass x acceleration F=ma This relationship between force, mass, and acceleration is called Newton’s second law of motion 1 newton is the force required to give a mass of 1kg an acceleration of 1m/s 2 (defining the newton)

Example: What is the acceleration of the model car on the picture?

First, work out the resultant force on the car. A force of 18N to the right combined with a force of 10N to the left 18N-10N=8N Next, work out the acceleration when F=8N and m=2kg F=ma 8N=2kg x a a=4m/s 2

Gravity and weight Gravitational force If you hang an object from a spring balance, you measure a downward pull from the Earth. This pull is called a gravitational force.

No one is sure what causes gravitational force All masses attract each other The greater the masses, the stronger the force The closer the masses, the stronger the force

Weight Weight=mass x g (g, gravitational field) W=mg ( W, gravitational force) Unit for weight is N (newtons)

Summary Newton’s first law of motion- law of inertia. If no external force is acting on it, an object will - if stationary, remain stationary - if moving, keep moving at a steady speed in a straight line Resultant force=mass x acceleration F=ma This relationship between force, mass, and acceleration is called Newton’s second law of motion

Example: What is the acceleration of the rocket on the right? W=mg=200kg x 10N/kg=2000N Resultant force=3000N-2000N=1000N F=ma 1000N=200kg x a A=5m/c 2

Action and reaction A single force cannot exist by itself. Forces are always pushes or pulls between two objects. So they always occur in pairs.

Newton’s third law of motion If object A exerts a force on object B, then object B will exert an equal but opposite force on object A. Another way of stating the same law: To every action there is an equal but opposite reaction.

More about vectors Quantities such as force, which have a direction as well as a magnitude (size), are called vectors. Quantities such as mass and volume, which have magnitude but no direction, are called scalars.

The parallelogram rule

The parallelogram rule is a method of finding the resultant (two vectors acting at a point can be replaced by a single vector with the same effect), where the vectors are not in line. The parallelogram rule also works in reverse: A single vector can be replaced by two vectors having the same effect.

Q 1) How is a scalar different from a vector? Give an example of each. 2) Force of 12N and 5N both act at the same point, but their directions can be varied. a) What is their greatest possible resultant? b) What is their least possible resultant? c) If the two forces are at right angles, find by scale drawing the size and direction of their resultant.

Moving in circles Centripetal force If someone whirling a ball around in a horizontal circle at a steady speed. F c=mv 2 /r This inward force Centripetal force isn’t produced by circular motion. It is the force that must be supplied to make something move in a circle rather than in a straight line

Centrifugal force When you whirl a ball araund on the end of some string, you feel an outward pull on your hand

Orbits Satellites around the Earth. A satellites travels round the Earth in a curved path called an orbit. Gravitational pull provides the centripetal force needed. Electron around the nucleus In atoms, negatively charged electrons are in orbit around a positively charged nucleus. The attraction between opposite charges provides the centripetal force needed