Forceful Physics

Resultant Force: the overall force of an object

To work out the resultant force: add forces that are acting in the same direction and subtract forces that are acting in the opposite direction.

When the resultant force of an object is zero:

• The object is at rest and will stay at rest
• The object is moving and will continue moving in the same direction at the same speed

When the resultant force of an object is not zero:

• If the object is at rest, it will accelerate in the direction of the resultant force
• If the object is moving in the same direction as the resultant force it will accerlerate
• If the object is moving in the opposite direction as the resultant force, it will decelerate

Objects always exert equal and opposite forces on each other. These are sometimes called 'action and reaction' forces.

Newton's Laws of Motion:

1) An object will only change speed when a resulatant force acts upon it.

2) the acceleration of an object is proportional to the force applied ( F=m x a)

3) For every action, there is an equal and opposite reaction.

If a vehicle is travelling at a steady speed, the resultant force is zero.

The faster the speed of a vehicle, the larger the deceleration needed to stop the car so the braking force needed is bigger.

Stopping distance= thinking distance + braking distance

Thinking distance: the distance the car travelas in the time it takes for the driver to react to the stimulus 

Thinking distance can be increased by: drugs, alcohol, distractions, fatigue, speed of the car

Braking distance: the distance the car travels before stopping while under the braking force

Braking distance can be increased by: poor roads, bad weather, the condition of the car, speed of the car

Car safety measures: crumple zones, air bags which slow down the momentum of the person GRADUALLY and seat belts which increase the impact time and reduce the force

If an object falls freely, the resultant force acting upon it is the force of gravity-it will make an object close to the earth accelerate at about 10m/s.

The force of gravity = weight

An object accelerates when it is first dropped because the resultant force is the force of gravity (weight)

When an object falls through a fluid, the fluid exerts a drag force which resists its motion.

• The faster an object falls, the bigger the drag force becomes
• The continues until the drag becomes equal to the weight of the object
• The resultant force is now zero so the object stops accelerating and moves at a constant velocity
• This is called terminal velocity

An elastic object is one that regains its original shape when the forces deforming it are removed. If we add weight to an elastic object, the increase in length from the original is called the extension.

The extension is directly proportional to the force applied. If we apply a force that is too big, the line of the graph becomes curved because we have exceeded the limit of propotionality.

Hooke's Law

Objects and materials that behave like this obey Hooke's Law. 

F= ke 

Where F is the force applied (N), k is the spring constant in N/m, e is extension in m

• The stiffer a spring is, the greater it's spring constant
• When an elastic object is stretched, work is done which is stored as elastic potential energy in the object
• When the stretching force is removed, the stored energy is released

• Reducing the speed of a vehicle reduces the amount of fuel it uses to travel a particular distance- this is called fuel economy.

• Reducing the air resistance of a car by making it more streamlined also improves fuel economy
• Skidding happens when the brakes of a vehicle are applied to harshly- the wheels lock and slide along the road surface
• Anti-skid surfaces are used to reduce or prevent skidding- they are rougher than normal road surfaces, increasing the friction
• Speed cameras are used to discourage motorists from speeding- they can determine the speed of a motorise at a particular point or can work in pairs to take an average speed between two points

Whenever an object starts to move, a force must have been applied to it.

When a force moves an object, energy is transeferred and work is done.

W = F x d

Kinetic Energy:

All moving objects have kinetic energy. The greater the mass and the speed of an object, the more kinetic energy it has. 

kinetic energy = ¹⁄₂ × mass × speed²

All moving objects have momentum. The greater the mass and the greater the velocity of an object, the greater its momentum.

p = m x v

Whenever objects interact (an interaction could be a collision or explosion),the total momentum before the interaction is the same as the total momentum afterwards so long as there are no external forces.

This is called the law of conservation of momentum.

Like velocity, momentum has both size and direction.

When two objects are at rest, their momentum is zero. In an explosion the objects move apart with equal and opposite momentum (one momentum is positive and one is negative)

An example of this is firing a gun. The bullet moves off with momentum when fired and the gun recoils in the opposite direction with equal momentum.

When a force acts on an object that is able to move or is moving, the momentum changes.

The longer the time taken for the change, the smaller the force that acts.

In a collision, the momentum of an object becomes zero during the impact-the object comes to rest

If the impact time is short, the forces on the object are large. As the impact time increases, the forces become less.

Crumple zones

Crumple zones in cars are designed to fold in a collision. This increases the impact time so reduces the force on the car and the people in it.

Gravitational potential energy is energy stored in an object because of its position in the earth's gravitational field.

Whenever an object is moved vertically upwards it gains gravitational potential energy equal to the work done on it by the lifting force.

GPE = m x g x h

Power is the rate of transfer of energy.

P = E/t 

The nuclei of radioactive substances are unstable.

They become stable by radioactive decay (emitting radiation and turning into other elements)

There are three types of radiation that can be emitted: alpha, beta and gamma

We cannot predict when an unstable nucleus will decay as it is a random process and is not affected by external conditions. For example, the rate of decay does not increase if temperature is increased.

Background radiation is around us all the time- this is radiation form radioactive substances in the environment, from space and from devices such as X-ray tubes.

The 'Plum Pudding' model

Scientists used to believe that atoms were spheres of positive charge with electrons stuck in them like plums in a pudding. 

The Nuclear Model

• Rutherford, Geiger and marsden did an alpha particle scattering experiment
• They fired alpha particles at thin gold foil

They found:

• Most of the particles passed straight through meaning that most of the atom is empty space
• Some of the alpha particles were deflected through small angles meaning the nucleus has a positive charge
• A few rebounded through very large angles meaning that the nucleus has a large mass and a very large positive charge

Alpha particles:

Consists of: two protons, two neutrons

Relative mass: 4

Relative charge: 2+

Alpha particles are the same as helium nuclei.

When a nucleus emits an alpha particle the atomic number goes down by two and the mas number goes down by four.

• Alpha particles are relatively large so they have lots of collisions with atoms so they are strongly ionising.
• Because of these collisions, alpha particles do not penetrate far into materials- they can be stopped by a thin sheet of paper, human skin, a few centimetres of air
• Alpha particles have a positive charge and are deflected by electric and magnetic fields

Relative mass: negligible

Relative charge: -1

A high speed electron from the nucleus emitted when a neutron in the nucleus changes to a proton and an electron.

The proton stays in the nucleus so the atomic number goes up by one and the mass number is unchanged.

The electron is instantly emitted. E.g. carbon-14 emits a beta particle when it becomes nitrogen

• Beta particles are smaller and faster than alpha particles so they are less ionising and penetrate further.
• They can be blocked by a few metres of air or a thin sheet of aluminium
• Beta particles have a negative charge so are deflected by electric and magnetic fields in the opposite direction to alpha particles

Gamma rays are electromagnetic waves.

They are weakly ionising (so they travel a long way through a material before colliding with an atom) 

They are very penetrating

• The radiation can be stopped by several centimetres of lead or several metres of concrete
• Gamma rays are not deflected by electric or magnetic fields

When a nucleus emits gamma radiation there is no change in the atomic or mass number

The gamma ray is an electromagnetic wave released from the nucleus- it has no charge and no mass

We can measure the radioactivity of a sample by measuring the count rate from it.

The radioactivity of a sample decreases over time- how quickly the count rate falls to nearly zero depends on the isotope

Half-life: the time taken for the count rate from the original isotope to fall to half its initial value/ the times it takes for the number of unstable nuclei in a sample to halve

The half-life is the same for any sample of a particular isotope

Smoke Alarms:

• Alpha sources
• Alpha sources are not dangerous because they are poorly penetrating
• The source needs a half-life of several years

Thickness monitoring in the manufacture of paper and metal foil:

• Beta particles
• Alpha particles would be stopped by a thin sheet of paper and gamma rays would pass through it
• The source needs a half-life of many years so that the decreases in count rate are due to changes in the thickness of the paper

Tracers in Medicine:

• Source injected or swallowed by the patient
• The source needs a half-life of a few hours so that the patient is not exposed to unnecessary radioactivity