Forces Chapter 7

What are forces? A force is a push, pull or twist that can change an objects motion

What can forces do? Acceleration: increase in speed deceleration: decrease in speed

Measuring forces Force is measured using a unit called newtons (N) The ability of springs to stretch and squash allow a weight force to be measured The larger he pulling force, the more the string is stretched, the higher the force

summary Not moving Speed

Note that the arrows are the same length Balanced forces Note that the arrows are the same length The downward force of gravity is balanced by the upward support force from the chair. The forces acting on this person are balanced This is an example of an object that is not moving/stopped

Note that the arrows are the same length Balanced forces This is an example of an object travelling at the same speed without changing directions The forward force from the pedalling is cancelled out by the friction force pushing backwards The forces at this stage are balanced forward force Note that the arrows are the same length

Unbalanced forces Motion will change when the forces are unbalanced Forces are unbalanced when: Start moving (pedalling fast and hard) Speed up (pedalling fast and hard) Slows down (using the brakes) Comes to a stop (using the brakes) Changes direction (turning the handle bars)

Note that the arrows are now different lengths Unbalanced forces Note that the arrows are now different lengths

Unbalanced forces summary Decelerate Accelerate Arrow is larger therefore moving forward Arrow is larger therefore moving backwards

Driving Force (thrust) Drag from caravan question Driving Force (thrust) 1. State which direction the car is moving 2. Predict what would happen to the speed of the car if the caravan gained extra weight 3. If the car is travelling at a constant speed. Compare the size of the thrust and drag forces acting on the car

Driving Force (thrust) Drag from caravan Driving Force (thrust) 1. State which direction the car is moving To left or forward 2. Predict what would happen to the speed of the car if the caravan gained extra weight Decrease as the drag from the caravan will increase 3. If the car is travelling at a constant speed. Compare the size of the thrust and drag forces acting on the car The thrust balances the drag. No overall force is acting. (Balanced forces = Constant Speed)

Inertia Inertia is an object’s resistance to changing its state of motion. The greater the mass of the object the greater the inertia. Therefore the greater the mass and inertia the harder and more force is required to change the objects state of motion.

Inertia The tendency of an object to resist any change in its motion (unless a force is acted upon it) If you put your school bag down, it will stay there until something happens to it Someone could lift it, push it or pull it to make it move. When left alone, the bag will not move

Newtons Law of motion Sir Isaac Newton proposed some statements to explain the way things move Anything that is not moving will stay that way unless a force makes it move Anything that is moving will keep moving at the same speed and in the same direction unless a force makes it change

The effect of mass The more massive something is, the greater its inertia This means it takes a greater force to change its motion Eg: it takes a smaller force to stop a pebble roll down a hill than to stop a massive rock

Types of Forces Non-Contact Contact Contact forces touch an object Friction Pull Push Non-Contact Gravity Magnetic Electrostatic Non-contact forces act at a distance

Friction 7.2

What is friction Friction occurs whenever one object tries to move over another. Because it occurs between surfaces in contact, friction is called a contact force. Friction acts in an opposite direction to motion

What affects friction? Friction depends on: How rough the surfaces in contact are How hard the surfaces are pushed together. The greater the weight of a sliding object, the greater the force of friction

Reducing friction By adding grease to ball bearings, or putting oil into a car, we reduce the friction between moving parts (Lubricant) Removalists use trolleys to shift refrigerators because rolling surfaces produce less friction than sliding surfaces Polishing a surfboard helps to make its surface smoother and will reduce friction Vehicles such as cars and aircraft are all designed to have a streamlined shape. These shapes allow air to flow over and around them more freely, and reduce drag.

Gravity 7.3

What is Gravity? Gravity is a non-contact force that pulls objects to the centre of the earth Gravity is this force of attraction The more mass a pair of objects have, the stronger the pulling force of gravity between them. distance - The further away from an object you get, the less gravitational pull it will have

How do Tides work? The tides are controlled by the gravitational pull of the moon on planet earth, as well as the gravitational pull of the sun. We have two high tides and two low tides per day

How do tides work?

How do tides work?

Spring tide A spring tide is caused by the moon and the sun pulling on the earth from the same side Spring tides are large because the gravitational pull of both the Moon and the Sun are in the same direction

Neap tide A neap tide is caused by the moon and the sun pulling at the earth at right angles Neap tides are smaller because the Moon and Sun are pulling in different directions

orbits An orbit is a pathway that a body (planet, moon or star) travels around another body. It is the gravitational pull of one body on another that holds the smaller body in an orbit

Magnetic and electric fields 7.4

Magnets A magnet pulls, or attracts, materials containing the metals iron, cobalt or nickel. A magnetic field is the space around a magnet where a magnetic force is experienced The ends of a magnet are called poles (north and south) The magnetic field is strongest at the poles of a magnet

Magnetic fields A magnetic field is normally invisible to us. Its shape and strength can be determined either by passing a compass around a magnet, or by examining a sprinkling of iron filings around a magnet

Attraction and repulsion of poles Unlike Poles Magnetic poles may be attracted to each other, or repelled by a magnetic force Like poles will push away/repel each other Unlike poles will pull together/attract each other Like Poles

Videos – Magnetic fields

Lines of force around magnets

Static Electricity When an object becomes charged (has unequal numbers of protons and electrons), we say that it has static electricity

Levers 8.1

Machines Simple Machines make it easier to do work over a longer period of time A simple machine can make a task easier in three different ways. It can: • change the size of a force • make things speed up • change the direction of a force. An effective machine is one that reduces the amount of effort (force) required

Key words Effort is the force applied to a lever to overcome the load or get a job done. Load what is lifted or moved by a simple machine. The force actually required to do the job. Fulcrum is the pivot point/balance/turning point of a lever

1st class levers Leavers increase the size of the effort that you can apply or reduce the effort need to get a job done First class levers – The fulcrum is between the load and the effort.

1st class levers

2nd class levers Second class levers – The load is between the effort and the fulcrum

2nd class levers

3rd class levers Third class levers – The effort is between the load and the fulcrum.

3rd class levers

summary