FORCES Chapter 7 WHAT ARE FORCES A force

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

BALANCED FORCES • 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

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

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

UNBALANCED FORCES Note that the arrows are now different lengths

UNBALANCED FORCES SUMMARY Accelerate Arrow is larger therefore moving forward Decelerate Arrow is larger therefore moving backwards

INERTIA • The tendency of an object to resist any change in its motion • 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 Contact Non. Contact Friction Gravity Pull Magnetic Push Electrostatic Contact forces touch an object Non-contact forces act at a distance

CHAPTER REVIEW Questions 1 -3 Moving Page 251 up Speeding False True False Stoping Slow down Change in direction

QUESTIONS 1. Label the four forces acting on these objects 2. Draw an arrow in RED to show these objects moving forward 3. Draw an arrow in BLUE to show the car slowing down 4. Draw an arrow in BLUE to show the plane going down

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

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

FRICTION 7. 2

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

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

REDUCING FRICTION o By adding grease to ball bearings, or putting oil into a car, we reduce the friction between moving parts (Lubricant) o. Removalists use trolleys to shift refrigerators because rolling surfaces produce less friction than sliding surfaces o. Polishing a surfboard helps to make its surface smoother and will reduce friction o. 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? o Gravity is a non-contact force that pulls objects to the centre of the earth o. Gravity is this force of attraction o. The more mass a pair of objects have, the stronger the pulling force of gravity between them.

QUES TIONS 1. Recall gravity by selecting the correct term to complete the following sentences. (a) Gravity is a contact/non-contact force. (b) Gravity pulls/pushes objects towards the Earth. (c) All objects naturally attract/repel each other. (d) Objects fall at different speeds due to their weight/surface area 2. What is the force pulling a bungee jumper down after she jumps off a bridge

MAGNETIC AND ELECTRIC FIELDS 7. 4

MAGNETS o A magnet pulls, or attracts, materials containing the metals iron, cobalt or nickel. o A magnetic field is the space around a magnet where a magnetic force is experienced o The ends of a magnet are called poles (north and south) o 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 POLESUnlike Poles o Magnetic poles may be attracted to each other, or repelled by a magnetic force o. Like poles will push away/repel each other o. Unlike poles will pull together/attract each Like Poles

VIDEOS – MAGNETIC FIELDS

LINES OF FORCE AROUND MAGNETS

QUESTION The figure shows a north pole of a magnetic and a south pole of another magnet, facing each other. The left-hand magnet is not able to move. The right-hand magnet can be allowed to slide to the left or right. What happens when the right-hand magnet is allowed to slide? A It moves to the right as a result of a contact force attract each other because like poles attract B It moves to the left as a result of a contact force C It moves to the left as a result of a non-contact force

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