Forces and their effects

What is a force? A force is a push or a pull. A force cannot be seen, but you can see how a force effects an object.

What type of force?

What is a force? A force is a “push” or a “pull”. Some common examples: ________ – pulls things downwards ___ ________ (drag) – acts against anything moving through air ______ – keeps things afloat _____ – acts against anything moving Words – upthrust, air resistance, friction, weight

A force can change the shape of an object What is a force? A force can make an object move faster or slower A force can make an object change direction The greater the force, the greater it’s effect All forces are measured in NEWTONS

Weight and Mass The MASS of an object is a measure of how much matter an object is made of. The WEIGHT of an object is the amount of force that pulls it downwards due to gravity. 1 K g This object has a MASS of 1Kg… …and a WEIGHT of 10N

What are mass and weight? Mass and weight are not the same! Mass is the amount of matter in an object and is measured in kilograms. Mass is not a force. Mass will have the same value anywhere in the Universe, including space. Weight is a force and is caused by the pull of gravity acting on a mass. Like other forces, weight is measured in Newtons and has both magnitude and direction.

Weight and mass on the Moon The force of gravity is less on the Moon than it is on Earth. This is because the Moon has a smaller mass than the Earth. Any object will weigh less on the Moon than it does on the Earth. An astronaut could jump 6 metres into the air on the Moon because gravity is less. However, the astronaut still has the same body and the same mass: he just weighs less because gravity is weaker on the Moon.

Talking about weight and mass A scientist would never say “She weighs 50 kilograms,” but should always say: “She has a mass of 50 kilograms”. Alternatively, the scientist could say: “The gravitational force acting on her mass is about 500 Newtons”. This the same as saying: “Her weight is about 500 Newtons”.

Balanced and unbalanced forces If two forces are the same size and pull in opposite directions, they are called balanced forces When forces push against each other and one force is bigger than the other, they are called unbalanced forces

Balanced and unbalanced forces Consider a camel standing on a road. Weight Reaction These two forces would be equal – we say that they are BALANCED. The camel doesn’t move anywhere. What forces are acting on it?

Balanced and unbalanced forces What would happen if we took the road away? Weight Reaction The camel’s weight is no longer balanced by anything, so the camel falls downwards…

Balanced and unbalanced forces What would happen if we took the road away? Weight The camel’s weight is no longer balanced by anything, so the camel falls downwards…

Balanced and unbalanced forces What would happen if we took the road away? Weight The camel’s weight is no longer balanced by anything, so the camel falls downwards…

Balanced and unbalanced forces What would happen if we took the road away? Weight The camel’s weight is no longer balanced by anything, so the camel falls downwards…

Balanced and unbalanced forces What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards…

Balanced and unbalanced forces What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards…

Balanced and unbalanced forces What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards…

Balanced and unbalanced forces What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards…

Balanced and unbalanced forces What would happen if we took the road away? The camel’s weight is no longer balanced by anything, so the camel falls downwards…

Balanced and unbalanced forces 1) The car is either ________ or moving at _______ _____. 4) The car is also _______ or moving at ________ ______. 2) The car is getting ________. 3) The car is getting _______. Words - Stationary, faster, slower or constant speed?

Some more type of forces Magnetic forces Can be felt by holding the end of the magnets together. Sometimes the forces are attractive forces (pulls), sometimes they are repulsive forces (pushes) depending on which way the magnets are facing.

Some more type of forces Impact forces A stationary object can be set in motion if a moving object collides with it. The object exerts force on it, an impact force.

Some more type of forces Strain forces Stretching, twisting or squashing exerts a strain force on an object.

Stretching materials When you pull on the end of a spring it gets longer. The size of the extension of the spring depends on the size of the force. Springs made from thicker wire need bigger forces to stretch them than springs made of thinner wire.

Stretching materials In string instruments, the string is stretched so it is tight enough to give the right note. Thicker strings give low notes; thinner strings give high notes. The top E string on an electric guitar needs the force of about 75N to stretch it into tune. When it is stretched into tune, it is about 7mm longer than when it is slack. The total force on the tailpiece of an electric guitar when it is in tune is about 530N – enough to pick up a 53kg person

What is friction? If you rub your hands together they get warm. There is resistance to the rubbing motion. What is this resistive force called? Your hands might look smooth but on a microscopic level they have rough surfaces. When you rub your hands together you feel the resistive force of friction. FRICTION What causes this force?

More about friction? Friction always tries to slow moving objects down. We say it opposes motion. Friction is created whenever two touching objects, or surfaces, move past each other. Friction also occurs when things move through air. This is called air resistance or drag. There is a maximum value for the frictional force which depends on: –The force pushing the two surfaces together. –The state of the surfaces in contact.

What are the sources of friction? Label all sources of friction that can act on this bike wheel bearingpedal bearingwheel bearing brake pad and rim air resistance or ‘drag’ the most important

Effects of frictional forces

Friction 2. Give 3 examples when it is annoying. 1. What is friction? 3. Give 3 examples where it is useful. 4. What are the three main ways to reduce friction?

Friction When two surfaces rub together they will eventually become worn down e.g. machine parts. Friction uses up energy – movement energy is converted to heat energy through friction. Rusty bolts sticking instead of turning: knots in natural fibre difficult to undo. 2. Give 3 examples when it is annoying: Friction is a force that is exerted when things rub together. 1. What is friction?

Friction 3. Give 3 examples where it is useful: Bikes and cars have brakes that use friction to slow them down or stop them. Without friction we could not walk. When you walk you push your foot backwards. The effect of friction between your foot and the floor is to move you forward. Drag (friction force when something slides through air) useful to slow down a parachutist.

Friction 4. What are the three main ways to reduce friction? Make the surfaces smooth. Use a lubricant e.g. oil. Design the moving parts so they roll on each other rather than slide.

Floating Whether or not an object will float depends on its DENSITY. For example: The wooden block will ____ because it is ____ dense than water The metal block will ____ because it is ____ dense than water

Floating Whether or not an object will float depends on its DENSITY. For example: The metal block will sink because it is more dense than water The wooden block will ____ because it is ____ dense than water

Floating Whether or not an object will float depends on its DENSITY. For example: The metal block will sink because it is more dense than water The wooden block will float because it is less dense than water

Ferry forces Label the forces acting on the ferry as it sails to Ireland upthrust or buoyancy air resistance friction thrust weight

Changing forces The ferry arrives at Dublin and is loaded with travellers What happens to the weight of the ship? What happens to the upthrust on the ship? Which is the largest force – weight or upthrust? increases both equal

Lift force (upthrust) and aircraft Air flowing over the wing has further to go than the air moving under the wing so it travels more quickly on the top of the wing. This creates a lift force which acts towards the region where air is travelling faster. Air flow Aircraft wing LIFT

Lift force (upthrust) and aircraft Upthrust – due to wings Forward thrust – due to engine Backward thrust – due to air resistance Weight

Lift force (upthrust) and aircraft Plane flies at a constant height when weight = upthrust (lift force) Plane flies at a constant speed when engine thrust = air resistance