Topic 5a, 5b, 5c, 5d

forces

What is a force?

A push or pull on an object that is caused by it interacting with something

Is a force a vector or scalar quantity?

vector.

All forces are either ____ or ______ forces

contact or non contact

What are examples of contact forces?

friction, air resistance, tension and normal contact force

What are examples of non contact forces?

Gravitational force
Electrostatic force
Magnetic force

What do vectors have that scalars don’t?

Vectors have magnitude and direction wheras scalars only have magnitude.

Give some examples of vector quantities

force, velocity, displacement, acceleration.

Give some examples of scalar quantites

speed, distance, mass, temperature.

What is an interaction pair?

a pair of forces that are equal and opposite and act on two interacting objects.

How to draw a vector?

A vector quantity may be represented by an arrow. The length of the arrow represents the magnitude, and the direction of the arrow the direction of the vector quantity.

What is weight?

Weight is the force acting on an object due to gravity.

What is the force of gravity close to the earth due to?

the gravitational field around the Earth.

What does the weight of an object depend on?

the gravitational field strength at the point where the object is.

What is the unit of force?

Newton

What is mass?

the amount of matter in an object.

What is the centre of mass?

the point at which the mass of the object may be thought to be concentrated

What would you use to measure weight?

a newtonmeter

Are weight and mass of an object directly proportional?

yasss

What is weight, w measured in?

Newtons, N

What is mass, m measured in?

Kg

What is a resultant force?

A number of forces acting on an object may be replaced by a single force (the resultant force) that has the same effect as all the original forces acting together.

How do you calculate the resultant of two forces that act in a straight line?

You can easily calculate the resultant force of two forces that act in a straight line in the same direction by adding their sizes together.

What is a free body diagram?

A free body diagram models the forces acting on an object.

e.g.: this is the free body diagram:

this is the real diagram

KNOW HOW TO DRAW SCALE DRAWINGS, BALANCED FORCES, RESOLVING FORCES

A single force can be resolved into two components acting at…

right angles to each other. The two component forces together have the same effect as the single force.

What is the definition for work done?

The energy transferred when a force moves an object through the distance

When a force causes an object to move through a distance work is done on the object. So…what?

So a force does work on an object when the force causes a displacement of the object.

When is one joule of work done?

when a force of one newton causes a displacement of one metre.

1 joule =

1 newton-metre

Describe the energy transfer involved when work is done

When you push something along a rough surface (like a carpet), you are doing work against frictional forces. Energy is transferred to the kinetic energy store of the object because it starts moving, but some is also transferred to thermal energy stores of the object, the surface, and the surroundings due to the friction.

This causes the overall temperature of the object and surface to increase.

What is the unit of distance?

metres (M)

What is the unit of work done?

joules (J)

describe the difference between elastic deformation and inelastic deformation caused by stretching forces

Elastic deformation is reversed when the force is removed. inelastic deformation is not fully reversed when the force is removed - there is a permanent change in shape.

give examples of the forces involved in stretching an object

Stretching is caused by pulling forces. To stretch a spring, we have to pull on it. This can be done by adding some small weights to the end of the spring, making it change in length.

give examples of the forces involved in bending an object

Bending forces can be push or pull. A force that causes a bend can either be a push force or a pull force.

The centre of an object will bend the most.

give examples of the forces involved in compressing an object

Compression is caused by pushing forces. To compress a spring, we have to push it. This can be done by pushing the two ends of the spring together, making it change in length.

What does applying a single force do?

causes movement, but not change in shape as only one force has been applied.

What does applying multiple forces do?

causes a change is shape.

Is the extension of an elastic object, such as a spring, directly proportional to the force applied?

yasss

What is spring constant measured in?

k, in newtons per metre, N/m

What is extension measured in?

e, in metres, m

A force that stretches (or compresses) a spring does work and… what?

elastic potential energy is stored in the spring. Provided the spring is not inelastically deformed, the work done on the spring and the elastic potential energy stored are equal.

Describe the difference between a linear and non-linear relationship between force and extension

Hooke's law is used to describe the relationship between force applied to an elastic object and the extension of the elastic object. It is a linear relationship, where force is directly proportional to the extension.

If an object doesn't obey Hooke's law, there is a non-linear relationship between force and extension.

What may a force or a system of forces cause an object to do?

rotate.

Describe examples in which forces cause rotation

For example, when you push a door, the door will turn on its hinges. The turning effect of the force causes the object to rotate.

The turning effect of a force is called..

the moment of the force.

what is moment in a force measured in?

newtonmetres, Nm

If an object is balanced, what will the total clockwise moment about a pivot equals?

the total anticlockwise moment about that pivot.

What can a simple lever or a simple gear system be used to?

to transmit the rotational effects of forces.

explain how levers transmit the rotational effects of forces

They allow a larger force to act upon the load than is supplied by the effort, so it is easier to move large or heavy objects.

explain how gears transmit the rotational effects of forces

As the small wheel turns, the rotational effect is transmitted to the larger wheel.

What can a fluid either be?

a liquid or a gas.

Why are fluids substances that can flow?

Because their particles are able to move around. As these particles move around, they collide with surfaces and other particles.

What happens when particles in a fluid collide with a surface?

They exert a force on it. As pressure is force per unit area, this means a pressure is exerted on the surface by the fluid. The pressure, and so the force, is exerted normal (at right angles) to the surface.

What is pressure measured in?

pascals, P

What is area measured in?

metres squared, m2

What will a denser liquid have in terms on particles?

A denser liquid will have more particles in a given volume. This means there are more particles that are able to collide - which means more collisions, a higher total force exerted and so a higher pressure.

Why does the depth of a liquid affect pressure?

As depth increases, the number of particles above that point increases. The weight of these particles adds to the pressure experienced at that point, so liquid pressure increases with depth.

What is upthrust?

A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards.

describe the factors which influence floating and sinking

• Density of the object – if an object is more dense that the fluid displaced, it will sink. If an object is less dense than the fluid displaced, it will float.

• Upthrust and weight of an object – if the upthrust on an object is less than the weight of the object, it will sink.

What is the atmosphere?

a thin layer (relative to the size of the Earth) of air round the Earth. The atmosphere gets less dense with increasing altitude.

Why does atmospheric pressure decreases with an increase in height?

Air molecules colliding with a surface create atmospheric pressure. The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases.

So as height increases there is always less air above a surface than there is at a lower height.

Describe a simple model of the Earth’s atmosphere

What is the definition for distance and is it a scalar or vector?

Distance is how far an object moves. Distance does not involve direction so is a scalar quantity.

What is the definition for displacement and is it a scalar or vector?

Displacement includes both the distance an object moves, measured in a straight line from the start point to the finish point and the direction of that straight line. Displacement is a vector quantity.

How do you work out somethings displacement?

measuring the final distance away from a point, and then subtracting the initial distance.

e.g. a person walks 5m north and 5m south. Calculate their displacement

0m as they ended up back at their starting position.

Does speed have direction and is it a vector or scalar?

Speed does not involve direction. Speed is a scalar quantity.

Is the speed of an object constant?

The speed of a moving object is rarely constant. When people walk, run or travel in a car their speed is constantly changing.

What is the typical speeds for walking, running, and cycling and what factors affect this?

walking ̴ 1.5 m/s

running ̴ 3 m/s

cycling ̴ 6 m/s.

The speed at which a person can walk, run or cycle depends on many factors including: age, terrain, fitness and distance travelled.

What is the typical speeds for a car, a train, and a plane?

car - 25m/s

train - 55m/s

plane - 250m/s

What is the speed of sound in air?

330m/s

What is velocity and is it a scalar or vector?

The velocity of an object is its speed in a given direction. Velocity is a vector quantity.

Explain qualitatively, with examples, that motion in a circle involves constant speed but changing velocity.

For example, a car travelling on a roundabout will move at a constant speed, but with a changing velocity, as its direction is constantly changing.

The centripetal force that acts inwards is due to the friction between the car's tyres and the road. This force keeps the car moving in a circular path.

An object that is slowing down is doing what?

decelerating

What is acceleration?

How quickly the velocity is changing.

This change in velocity can be a change in speed, a change in direction, or both.

important facts about distance-time graphs:

1. gradient = speed

2. straight sections mean its travelling at a steady speed

3. flat sections are where it stationary

4. curves represent acceleration

5. an increasing curve means its accelerating

6. a decreasing curve means its decelerating

important facts about velocity-time graphs:

1. the gradient gives the objects acceleration

2. flat sections represent steady speed

3. the steeper the graph = the greater acceleration or deceleration

4. uphill sections are acceleration

5. downhill sections are deceleration

6. a curve means changing acceleration

Near the earths surface, falling freely under gravity has an acceleration of about?

9.8 m/s squared

What will an object falling through a fluid initially do?

An object falling through a fluid initially accelerates due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity.

What are the ways of increasing the top speed of a vehicle?

1. reducing drag - this can be done by altering the shape of the vehicle to make it more streamlined

2. increasing the power of the vehicles engine - this way the driving force becomes larger so the drag force on the vehicle will equal the driving force at higher speed

What are some factors affecting terminal velocity?

The two main factors which affect the terminal velocity of an object falling through a fluid are the mass and the shape of the object.

The larger the mass of the object, the greater the weight. Objects with large surface areas will often experience a large amount of air resistance when they move.

Give an example of terminal velocity?

As the skydiver gains speed, their weight stays the same but the air resistance increases. There is still a resultant force acting downwards, but this gradually decreases.

Eventually, the skydiver's weight is balanced by the air resistance. There is no resultant force and the skydiver reaches terminal velocity.

What is newtons first law?

an object remains in the same state of motion unless a resultant force acts on it. If the resultant force on an object is zero, this means: a stationary object stays stationary.

What happens if their is a non-zero resultant force?

The velocity will change.

What is inertia?

The tendency of objects to continue in their state of rest or of uniform motion.

What is newtons second law?

force = mass x acceleration (m/s)

f=ma

What is the definition for inertial mass?

inertial mass is a measure of how difficult it is to change the velocity of an object.

What is newtons third law?

every action has an equal and opposite reaction

explain objects in equilibrium of newtons third law

Newton's third law can be applied to examples where bodies are in equilibrium . A body is in equilibrium when the forces acting on it are all balanced, so there is no overall force acting on the body.

This means that the body will either be stationary or moving at a constant speed.

What are the safety features in a car? (momentum)

-The main vehicle safety features are crumple zones, seat belts and airbags.

-For a given force upon impact, these absorb the energy from the impact and increase the time over which the force takes place

-This, in turn, increases the time taken for the change in momentum of the passenger and the vehicle to come to rest

-The increased time reduces the force and risk of injury on a passenger

What is stopping distance equation?

thinking distance + braking distance

What is thinking distance?

The distance the vehicle travels during the drivers reaction time (the time between seeing a hazard and applying the breaks).

What is the braking distance?

The distance the vehicle travels after the brakes are applied until it comes to a complete stop, as a result of braking force.

What factors is thinking distance affected by? (2)

-How fast you’re going - whatever your reaction time, the faster you’re going, the further you’ll go in that time.

-How quick to response you are, (your reaction time) - this can be affected by tiredness, drugs, alcohol, and a lack of concentration.

What is braking distance affected by? (2)

-How fast you’re going - the faster you’re going, the further it takes to stop.

-How good your brakes are - worn or faulty brakes won’t be able to apply as much force as well-maintained breaks

typical stopping distance for 30mph?

thinking: 9m

braking: 14m

typical stopping distance for 50mph?

thinking: 15m

braking: 38m

typical stopping distance for 70mph?

thinking: 21m

braking: 75m

What happens when a force is applied to the brakes of a vehicle?

there is work done by the friction between the brakes and the wheel. This reduces the kinetic energy of the vehicle, slowing it down and causing the temperature of the brakes to increase.