GCSE Physics- Forces

What is a fluid

A liquid/gas

In any fluid, at what angle do the forces due to pressure act on a given surface?

Right angles normal to the surface

What is pressure?

The force per unit area

Why does pressure in a liquid increase with depth?

-As depth increases, the mass of liquid above that depth also increases

-Therefore the force due to the mass increases

-Since the force has increased but the area has stayed the same, the pressure will increase

What is the unit of pressure

Pascals (Pa)/ N/m²

What is the difference between the pressure of a submerged object and a floating object?

-A submerged object experiences a greater pressure on the bottom surface than on the top surface

-This creates a resultant force upwards called upthrust

What are factors that affect floating and sinking?

-Amount of upthrust

-Density

-Mass

What is the atmosphere?

A thin layer (relative to the size of the Earth) of air round the Earth

What is the trend of density of the atmosphere as altitude increases?

It gets less dense with increasing altitude

Why does atmospheric pressure decrease 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

-So atmospheric pressure decreases with an increase in height

What is a force?

A push or a pull that acts on an object due to an interaction with another object

What is distance?

The length of the space between two points

What is displacement?

Displacement is the direct length between any two points when measured along the minimum path between them

What is the difference between distance and displacement?

Displacement is a vector so it has both magnitude and direction but distance is scalar so it only had magnitude

Scalar

Only has magnitude

Vector

Has magnitude and direction

What can forces do?

Change the motion of an object

How are forces measured?

Newtons

What is a contact force?

Any force that occurs as a result of two objects making contact with each other

What is a resultant force?

A single force which replaces several other forces

What would happen if the resultant force was 0?

The object would remain stationary

How to you calculate the resultant force when the two forces are opposite?

Subtract the smaller force from the larger force

What is the centre of mass of an object?

The average position of all the parts of the system weighted according to their masses

How do you calculate the centre of mass on a shape?

-Hang a shape from two different points (one at a time)

-Draw a line straight down from each point

-The center of mass is where those lines intersect

How does centre of mass affect balance?

-The lower center of mass the better stability

What is the parallelogram of forces used for?

A method for solving the results of applying two forces to an object.

Equilibrium

When all external forces are balanced

Newton’s Third law of Motion

Every action there is an equal and opposite reaction

How does acceleration depend on the size of resultant force?

The greater the resultant force on the object, the greater the object’s acceleration

What is the effect of mass on acceleration?

The greater the mass of an object, the smaller its acceleration for a given force

What is the formula for resultant force?

mass (kg) x acceleration (m/s2)

What is inertia?

How difficult it is to change an object’s speed or direction

What is the difference between mass and weight?

The weight of an object is the force acting on the object due to gravity, mass is the quantity of matter in the object

What is the motion of a falling object in terms of forces involved?

-First the object accelerates due to gravity

-As the object speed increases, frictional forces like air resistance/drag increase

-At terminal velocity, the weight of an object due to gravity is balanced by the frictional forces, so the resultant force is 0

What is terminal velocity?

The maximum speed of an object when the forces acting upon it are balanced

What are the forces acting upon an object at terminal velocity?

Gravity and Friction

What are the forces opposing a moving vehicle?

Friction and Air Resistance

What are the factors affecting stopping breaking distance?

-Speed

-Poor weather conditions

-Poor vehicle maintenance

-Mass of vehicle

-Tire condition

-Quality of Breaks

What are the factors affecting stopping thinking distance?

-Speed

-Reaction time of driver (tiredness, alcohol, drugs, phone usage)

What is stopping distance?

The distance travelled before a vehicle stops

What is thinking distance?

The distance travelled by the vehicle in the time it takes the driver to react

What is breaking distance?

The distance travelled by the vehicle during the time the breaking force acts

How can you estimate stopping distance?

Thinking distance + Breaking distance

How can you calculate the momentum of an object?

mass (kg) x velocity (m/s)

What is momentum?

The product of an objects mass and velocity

What is the unit for momentum?

kg m/s

What is law of conservation of momentum?

In a closed system, the total momentum before an event is equal to the total momentum after the event

How is momentum is conserved in an explosion?

As momentum is a closed system, as long as no outside forces act on the object it will be conserved

What does elastic mean?

If an object returns to its original shape after removing the force deforming it

What is the relationship between the extension of a spring and the force applied? (Hooke’s Law)

Directly proportional

What is meant by the limit of proportionality/the elastic limit of a string?

The point which Hooke’s law is no longer true when stretching a material

Why is momentum classified as a vector quantity?

It has both size and direction

Why do two objects in an explosion move away at different speeds?

Unequal masses

What happens to the total momentum of two objects when they collide?

When two objects collide the total momentum before the collision is equal to the total momentum after the collision (in the absence of external forces)

What can affects force of impacts during a collision?

Time and Speed

What is impact force?

The force applied to a person during a collision

Why does impact force depend on impact time?

If the impact time is increased the force on the vehicle will be less

Why do cycle helmets and cushioned surfaces reduce impact forces?

They increase the impact time

Why do seat belts and air bags reduce the impact forces in car accidents?

They spread the force across the chest and increase the impact time

Hoe do side impact bars/ crumple zones work?

They give way in impact, and so increase the impact time

Equation for speed

Speed = Distance/Time

Speed

Rate of change of distance

Velocity

Speed in a direction

Acceleration

Rate of change in velocity

Decceleration

Negative acceleration

Displacement

Distance from start to end point

What is the gradient of the line on a distance time graph used to determine?

Speed

What is the difference between speed and velocity?

Velocity has direction and magnitude

Acceleration equation

Acceleration = Change in velocity/Time taken for change

What is the difference between Acceleration and Deceleration?

Acceleration is speeding up and acceleration is slowing down