Forces, Movement, Shape and Momentum (1.11-1.24)

What is a force?

A push or pull that arises from the interaction between objects

How can forces affect an object?

They can change the velocity and shape of an object by stretching, compressing or twisting it

How to calculate resultant force?

• Forces acting in the opposite direction are subtracted from each other

• Forces acting in the same direction are added together

• If forces are acting in opposite directions are equal in size then there will be no resultant forces and the forces are said to be balanced

What is friction?

A force which opposes the motion of an object

• always acts in the opposite direction to the object’s motion

• occurs when 2 or more surfaces rub against each other

Equation linking force, mass and acceleration:

F = m x a

resultant force (N) = mass (kg) x acceleration (m/s²)

This equation is also known as Newton’s 2nd law of motion

Equation for weight:

W = m x g

Weight (N) = mass (kg) x gravitational field strength (N/kg)

The gravitational field strength on Earth is 10N/kg

What is stopping distance?

The total distance travelled during the time it takes to stop the car

Formula for stopping distance:

Stopping distance = thinking distance + braking distance

(all in metres)

What is thinking distance?

The distance travelled in the time it takes for the driver to react to an emergency and prepare to stop

Main factors affecting thinking distance:

• speed of car

• reaction time of driver

What is reaction time?

A measure of how much time passes between seeing something and reacting to it

Factors affecting reaction time:

• tiredness

• intoxication (i.e. alcohol, drugs)

• distractions (e.g. loud music, texting)

What is braking distance?

The distance travelled under the braking force

Factors affecting stopping distance

• vehicle speed

  • the greater the speed, the more work the brakes need to do

• vehicle mass

  • the more massive the vehicle, the more distance it will travel until it stops

• road conditions

  • wet/icy roads make the brakes less effective and cause further travel

• driver reaction time

  • increases thinking distance

What is terminal velocity? When does it happen?

The fastest speed an object can reach when falling

It is reached when the upward and downward acting forces are balanced: the resultant force on the object reaches 0

The object is no longer accelerating

How does an object reach terminal velocity?

• initially, a falling object accelerates since the initial resultant force is very large

  • this is because there is a large unbalanced force acting downwards (weight)

• the faster it falls, the larger the force of air resistance is which acts on it

  • this is due to the force of friction between the object’s motion and collisions with air particles

• eventually it falls at a steady speed when the force of air resistance equals the force of weight - the resultant force is 0 and the object no longer accelerates

What is Hooke’s Law?

The extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality

What is the limit of proportionality?

The point beyond which the relationship between force and extension is no longer directly proportional

What is extension?

The initial length of an unstretched object subtracted from the length of the same stretched object

PRACTICAL: investigate how extension varies with applied force for helical springs, metal wires and rubber bands

Method

1. record initial length of rubber band

2. add 100g mass hanger onto the spring

3. record mass (in kg) and extended length from original (in cm)

4. add another 100g to mass hanger

5. record new mass and position now that spring has extended further

6. repeat process until all masses have been added

7. remove masses and repeat entire thing again

Showing the limit of proportionality on a force-extension graph

Any material beyond its limit of proportionality will have a non-linear relationship between force and extension

What is elastic behaviour?

The ability of a material to recover its original shape after the forces causing the deformation

have removed

What is elastic deformation?

When the object DOES return to its original shape after the deforming forces are removed

It results in a change to the object’s shape that is NOT permanent

Examples of materials that undergo elastic deformation:

• rubber bands, fabrics, steel springs

What is plastic deformation?

When the object DOESN’T return to its original shape after the deforming forces have been removed

It results in a change to the object’s shape that is PERMANENT

Examples of materials that undergo plastic deformation:

• plastic, clay, glass