Forces

What is a scalar quantity?

Magnitude only

What is a vector quantity?

Magnitude and an associated direction

What does the length and direction of an arrow representing a vector quantity show?

Length: magnitude
Direction: direction of the vector quantity

What is a force?

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

What is a contact force?

Objects that are physically touching

What is a non-contact force?

Objects are physically separated

What are some examples of contact forces?

• Friction

• Air resistance

• Tension

• Normal contact force

What are some examples of non-contact forces?

• Gravitational force

• Electrostatic force

• Magnetic force

What is weight?

The force acting on an object due to gravity

What is the force of gravity close to the Earth caused by?

The gravitational force around the Earth

The weight and mass of an object are… what?

Directionally proportional

How is weight measured?

By using a calibrated spring-balance

What is a resultant force?

A single force that has the same effect as all the original forces acting together

How can a single force be resolved?

It can be resolved into two components acting at right angles to each other

When is work done?

When a force causes an object to move through a distance. A force does work when the force causes a displacement.

What is one joule of work done?

When a force of one newton causes a displacement on one metre
1 joule = 1 newton-metre

What is the energy transfer involved when work is done?

Kinetic energy

When you apply a force to an object, what might happen?

Stretch, compress or bend

What is elastic deformation?

When an object can go back to its original shape and length after the force has been removed

What is inelastic deformation?

When an object doesn’t return to its original shape and length after the force has been removed

What is the extension of an elastic object proportional to?

The force applied, provided that the limit of proportionality is not exceeded

Where is elastic potential energy stored?

In a spring

What is a linear relationship between force and extension?

The change in the force is directly proportional to the change in extension

What is a non-linear relationship between force and extension?

The change in force is not directly proportional to the change in extension. Could be exponential, logarithmic etc…

What is a moment?

The turning effect of a force

If an object is balanced the what is equal to the what?

The total clockwise moment about a pivot equals the total anticlockwise moment about that pivot

What can be used to transmit the rotational effects of forces?

A simple lever and a simple gear system

How do levers transmit the rotational effects of forces?

A force is applied to one end of the lever and rotates around the fulcrum. The lever amplifies or changes the direction of the force applied.

How do gears transmit the rotational effects of forces?

When one gear it turned, it causes other connected gears to rotate

What does the pressure in a fluid cause?

A force normal at right angles to any surface

Why does the pressure point of a liquid increase with the height of the column of liquid above that point?

As you go deeper into a liquid the weight of the liquid above that point exerts a downward force. This results in an increase in pressure at that point.

Why does the pressure point of a liquid increase with the density of the liquid?

A denser liquid has more mass packed into a given volume leading to more particles in close proximity, leading to a higher pressure as there are more frequent collisions.

What is upthrust?

When 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.

What is the atmosphere?

A thin layer of air round the Earth. It gets less dense with increasing altitude

What creates atmospheric pressure?

Air molecules colliding with a surface

Why does atmospheric pressure decrease with an increase in height?

As height increases the air is less dense above a surface than it is at a lower height. This means there are fewer particles colliding with a given area.

What is distance? Is it scalar or vector?

Distance is how far an object moves. It’s a scalar quantity.

What is displacement? Is it scalar or vector?

The distance an object moves, measured in a straight line from the start to the finish point, and the direction of that straight line. It’s a vector quantity.

Is speed scalar or vector?

Scalar

What is the typical speed for walking?

1.5 m/s

What is the typical speed for running?

3 m/s

What is the typical speed for cycling?

6 m/s

Are most human speeds and the speed of sound/wind constant or do they vary?

They vary

What is a typical value for the speed of sound in air?

330 m/s

What is velocity? Is it scalar or vector?

The velocity of an object is its speed in a given direction. It’s a vector quantity.

What happens when an object travels around a circular path?

The object moves around a circle at a consistent speed but the velocity changes continuously as the direction of motion changes, leading to a changing velocity.

How can the speed of an object at a particular time be determined if the object is accelerating?

By drawing a tangent

How can the acceleration of an object be calculated?

From the gradient of a velocity-time graph

How can the distance travelled by an object be calculated?

From the area under a velocity-time graph

What is the acceleration of a free falling object near the Earth’s surface?

9.8 m/s²

Why does an object falling through a fluid initially accelerate?

Due to the force of gravity

What’s terminal velocity?

The highest velocity than an object in free fall can achieve when the drag force equals the gravitational force pulling the object down

What factors affect terminal velocity?

Size, shape, mass, density of fluid (liquid or gas) its falling through

What is Newton’s First Law?

If the resultant force acting on a stationary object is zero, the object will remain stationary.
If the resultant force on a moving object is zero, it will continue moving at the same velocity.

What is inertia?

The tendency of objects to continue in their state of rest or of uniform motion is called inertia

What is Newton’s Second Law?

When the acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.

What is inertial mass?

How difficult it is to change the velocity of an object

How can inertial mass be defined, in regards to a ratio?

It can be regarded as the ratio of force acting on an object to the acceleration produced by that force.

What is Newton’s Third Law?

Whenever two objects interact, the forces they exert on each other are equal and opposite

What is stopping distance?

The sum of the distance the vehicle travels during the driver’s reaction time (thinking distance) and the distance it travels under the braking force (braking distance)

What is the range of a typical reaction time?

0.2s to 0.9s

What can reaction time be affected by?

• Tiredness

• Drugs

• Alcohol

• Distractions

What can affect the braking distance of a vehicle?

• Wet and icy conditions (adverse road conditions)

• Poor condition of the vehicle e.g. brakes or tyres

What happens during braking?

A force is applied to the brakes of a vehicle so work is done by the friction between the breaks and wheel. This reduces the kinetic energy and the brake temperature increases.

What increases as the speed of a vehicle increases?

The braking force needed to stop a vehicle in a certain distance

What may large decelerations lead to?

Overheating breaks and/or loss of control

What are some of the forces involved in the deceleration of road vehicles?

• Friction

• Air resistance

• Gravity

What is momentum?

The product of an objects mass and velocity. It’s a vector quantity.

What is the conservation of momentum?

When the total momentum before an event is equal to the total momentum after the event

When does a change in momentum occur?

When a force acts on an object that is moving or is able to move

What are some safety feature in a car?

• Crumple zones increase time taken for car to stop

• Seat belts stretch, increasing time taken for wearer to stop

• Air bags inflate before passenger hits the dashboard

What is a safety feature of bike helmets?

Contain a crushable layer helping to lengthen the time for your head to stop in a crash

What is a safety feature of crash mats and cushioned playground flooring?

Increase time taken for you to stop if you fall because they’re made from soft, compressible materials.