PMT FORCES

What is a scalar quantity?

A quantity that only has a magnitude

A quantity that isn’t direction dependent

What is a vector quantity?

A quantity that has both a magnitude and an associated direction.

How can a vector quantity be drawn and what does it show?

* As an arrow


* The length of the arrow represents the magnitude


* The arrow points in the associated direction

What is a force?

A push or pull acting on an object due to an interaction with another object.

What are the two categories that all forces can be split into?

1\. Contact forces (objects touching)

2\. Non-contact forces (objects separated)

Give three examples of contact forces.

1\. Friction

2\. Air resistance

3\. Tension

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Give three examples of non-contact forces.

1\. Gravitational forces

2\. Electrostatic forces

3\. Magnetic forces

Is force a vector or a scalar quantity?

* Vector


* It has both a magnitude and an associated direction

Give three examples of vector quantities.

1\. Velocity

2\. Displacement

3\. Force


4. acceleration
5. momentum

Give three examples of scalar quantities.

speed

distance

temperature

mass

time

Give three examples of scalar quantities.

● Temperature

● Time

● Mass

● Speed

● Distance

● Energy

What is weight?

The force that acts on an object due to gravity and the object’s mass.

What quantities does weight depend on?

Weight = mass x gravitational field strength

The object’s mass

The gravitational field strength at the given position in the field

What is the unit used for weight?

The Newton (N).

What is the unit used for gravitational field strength?

N/kg

What is meant by an object’s centre of mass?

The single point where an object’s weight can be considered to act through.

What piece of equipment can be used to measure an object’s weight?

A calibrated spring-balance or newton- meter.

What is the name given to the single force that is equivalent to all the other forces acting on a given object?

The resultant force.

Relationship between mass and weight

If you increase mass, weight will increase= directly proportional

What does it mean if a force is said to do ‘work’?

The force causes an object to be displaced through a distance.

What is the equation used to calculate work done? Give appropriate units.

Work done = Force x Distance

Work done (Joules), Force (Newtons), Distance (metres)

What distance must be used when calculating work done?

It must be the distance that is moved along the line of action of the force.

Under what circumstance is 1 joule of work done?

When a force of 1 Newton causes a displacement of 1 metre.

How many Newton-metres are equal to 1 joule of energy?

1 Nm = 1 J

How much work is done by the force acting on the below object over a distance of 5m left ?

10 x 5 = 50 Nm = 50 J

What occurs when work is done against frictional forces?

● A rise in temperature of the object occurs

● Kinetic energy is converted to heat

what is a resultant force

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* When you have a number of forces acting on a single point and you replace them with a single force (aqa)
* The overall force/ interaction the forces have on a single object

distance time graphs

increasing slope- moving, steeper slope, faster movement

flat line- not moving

gradient- speed = up/across= distance/ time

velocity time graphs

increasing slope- increasing acceleration

flat line- moving at steady speed

gradient- acceleration = up/across= velocity/ time

area under graph- distance travelled

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triangle= 1/2bh

rectangle=bh

Why does air resistance slow down a projectile?

● The object does work against the air resistance

● Kinetic energy is converted in to heat, slowing down the object

Explain the relationship between the force applied and the extension of an elastic object.

The extension is directly proportional to the force applied, provided that the limit of proportionality is not exceeded.

What is meant by an inelastic (plastic) deformation?

● A deformation which results in the object being permanently stretched

● The object doesn’t return to its original shape when the force is removed

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* Cannot return to its original shape and length after force is removed 

State the equation relating force, spring constant and extension. Give appropriate units.

Force = Spring Constant x Extension

Force (N), Spring Constant (N/m) Extension (m)

What type of energy is stored in a spring when it is stretched?

Elastic potential energy.

Compress-

object stays the same

Stretch

doesn't/ past elastic limit 

Bend

dependent on force applied

Relationship between to extension and force

Force is directly proportional to extension up unit limit of proportionality

limit of proportionality-

force is no longer proportional to extension (elastic limit)

What can extension be replaced with in the equation for spring force?

Compression.

What is an alternative name for the turning effect of a force?

A moment.

Relationship between a moment and a force

Larger force = larger moment

State the equation used to calculate the moment of a force. Give appropriate units.

Moment of force = Force x Distance

Moment (Nm), Force (N), Distance (m)

What distance measurement is used when calculating a moment?

The perpendicular distance from the pivot to the line of action of the force.

If an object is in equilibrium, what can be said about the moments acting on the object?

The clockwise moments are equal to the anticlockwise moments.

A FD = C FD

* Increase the distance from the pivot at which the force is applied 
* Less force needed for same moment

What three parts make up a lever system?

1\. Load 2. Effort 3. Pivot

what are levels

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* Increase the distance from the pivot at which the force is applied 
* Less force needed for same moment

what are gears

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* Larger gear bigger moment as distance from pivot is greater
* Lower
* Rotational effect at a force from one gear to another

How can a lever be used as a force multiplier?

If the distance between the effort and pivot is greater than the distance between the pivot and load, the force applied on the load is greater than the effort force. This is since the moment on both the effort and load must be the same.

Give an example of when a lever may be used to multiply a force.

A wrench, which has a long handle so that the force applied by the user is multiplied.

What determines the moment of a gear wheel?

The size of the wheel.

Describe the moment and speed of a larger gear being driven by a smaller gear.

The larger wheel will rotate more slowly but will also produce a larger moment.

What is meant by the term ‘fluid’?

A liquid or a gas.

* substances that can flow as particles can move around

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

At right angles (normal to) the surface.

* Forces can be exerted at right angles to  any surface in contact with fluid 

State the equation used to calculate pressure. Give appropriate units.

Pressure = Force/Area

Pressure (Pascals), Force (Newtons), Area (metre2)

Fluid pressure is dependent on:

Density

\- how close particles are/ compactness, same (size and shape)  

\-More dense = more particles in a certain area= collisions more frequent= higher pressure

Depth

\- higher depth, no. of particles above increases

\-Height of tehse adds to pressure

GFS

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What happens to the density of the atmosphere with increasing altitude?

The atmosphere becomes less dense as altitude increases.

Explain why atmospheric pressure decreases with an increase in height.

● Pressure is created by collisions of air molecules

● The quantity of molecules (and so weight) decreases as the height increases

● This means atmospheric pressure decreases with an increase of height

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* Ap is created on a surface by air molecules colliding with that surface
* As altitude ( height above earth) increases, AP decreases 
* Atmosphere is less dense, so there are fewer air molecules which are able to collide to surface
* As altitude increases, weight of particles decrease

What is the Earth’s atmosphere?

A thin (relative to the magnitude of the Earth) layer of gas surrounding the Earth.

what is terminal velocity

reached when all forces are balanced

\-very fast acceleration as object initially starts to fall

\-objects stars to balance out it would slow

\-once terminal velocity reached, no further increase

what is a persons freewill acceleration

due to gravity

9\.8 m/s

How does pressure in fluids increase with depth?

As the depth increases, the mass of liquid above that depth also increases. This means that the force due to the mass increases. Since the force has increased whilst the area has remained constant, the pressure will increase.

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* Pressure increases with increase in depth
*  the force exerted on bottom is larger than the force exerted on top
* This causes an upwards resultant force (upthrust)

Upthrust

weight of fluid that has been displaced by object

Why does pressure in fluids increase with density?

As the density of a fluid increases, the number of particles in a given volume increases. Consequently the weight of the fluid is greater. This means that the force from the fluid above a certain point is larger. Since the force has increased, the pressure also increases.

What equation shows the magnitude of pressure in liquids at different depths?

Pressure due to a column of liquid(Pa) = Column height(m) x density(kg/m3) x gravitational field strength(N/kg)

P = hρg

Why are objects in a fluid subject to an upthrust?

* When an object is submerged in a fluid, it has a higher pressure below it than it does above
* as there is more weight above it at the bottom than there is at the top.
* This leads to an upwards force called upthrust.
* You float or sink depending on whether the upthrust is more or less than your weight.

What is upthrust always equal to?

The weight of the fluid that the object displaces.

What factors influence whether an object will sink or float?

● Upthrust

● Weight

● Density of fluid

Explain why an object with a density greater than that of water can never float.

Upthrust is equal to the weight of the displaced fluid. If the density of the object is high, there would not be enough volume displaced to produce an upthrust larger than the object's weight. This means that it will sink.

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* If an object is denser than the fluid its placed in, this means that is unable to displace enough fluid equal to weight
* The weight is more than the upthrust therefore it sinks

Explain why an object with a density less than that of water can float.

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* If an object that is less dense than the fluid its placed in, this means that it weighs less than the fluid
* This means that it displaces a volume of fluid that's equal to its weight before it completely submerges 
* Weight = upthrust therefore floats

Does a distance quantity require a specific direction? i.e. Is it a scalar or vector quantity?

No specific direction is required so it is a scalar quantity.

If an object moves 3 metres to the left and then 3 metres back to its initial position, what is the object’s total displacement?

● The object has zero displacement

● Displacement is a vector quantity so it also involves direction

● The object starts and ends at the same point

State a typical value for the speed of sound.

330 m/s

What is a typical value for human walking speed?

1\.5 m/s

What is a typical value for human running speed?

3 m/s

What is a typical value for human cycling speed?

6 m/s

State the equation linking distance, speed and time. Give appropriate units.

Distance = Speed x Time

Distance (m), Speed (m/s), Time (s)

Why can an object travelling at a constant speed in a circle not have a constant velocity?

● Speed is a scalar quantity

● Velocity is a vector quantity which means it can only be constant if the direction is constant

● In circular motion, the direction is continuously changing

How can speed be calculated from a distance-time graph?

The speed is equal to the gradient of the graph.

What must be done to calculate speed at a given time from a distance-time graph for an accelerating object?

● Drawing a tangent to the curve at the required time

● Calculating the gradient of the tangent

State the equation for the average acceleration of an object. Give appropriate units.

Acceleration = (Change in Velocity)/(Time Taken)

Acceleration (m/s2), Velocity (m/s), Time (s)

How can the distance travelled by an object be calculated from a velocity-time graph? (Higher)

It is equal to the area under the graph.

Give an approximate value for the acceleration of an object in free fall under gravity near the Earth’s surface.

9\.8 m/s2

law of conservation of momentum

momentum is always conserved

What can be said about the resultant force acting on an object when it is falling at terminal velocity?

● The resultant force is zero

● When at terminal velocity, the object is moving at a constant speed and so isn’t accelerating

what is acceleration

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* Change in velocity in a given time

State Newton’s first law for a stationary object.

If the resultant force on a stationary object is zero, the object will remain at rest.

State Newton’s first law for a moving object.

If the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction).

State Newton’s first law for a stationary object.

If the resultant force on a stationary object is zero, the object will remain at rest.

State Newton’s first law for a moving object.

If the resultant force on a moving object is zero, the object will remain at constant velocity (same speed in same direction).

What can be said about the braking forces and driving forces when a car is travelling at constant velocity?

The braking forces are equal to the driving forces.

If an object changes direction but remains at a constant speed, is there a resultant force?

Since there is a change in direction, there is a change in velocity and so there must be a resultant force.

What is inertia?

The tendency of an object to continue in its state of rest or uniform motion.

State the defining equation for Newton’s Second Law.

Resultant force = Mass x Acceleration

F = ma

State Newton’s Second Law in words.

An object’s acceleration is directly proportional to the resultant force acting on it and inversely proportional to its mass.

What is inertial mass?

● A measure of how difficult it is to change a given object’s velocity

● The ratio of force over acceleration

What is the symbol used to represent an approximate value?

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State Newton’s Third Law.

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

* every action has an equal and opposite reaction

What is the stopping distance of a vehicle equal to?

The sum of thinking distance and braking distance.

For a given braking distance, if the vehicle’s speed is increased, what can be said about its stopping distance?

The stopping distance and thinking distance is increased with an increase in speed.