Forces, movement, shape and momentum

(as momentum is for Single Science, I won't be talking about it unless you want smth on it, anything for Single Science is labelled SS in front of the specification point.)

1.8 express a force as a push or pull of one body on another

A force is a push or pull that one object exerts on another
3 things a force can do to an object:

• change its speed
• change is shape
• change its direction

1.9 identify various types of force (for example gravitational, electrostatic etc.)

various types of force:

• push/pull (contact force)
• tension-the pull at both ends of a stretch spring, string or rope
• compression
• thrust/upthrust
• load
• effort
• *weight/gravitational
• *electrical/electrostatic
• *magnetic

*the last three forces are non-contact, they can act without touching an object

SS 1.10 distinguish between vector and scalar quantities 

Scalars have magnitude (size), Vectors have both magnitude and direction. 

Scalars	Vectors
Speed	Velocity
Distance	Displacement
Mass	Weight/tension/compression à forces
Temperature	Thrust
Energy	Drag
Charge	Upthrust
Volume	Acceleration
Area	Field strength – magnetic/electrical/gravitational

SS 1.11 appreciate the vector nature of a force

Forces have magnitude and direction, it is therefore a vector. For example, weight is a force with magnitude, and it acts downwards. It is a vector, and you would normally use an arrow pointing downwards with its magnitude ("xN" N being the unit-Newtons) to represent it in a diagram.

SS 1.12 add forces that act along a line

The forces act along a line, meaning they are collinear. You can add the forces together like scalars. For instance if two forces are acting on a box, both pushing towards the right side, one with a force of 4N and the other with 6N, then 4N+6N=10N (resultant force). So a total of 10N is acting on the box, pushing it to the right. 

However, if one of the forces was acting to the left with say, 2N and the other to the right with 7N, then the resultant is 5N to the right. The 2N to the left cancels out 2N from the right.  You can think of the force acting to the left as a negative value, like the reverse direction, so 7N + (-2N) = 5N. It's good to use diagrams to help you out. 

1.13 understand that friction is a force that opposes motion

Friction is a force that always opposes motion between 2 surfaces in contact.

1.14 recall and use the relationship between unbalanced force, mass and acceleration:

force=mass x acceleration

F= m.a

1.15 recall and use the relationship between weight, mass and g:

weight= mass x g

W= m x g

g=gravitational field strength, this is the gravitational force exerted per unit mass at a point in the field, it is a vector quantity. unit= N/kg

on earth, we consider g as 10N/kg, or g = 9.8 m/s² but we use g = 10 m/s² this is how an object in the Earth's gravitational field would accelerate if it was free fall-without friction (this doesn't happen in the real world). 

( to explain how we get to this:

a= F/m = mg/m = g ) 

1.16 describe the forces acting on falling objects and explain why falling objects reach a terminal velocity

2 forces act on falling objects: 

• Earth's gravity-Weight (W): has direction (vector quantity), pulls object downwards towards the centre of the Earth
• air resistance/drag (D)--upwards force, pushes object upwards

Air resistance increases with speed. When an object first starts falling, their weight is greater than air resistance. (W>D) Hence it falls faster, accelerating towards the ground. However, as it gains speed, the air resistance increases until it eventually equals the object's weight. (W=D) Acceleration then becomes zero (a=0) and the object will have reached terminal velocity. 

1.17 describe the factors affecting vehicle stopping distance including speed, mass, road condition and reaction time

vehicle stopping distance= *( reaction time (a.k.a thinking time) x constant speed ) + breaking distance 

vehicle stopping distance= thinking distance + braking distance

*d=s x t, so it becomes thinking distance

thinking distance: how far the car travels at constant speed before the driver reacts by applying the car brakes

braking distance: distance travelled by the car as it decelerates to a stop

• as speed increases, stopping distance increases
• as mass increases, force needed to stop car increases (F=ma)
• dry weather=more friction, rainy=less friction, as water acts as lubricant
• as reaction time increases, stopping distance increases (If driver was drunk, their reaction would be slower, so reaction time increases, hence stopping distance increases)

SS 1.18 recall and use the relationship between momentum, mass and velocity:

momentum = mass x velocity

p= m x v

SS 1.19 use the ideas of momentum to explain safety features

SS 1.20 use the conservation of momentum to calculate the mass, velocity or momentum of objects

SS 1.21 use the relationship between force, change in momentum and time taken:
force= change in momentum/time taken

SS 1.22 understand Newton's third law

Newton's Third Law of Motion:

For every action, there is an equal and opposite reaction, and these forces act on mutually opposite bodies. 

i.e. forces occur in pairs.

Fab = -Fab

E.g. when an object is falling, not only is weight acting on it downwards, but there is also air resistance pushing it upwards -- the forces act in opposite directions on the same object.
Or for instance, when a car is driving, the driving force pushes the car forwards, but there is also friction acting in the opposite direction, if the driving force wasn't bigger than friction, the car would be slowing down...

1.23 recall and use the relationship between moment of a force and its distance from the pivot

moment= force x perpendicular distance from the pivot

moment= F x d

1.24 recall that the weight of a body acts through its centre of gravity

SS 1.25 recall and use the principle of moments for a simple system of parallel forces acting in one plane

SS 1.26 understand that the upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam

1.27 describe how extension varies with applied force for helical springs, metal wires and rubber bands

they all obey Hooke's Law up to their elastic limit
-thanks to a correction by an anonymous person (see comments), it apparently should be:
helical springs and metal wires do obey Hooke's Law, but rubber bands do NOT follow Hooke's Law  and the extension is NOT directly proportional to the force causing it. 

(y)

1.28 recall that the initial linear region of a force-extension graph is associated with Hooke's Law

Hooke's Law: The extension is directly proportional to the stretching force. 

 

Hooke's Law only applies to the straight part of the graph-initial linear region, up to the limit of proportionality. 

Elastic limit-if a spring is taken beyond this limit, it won't return to its old shape=deformed.

1.29 associate elastic behaviour with the ability of a material to recover its original shape after the forces causing deformation have been removed

Deformation from the limit of proportionality to the Elastic limit is still reversible; but 

Beyond the elastic limit, deformation is irreversible=permanent deformation or plastic deformation.