Forces

1) What is a Force ?
They are usually pushes or pulls. (they can also be twists)
A force always acts in a certain direction.
Forces normally occur between two objects that are touching (or in contact)
However some forces can occur without contact.

2) Forces can be used to make objects do 7 things, what are they ?
make an object start moving / constant speed
make an object stop moving
speed up / accelerate
slow down / decelerate
change direction
turn/pivot/twist
change shape/deform

3) Can you give some example of Contact Forces ?
Friction, Upthrust

4) Can you give some examples of Non-Contact Forces ?
Gravity - (gravitational force) everything falls to the floor
Magnet - (magnetic force) magnets will attract paper clips towards it
Static Electricity - (electric force) electric charge that is not flowing (static charge on a balloon)

5) What is a Field ?
A field is a region where an object experiences a non-contact force.

6) How are forces measured, what is the unit ?
Newtons (N)

7) What is the difference between balanced and unbalanced forces ?
Balanced forces - are equal in size but opposite in direction. The net force is zero. Objects can be stationary or moving at a constant velocity. Also called a Reaction force.
Unbalanced forces - are not equal in size or not in the same direction.

8) Can you describe a Gravitational Force ?
This is the force of attraction between masses.
The larger the mass, the greater the gravitational force.

9) Can you describe Gravitational Field Strength ?
This varies depending how close you are to the mass that is creating the gravitational force.
The closer you are, the stronger the field.

Weight (N) = Mass (kg) x Gravitational Field Strength (N / Kg) 

10) What is Momentum ?
Momentum is a measure of how much motion a moving object has.
Momentum is not a force.
It is mass times velocity.

Momentum (KG m/s) = Mass (Kg) x Velocity (m/s) 

11) What is the Concentration of Momentum ?
This states that the total momentum of a closed system remains constant.
The total momentum before a collision is the same as the total momentum after the collision.

12) What causes a change in momentum ?
When a force acts on an object, it causes the object to change momentum.
The larger the force, the faster the change in momentum.

13) What is Newton's First Law ?
A resultant force is only needed to start, stop, move, speed up, slow down, change direction.
Objects will continue at a constant speed until they are acted upon by a force.

14) What is Newton's Second Law ?

F = ms 

The acceleration of an object is directly proportional to the net force acting on it.
The same force on a larger mass, results in a smaller acceleration.

15) What is Newton's Third Law ?
When two objects interact, the forces they exert on each other are equal and opposite.

16) What is Inertia ?
This is the tendency for objects to continue in the same state of motion.
Objects will stay still until acted on by a force.
Objects that are moving will continue at a constant speed until acted upon by a force.

17) What is Speed ?
Speed is a measure of how far you travel in a given amount of time.
Speed is Distance over Time.

18) What is Velocity ?
Velocity is speed in a particular direction.

19) What are the 3 common units of speed ?
metres per second (m/s), miles per hour (mph), kilometres per hour (km/h).

20) What is Friction ?
Friction is a contact force that always acts in the opposite direction to the movement.
For an object to move the force must be larger than the friction (or resisting force).
Friction only occurs when an object is moving and two surfaces rub together.
When an object tries to pass through a gas or liquid there will be some friction/resistance.
The rougher the surface, the more friction there is.

21) What is Air Resistance ?
This is a frictional force that pushes against the object trying to move through the air, also referred to as drag.
The faster the object moves the greater the resistance.
When the frictional force equals the opposing force the object moves at a steady speed.
The fastest car on land travelled at 763 mph.

22) What is Water Resistance ?
This is a frictional force that slows objects down when they pass through water, also referred to as drag.
The fastest boat on water travelled 316 mph.
The fastest submarine travelled at 51 mph (44 knots).

23) Whas is Mass ?
This is the amount of stuff in an object.
For any object this is constant.

24) What is Weight ?
The force acting on an object because of gravity.

25) What is Density ?
This is how heavy a material is for its size.
This is a measure of how many particles there are in a given space.
Density is the amount of mass in a given volume. How closely packed they are.

26) What is the formula that connects density, mass and volume ?

density = mass / volume 

27) Can you describe the density in solids, liquids and gases ?
Solids usually have a higher density than liquids.
Liquids usually have a higher density than gases.
When a solid is heated, the particles move around more and move further apart
The substance expands and becomes less dense.
Solid to Liquid, density decreases, volume increases
Liquid to Solid, density increases, volume decreases.

28) When H2O changes from a liquid to a solid, does the density increase ?
No. H2O is the only known non-metallic substance that actually expands when it changes from liquid to solid.
Its density actually decreases and it expands approximately by 9% volume.

29) When ice melts, does its volume increase or decrease ?
This is solid to liquid, density decreases, volume increases.

30) Why do icebergs Float ?
The density of ice is lower than the density of water.

31) What is the opposite of Expand ?
Contract.

32) What is a Force Diagram ?
It is a drawing with arrows to show all the forces acting on an object.

33) What is Upthrust ?
This is a contact force that pushes things upwards.
In liquids the pressure increases with depth due to the weight of the water above it.
Liquid pressure causes upthrust and makes things float.

34) What are the units of Pressure ?
Pascals, Pa (N/m2)

35) When does an object Sink ?
If the object is denser than the liquid, the object will sink.
Salty water is denser than pure water, which makes it easier for objects to float.

36) When is there a change in speed or direction ?
There is a change in speed or direction when the forces are unbalanced

37) When forces make an object turn, what is the Moment ?
The moment is the name given to the force that is creating the turning effect
When a force acts on something that has a pivot, it creates a moment (or turning effect)

Moment = Force x Distance 

38) What are Levers, Pulleys and Gears ?
A lever uses force on a pivot to lift the load.
A pulley uses a flexible cable across a wheel to lift the load.
A gear uses wheels with teeth to transfer motion and power from one to another.

39) What is Tension ?
State of being stretched tightly.

40) How can you Deform an object ?
You can use forces to stretch or compress objects
When an object deforms it changes its shape
A spring is an example of an object that can compress

41) What is Hooke's law ?
This only applies to Springs.
This says the amount it stretches (the extension) is directly proportional to the force applied (the weight).
For springs, the force at which Hooke's law stops working is much higher than for most materials, springs are unusual.

F = ke 

42) What is the Limit of Proportionality ?
This is the maximum force that can be applied to a spring for Hookes law to apply.
The extension of a spring is directly proportional to the force.

43) What is Compression ?
A force that is squashing or pushing that changes the shape of an object

44) What does the gradient show on a Distance-Time Graph ?
The gradient on a distance-time graph is the speed of the object.
The steeper the line, the faster the object is moving.

45) What does a straight line show on a distance-time graph ?
A straight line represents a constant speed.

Forces Questions

46) How can a large ship float ?
There is a lot of air inside the ship which means that it is not as dense as water.
If the upthrust is equal to the objects weight, the object will float.
The force pushing upwards at the bottom of the object is greater than the force pushing down at the top of the object.

47) Why do helium balloons float in air ?
Anything that is less dense than air will float, although air is not very dense.

48) Two identical cranes can transfer 20 KJ of energy to lift an object. Which crane can lift the object the highest ?
One crane applies a big force and the other crane applies a small force.
A large force can be applied over a small distance.
A small force can be applied over a large distance.

49) What is aerodynamic lift ?