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P8 Forces Flashcards

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1
Q

What do we mean by displacement?

A

Displacement is the distance in a given direction, so there are no changes of direction (example of bus route)

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2
Q

What are vectors?

A

Vector are physical quantities that have both magnitude and an associated direction.

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3
Q

Example of vector quantities

A

Some examples of vector quantities include:

force, eg 20 newtons (N) to the left
displacement, eg 50 kilometres (km) east
velocity, eg 11 metres per second (m/s) upwards
acceleration, eg 9.8 metres per second squared (m/s²) downwards
momentum, eg 250 kilogram metres per second (kg m/s) south west

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4
Q

How are vector quantities represented?

A

The direction of a vector can be drawn as an arrow. The length of an arrow represents the magnitude of the quantity. The direction of the arrow represent the direction of the vector

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5
Q

What is a scalar quantity?

A

It is a quantity that has magnitude but no direction

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6
Q

What are examples of scalar quantities?

A

Some examples of scalar quantities include:

temperature, eg 10 degrees Celsius (°C)
mass, eg 5 kilograms (kg)
energy, eg 2,000 joules (J)
distance, eg 19 metres (m)
speed, eg 8 metres per second (m/s)
density, eg 1,500 kilograms per metre cubed (kg/m³)
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7
Q

What is meant by magnitude of a vector quantity?

A

The magnitude is the size

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8
Q

What is a force?

A

A force is a push or a pull that acts on an object because of an interaction with another object

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9
Q

What is a contact force?

A

Contact forces are forces that act between two objects that are physically touching each other

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10
Q

Example of contact forces

A

Friction
Air resistance
Tension

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11
Q

Example of non contact forces

A

Magnetic force
Electrostatic force
Gravity

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12
Q

Newton Third Law of Motion

A

According to Newton’s Third Law of motion, whenever two objects interact, they exert equal and opposite forces on each other

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13
Q

What are non contact forces?

A

Non-contact forces are forces that act between two objects that are not physically touching each other

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14
Q

What is the driving force on a car?

A

It is the force that makes it move.

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15
Q

How does driving force move a car forward?

A

This force pulls the car forward because of friction between the tyre and the ground. When the car moves forward:
- force of the friction of the road on the tyre is in the fwd direction
force of friction of the tyre on the road is in the reverse direction

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16
Q

What is a resultant force?

A

The resultant force is a single force that has the same effect as two or more forces acting together

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17
Q

What is the resultant force of two forces in the same direction?

A

Two forces that act in the same direction produce a resultant force that is greater than either individual force. Simply add the magnitudes of the two forces together.

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18
Q

Two forces, 3 newtons (N) and 2 N, act to the right. Calculate the resultant force

A

3 N + 2 N = 5 N to the right

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19
Q

Newton’s first law of motion

A

It states that if the forces acting on an object are balanced, the resultant force on the object is zero; and:

  • if the object is at rest it stays stationary
  • if the object is moving, it keeps moving in the same direction and at the same speed
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20
Q

If the resultant force on an object is zero, this means what?

A

a stationary object stays stationary

a moving object continues to move at the same velocity (at the same speed and in the same direction)

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21
Q

How do you calculate the resultant force of two forces act on an object on the same line and different directions?

A

Two forces that act in opposite directions produce a resultant force that is smaller than either individual force. It is often easiest to subtract the magnitude of the smaller force from the magnitude of the larger force.

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22
Q

A force of 5 N acts to the right, and a force of 3 N act to the left. Calculate the resultant force.

A

5 N - 3 N = 2 N to the right

23
Q

If forces acting on an object are balanced how much is the resultant force?

A

Zero

24
Q

What happens when the resultant force on an object is not zero?

A

When the resultant force on an object is not zero, the forces on the object are not balanced. The movement of the object depends on the size and direction of the resultant force

25
Q

What happens when a driver brakes?

A

The braking force is greater than the engine force. The resultant force acts in the opposite direction to the car direction so it slows it down

26
Q

What happens to the resultant force when an object is acted on by two unequal forces in opposite directions (car breaking example)?

A

The resultant force is:

  • equal to the difference between the forces
  • in the direction of the larger force
27
Q

What is a free-body force diagram?

A

Free body diagrams are used to describe situations where several forces act on an object. It shows the forces acting on the object

28
Q

What does the moment of a force measure?

A

A moment is the measure of the turning effect of a force on an object

29
Q

How is the moment of a force calculated?

A

moment of a force = force × distance (perpendicular distance from the line of action of the force to the pivot.
This is when:
moment (M) is measured in newton-metres (Nm)
force (F) is measured in newtons (N)
distance (d) is measured in metres (m)

30
Q

How can the moment of a force be increased?

A

increasing the size of the force

using a spanner with a longer handle

31
Q

A force of 15 N is applied to a door handle, 12 cm from the pivot. Calculate the moment of the force.

A

First convert centimetres into metres:

12 cm = 12 ÷ 100 = 0.12 m

Then calculate using the values given in the question:
M= Fd
M=150.12
M=1.8 N

32
Q

What are levers?

A

They are objects that can be used to exert a force that is greater than the effort. They are force multipliers because the effort moves a much bigger load. They can increase the size of a force acting on an object or to make the object turn more easily

33
Q

What does a lever consist of?

A

A lever consists of:

a pivot
an effort
a load

34
Q

Levers: what are the effort and the load?

A

Effort is the force applied

Load is the weight of the object

35
Q

A solid beam 0.5 m long is laid across a pivot to form a simple lever. The pivot is 0.1 m from the end of the beam. Calculate the heaviest load that could be lifted using a force of 500 N.

A

First, calculate the moment due to the 500 N force. To do this, distance will also need to be calculated. To lift the greatest load, the effort must be applied furthest from the pivot.
Calculate the greatest distance from the pivot:
0.5 − 0.1 = 0.4
M=Fd
M=5000.4
M=200Nm

Second, use the answer above to calculate the maximum force 0.1 m from the pivot.
Rearrange M=F*d to find F:
F=M/d
M=200/0.1
M=2000N
The heaviest load that could be lifted by this arrangement is 2,000 N. The lever has the effect of multiplying the force by 4 times - it is a 4× force multiplier.

36
Q

Where does a force applied to a lever must act?

A

The force applied to a lever must act further away from the pivot than the force it has to overcome (think about scissors or bottle opener)

37
Q

What is a gear?

A

Gears are wheels with toothed edges that rotate on an axle or shaft. The teeth of one gear fit into the teeth of another gear. This lets one gear turn the other, meaning one axle or shaft can be used to turn another shaft.
They are like levers because the multiply the effect of a turning force.

38
Q

What need to happen for the moment of a turning effect to increase?

A

A small gear needs to drive a large gear

39
Q

Why does a force transmitted to a larger gear cause bigger moment?

A

Because the distance to the pivot is larger

40
Q

What happens when a lower gear is chosen in a car?

A
  • a small wheel driven by the engine shaft is used to turn a large wheel on the output shaft. So the output shaft turns lower than the engine shaft
  • the turning effect of the output shaft is greater than the turning effect of the engine shaft
  • Low gear gives low speed and a high turning effect
41
Q

What happens when a higher gear is chosen in a car?

A
  • a large wheel driven by the engine shaft is used to turn a small wheel on the output shaft. So the output shaft turns faster than the engine shaft
  • the turning effect of the output shaft is less than the turning effect of the engine shaft
  • High gear gives high speed and a low turning effect
42
Q

What is the centre of mass of an object?

A

The centre of mass of an object is the point where its mass can be thought as being concentrated

43
Q

When can you say that an object is freely suspended?

A

An object is freely suspended if it returns to its equilibrium position after the turning force has been taken away.
Explanation: if you suspend an object and then release it, it will sooner or later come to rest with its centre of mass directly below the point of suspension. The object then is in equilibrium, which means it is at rest. Its weight does not exert a turning effect because its centre of mass is directly below the point of suspension

44
Q

Where is the centre of mass of a symmetrical object?

A

The centre of mass is along the line of symmetry.

If there is more than one line of symmetry it will be where the axis of symmetry meet.

45
Q

What does the principle of moments state?

A

For an object that is not turning:
Sum of all clockwise moments about any point
=
Sum of all anticlockwise moments about any point

If a seesaw is balanced the moments on both sides are the same

46
Q

What is a parallelogram of forces?

A

A parallelogram of forces is scale diagram of two force vectors

47
Q

What is a parallelogram of forces used for?

A

It is used to find the resultant two forces that do not act along the same line. The resultant force would be the diagonal of the parallelogram

48
Q

What is meant by resolution of forces?

A

Resolving a force means finding the perpendicular components that have a resultant force that is equal to the force

49
Q

How do you resolve a force in two perpendicular directions?

A

Draw a rectangle with adjacent sides along the two directions so that the diagonal represents the force vector

50
Q

When is an object in equilibrium?

A

When the resultant force on it is zero

51
Q

What are the conditions for an object to be in equilibrium?

A
  • the resultant force on the object is zero

- the forces acting on the object have no overall turning effect

52
Q

How do you work out whether or not an object is in equilibrium?

A
  • If the lines of action of the forces are parallel, the sum of the forces in one direction must be equal to the sum of forces in the opposite direction. This means that the resultant force on the object is zero
  • if the lines of action of the forces are NOT parallel, the forces can be resolved into two components along the same perpendicular lines. The components along each line must balance out if the resultant force is zero.
53
Q

What is the formula for calculating moment with gears?

A

Moment = force x gear wheel radius

Physics (9 decks)

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