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Physics > Forces 1 > Flashcards

Forces 1 Flashcards

Scalar and vector quantities, Contact and non-contact forces, Gravity, Resultant forces, Vector diagrams.

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1
Q

What is the difference between scalar and vector quantities?

A

Scalar quantities have magnitude only, whereas vector quantities have both magnitude and direction.

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2
Q

How might we represent a vector quantity?

A

With an arrow.

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3
Q

When representing vector quantities with an arrow, what does the length of the arrow represent?

A

The magnitude.

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4
Q

When representing vector quantities with an arrow, what does the direction of the arrow represent?

A

The direction of the vector quantity.

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5
Q

The length of the arrow is proportional to…

A

its magnitude. (size)

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6
Q

List some vector quantities

A

-velocity
-displacement
-force
-weight
-acceleration

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7
Q

List some scalar quantities

A

-speed
-distance
-length
-mass
-power
-time
-temperature

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8
Q

What is displacement as a vector quantity?

A

It gives the distance travelled and in what direction, for example travelling 50 miles East.

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9
Q

What is a force?

A

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

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10
Q

What are the two types of forces between objects?

A

Contact forces
Non-contact forces

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11
Q

What is the difference between a contact and non-contact force?

A

For a force to be contact, the objects interacting must be touching, whereas for non-contact forces, the objects interacting are physically separated.

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12
Q

Examples of non-contact forces (hint: there are only three!)

A

-gravitational force

-magnetic force

-electrostatic force (between + and -)

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13
Q

Examples of contact forces

A

-friction
-air resistance
-water resistance
-normal contact force
-tension
-lift

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14
Q

Normal contact force

A

(picture the box on the table).

The force an object at rest on a surface exerts on the surface.

Also called a reaction force, normal contact force acts at right angles to the surface.

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15
Q

Newton’s third law

A

‘Every action has an equal and opposite reaction’…

which means that whenever two objects interact, they exert equal and opposite forces on each other.

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16
Q

Give an example that shows ‘every action has an equal and opposite reaction’

A

A satellite in Earth orbit:

There are non-contact gravitational forces between Earth and the satellite.

The Earth pulls the satellite.

The satellite pulls Earth.

These forces are equal in size and opposite in direction.

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17
Q

Why does it hurt when you kick a door hard?

A

The door exerts an equal and opposite force on your foot, which causes it to hurt.

18
Q

Weight

A

The force acting on an object due to gravity.

19
Q

What is the unit for weight?

A

N, Newtons.

20
Q

What does the gravitational field around the Earth cause?

A

The force of gravity close to the earth.

21
Q

What two things does the weight of an object depend on?

A

-The gravitational field strength at the point where the object is

-The mass of the object

22
Q

What is the formula for weight?

A

W = mg

23
Q

What is the unit for mass?

A

kg

24
Q

What is the unit for gravitational field strength, (g)?

A

N/kg

25
Q

Mass

A

The amount of matter or substance in an object.

It is measured in units called kilograms (kg).

Mass always stays the same (it is an intrinsic property), while weight changes with changes in gravity.

26
Q

Centre of mass

A

A single point through which the weight of an object may be considered to act.

27
Q

The weight of an object and the mass of an object are ________ proportional.

A

directly

28
Q

How can we measure the weight of an object?

A

Using a newtonmeter (a calibrated spring-balance)

29
Q

How can we find the centre of mass of an irregular object?

A

By hanging a plumb line from a bung between the arms of a clamp on a clamp stand.

Draw crosses where the plumb line falls on the object.

Repeat hanging the object (we used whiteboards) from different points.

30
Q

Resultant force

A

A single force that describes all of the forces acting on a body.

31
Q

How do we calculate the resultant of two forces that act in a straight line? (so from a scaled vector diagram?)

A

Work out the difference in the two forces.

32
Q

If a car has more force (N) acting on the way it is facing than backwards, what is the car doing?

A

Accelerating

33
Q

If a car has equal force (N) acting on the way it is facing as backwards, what is the car doing?

A

Moving at a constant speed
or
Is stationary

34
Q

If a car has more force (N) acting backwards than the way it is facing, what is the car doing?

A

Decelerating

35
Q

Newton’s first law

A

An object at rest remains at rest, or if in motion, remains in motion at a constant velocity unless acted on by a net external force (resultant force).

36
Q

When describing resultant forces we must …

A

-state the resultant force

-state in which direction the force is acting

37
Q

When would we say an object is in a state of equilibrium?

A

When all the forces that act upon the object are balanced.

38
Q

A cyclist pushes on his pedals moving north with a force of 30N against a wind pushing him east with a force of 40N.

Calculate the resultant force and his direction.

[what are the steps to working out this type of question?]

A

-use a vector diagram
-find a scale factor (e.g. 1cm = 10N)

-in this case, draw a line with an arrow pointing north (3cm)

-from the TOP of this line draw another arrow pointing east (4cm)

-turn this into a triangle and measure the line (5cm)

-use Pythagoras to check your answer
-state the direction

39
Q

How would we find the bearing from a vector diagram?

A

-Draw an angle curve thing in a clockwise direction on your triangle from the (north) facing line to the one you drew.

-Use a protractor to measure the angle

-Use 3 figures e.g. 037°

40
Q

What is the equation for resultant force?

A

mass x acceleration

Physics (9 decks)

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