Mechanics - Forces Flashcards

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1
Q

Define a force

A

Pushes, pulls or twists.

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2
Q

How do forces change objects?

A

Forces can change an object’s motion and shape.

  • Change in motion = either change in speed or direction of motion. Change in speed = acceleration/deceleration.
  • Sometimes forces can change both motion and direction. Eg when a ball is thrown up, gravity pulls downwards on the ball (weight is the force), so it slows down on the way up, changes direction and accelerate downwards back onto the ground.
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3
Q

How to describe forces

A
  • label the force
  • label/state direction of force (using an arrow)
  • show the size of the force
  • UNIT = N, newtons.
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4
Q

What is net force?

A

When there is more than one force acting on an object, it is the combination of forces that affects the object’s motion.

The combined force is called net force. F_net.

  • Forces acting in the same direction are added together
  • Forces acting in opposite directions are subtracted to give the net force.
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5
Q

What is thrust?

A

It is a mechanical force, meaning the propulsion system must be in physical contact with the object

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6
Q

Newton’s Third Law

A

If object A exerts a force on object B, then object B will exert an equal and opposite force on object A.

Eg. when you stand on the ground the force you are applying to the Earth is your weight, so Earth is also supporting you with an equal and opposite force so you aren’t moving vertically.

When you throw a ball up, you are exerting thrust onto the ball and we can see it accelerate due to this force. However, the ball also exerts an equal and opposite force to your hands, so your hands/arms (whole body) also move in the opposite direction. However, due to f=ma, and the two forces being equal, the ball has a smaller mass than your arm therefore it will accelerate more than your hand will.

When you push a trolley, the trolley is pushing you back with the equal and opposite force. However you do not move because the opposite force is not able to overcome your weight and friction (between you and the ground). Friction is a contact force which opposes motion, and in this case, the friction force is greater than the thrust force causing the motion so you do not move.

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7
Q

Newton’s second law

A

The acceleration of an object is directly proportional to the unbalanced external force and is in the same direction as that force.

F_net = ma

When the forces acting on an object are unbalanced, the net force will make the object accelerate. If the unbalanced force is in the same direction as motion, the accelerations will be positive. If the unbalanced force is against the motion, the acceleration will be negative, therefore a deceleration.

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8
Q

Units of F = ma

A
Force = N, newtons
m = kg
a = ms^-2
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9
Q

Mathematical relationship of net force (unbalanced force)

A

F_net is proportional to mass and acceleration. F_net increases as mass and acceleration increases.

This means that when different net forces are applied to objects of the same mass, a greater net force will produce a greater acceleration.

Also, when the same force is applied to objects of different masses, the smaller mass has the greater acceleration.

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10
Q

Labelling opposite forces

A

Use “-“ to represent force in the opposite direction but of the same size.

Eg upward force = -mg because weight is mg

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11
Q

Newton’s first law

A

An object will remain at rest or constant velocity unless acted upon by an UNBALANCED external force

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12
Q

What is friction?

A

A REACTION contact force which opposes motion.

Friction is created as soon as something starts SLIDING past another object. It is a reaction force as friction doesn’t exist until the sliding starts.

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13
Q

What is the difference between mass and weight?

A

Mass is the amount of material in an object (and therefore stays the same wherever the object is) whereas weight is the force with which an object pushes down on to whatever it is resting on. WEIGHT IS CAUSED BY GRAVITY PULLING AN OBJECT DOWNWARDS.

UNITS : mass in kg or g | weight in N

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14
Q

How to calculate weight?

A

weight = mg

This means weight is calculated by multiplying mass by the STRENGTH OF GRAVITY. The Earth attracts EACH kilogram towards itself with a force of 10N.

Therefore g = 10N kg^-1

The value of gravity is often called the acceleration of gravity. This is because if an objet is dropped it accelerates downwards with an acceleration that has the same value as gravity, ie at 10ms^-1.

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15
Q

Relationship between friction and an object

A

Friction always opposes the motion of the object.
The more the two sliding surfaces press together, the more friction generated. The rougher the sliding surfaces, the greater the friction that is generated (because there are more irregularities in the surfaces meaning the “peaks” of one surface can fall into the “valleys” of the other, meaning that to keep moving either something needs to break, or the surfaces would need to push apart briefly)

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16
Q

Friction between states of matter

A

Friction can happen between:

  • 2 solid objects
  • one solid and one liquid | eg person wading through water
  • solid and air | eg air resistance
  • liquid and air | eg water falling over a waterfall is turned into spray by friction with the air.
17
Q

Situations where friction is desirable and undesirable

A

Desirable: car and bicycle tyres have tread to grip the road | parachutes work because of friction with the air | brakes on cars and bicycles use friction to slow down and stop them (brake pads causes friction on the motor which slows it down to stop).

Undesirable: causes surfaces in machinery to heat and wear out therefore WASTES ENERGY AND GENERATES HEAT | air resistance slows moving vehicles down therefore WASTES ENERGY AND GENERATES HEAT | can cause abrasions if a person falls over.

18
Q

How to reduce friction?

A
  • Lubricating oil and grease between surfaces (surfaces slide easier past each other)
  • wheels or ball bearings to roll the surfaces past each other (only a small point of contact at any one point)
  • streamlined shapes to reduce air resistance
19
Q

What is STATIC FRICTION?

A

Static friction is the force of friction which is stopping a sliding motion from starting.

For example, when a block is resting on the floor is pulled gently, friction force matches the pulling force and acts in the opposite direction (Newton’s 3rd), so the block doesn’t move. The harder the pull, the greater friction force is generated.
- there is a maximum friction force that can be generated between two surfaces and this is called the STATIC FRICTION FORCE and is varied between different surfaces.
If the pull gets larger than the static friction force between block and floor, the block will accelerate across the floor.

20
Q

Sliding friction vs static friction

A

Between any 2 surfaces, static friction is LARGER than sliding friction meaning that the force needed to keep an object moving is less than the force needed to make it start moving.

21
Q

What does an unbalanced force produce?

A

An acceleration

22
Q

Air resistance - Terminal velocity

A

Terminal speed is the speed an object will constantly travel at when forces are balanced.

For example, a skydiver jumping out of a plane will initially accelerate at 10ms^-1 due to the pull of gravity. However, as the object moves faster the air resistance (upwards friction) increases, meaning that the net downwards force will decrease. Eventually the upwards air resistance will balance the downwards pull of gravity, resulting in 0 F_net meaning there is no more acceleration and the object will continue to fall at constant speed. This constant speed is called terminal speed.

23
Q

What happens when forces are balanced?

A

When forces are balanced, it means all forces combine to cancel each other out so there is no net force and therefore no acceleration.
No acceleration means no change in speed, meaning the object can be either stationary or travelling at constant velocity.

24
Q

Examples of when forces are balanced

A
  • when a power boat is moving at a constant speed, the driving force of the engine balanced out the friction force of the water on the boat.
  • when a book rests on the table, the upwards push from the table balance out the downward push of the weight. Upward push from the table is a REACTION force because it only occurs if the book pushes down.
  • in space there is no friction to stop a space craft down so it does not need a rocket force to keep it moving at a constant speed. The forces are balanced because they are zero.
25
Q

What effect does pressure have on an object

A

Pressure is a force, and it is able to change the shape of the object it is acting on.

The amount of change a force can make to the shape of the object is it acting on depends on the pressure the force is able to create.

26
Q

Equation for pressure

A

P = force / area

UNITS:
Pressure = Pa, Pascal ( 1 Nm^-2, ie Newtons per m^2)
Force = N
Area = m^2

CONVERT ALL INFORMATION TO CORRECT UNITS BEFORE EXECUTING THE EQUATION.

27
Q

What is pressure?

A

Pressure is the ratio of force to area applied in a direction perpendicular to the surface of an object.

28
Q

What is air pressure?

A

It is the continual bombardment of air gas molecules. Molecules of gases in the air are constantly moving in random directions and hitting against anything that is in the air.

29
Q

What is the pressure of air at sea level?

A

101 kNm^-2