Physics: Topic 5 - forces

What is a vector quantity?

Had a magnitude and a direction

E.g. forces, velocity, acceleration

What is a scaler quantity?

Has a magnitude (size) but no direction

E.g. energy, speed, mass, temp

What is a force?

A push or pull that acts on an object due to the interaction with another object (particles, humans, earth etc)

How do forces affect motion?

• change speed

• Change direction

• Change shape

Contact forces

• normal reaction force

• Tension

• Air resistance

• Friction

Non-contact forces

• electrostatic

• Magnetic

• Gravitational

What is normal contact force?

Acts on any object while in contact with a surface

What is mass?

• amount of matter an object has

• Stays the same

• Kg

What is weight?

• force or object due to gravity

• Changes based on what gravitational field it’s in

• Newtons

What is gravity?

A force caused by a larger object, causes/changes weight of a smaller object in its field

Weight equation?

• weight = mass x gravitational field strength (depends on distance between objects)

• N= kg x N/kg

What is Newton’s first law?

An object at rest will remain at rest, or a moving object will continue moving, unless there is a resultant force acting on the object

What is a resultant force?

• Affect of all the forces acting on us

• At least 2 forces acting on you at all time

  • weight + normal reaction force

When are forces balanced ?

• the forces are opposite each other

• The resultant force acting= 0N

When are forces unbalance?

• not equals to one another and/or not opposite

• If resultant force doesn’t equal 0N, we have a change in motion

When does an object accelerate?

If the resultant force is acting with motion

When does an object decelerate?

When resultant force is acting against motion

In what direction does the resultant force act?

Always acts in the direction of the bigger force

What does equilibrium mean?

When all forces are balanced, object not:

• accelerating

• Changing direction

• Going up or down

Resolving + resultant forces in 2d?

PRACTICE QUESTIONS

Elastic deformation?

When we stretch an object, remove the force, and it returns to its original shape (happens up to the objects elastic limit)

Inelastic (permanent) deformation?

Past the elastic limit, no longer returns to original shape after the forces are removed from the stretched shape

Hooke’s Law?

The extension of a spring (x) is directly proportional to the force applied to it (f)

Only applies up to elastic limit

Hookes law equation

Force = spring constant x extension

F = k x x

Hooke’s law graph

Hooke’s law practical?

• measure length of spring with meter ruler

• Add 100g to end of spring

• Measure new length

• Repeat adding 100g until 700g

• Repeat entire experiment 2x more

• Calculate average extension and force (mass/ 1000)

• Plot graph

Energy in springs?

Work is done when we extend or compress an object

Elastic potential energy equation

• Epe: 0.5 x spring constant x extension (squared)

• Spring constant : 2 x epe / extension (squared)

• Extension: (square root) 2xepe / spring constant

What is a moment?

A turning motion/force on an object

Moment equation

Moment (N/M) = force (N) x perpendicular distance from pivot (m)

Principle of Moments

If an object is balanced the total clockwise moment about a pivot is equal to the total anti-clockwise moment about a pivot

What does a lever consist of?

• pivot

• Effort

• Load

What is a lever?

A force multiplier

What are gears?

Toothed wheels used in machines to transfer the Turing affects of forces

What do gears do?

Transfer rotations and increase moments

What is the relationship between the size of gears and the moment?

the smaller the gear the smaller the moment as it has a smaller radius

Pressure in fluids?

• fluids are liquids and gases

• pressure increases with depth

• Larger force is exerted over the water (weight)

• Pressure (due to liquid) : depth x density x gravitational field strength

• Pa: m x kg/m3 x N/kg

Gas pressure?

The force per unit area exerted by moving gas particles

What is the atmospheric pressure?

101,000 Pascals / 101 Kap

How to increasegas pressure

• increase temp

• Increase concentration

What is atmospheric pressure?

The pressure that acts on us because of the atmosphere above us

What is displacement?

The total distance moved from the starting point, vector quantity

Typical speed to know?

• walking = 1.5 m/s

Running. 3 m/s

• cycyking = 6 m/s

What is instantaneous speed.

The speed of a particular instant can change

What is velocity?

The speed of an object in a particular direction (vectors)

Velocity equation?

Displacement / time

Average speed and average velocity equation?

• average speed = total distance/ total time

• Average velocity = total displacement/ total time

Instantaneous velocity in a circular path?

• constantly changing as the direction of movement is changing

What does the gradient show on a displacement time graph ?

• velocity

What does a curved line of a displacement graph represent?

• changing velocity

What is acceleration?

• the acceleration of an object is the rate of change of velocity

What effects acceleration ?

• the speed (increasing or decreasing)

• The direction

What type of quntaity is acceleration ?

• vector

Acceleration equation ?

Accelortaion = final velocity- initial velocity/ time

Unit for acceleration?

M/s2

What does the gradient represent on a velocity time graph?

• the instantaneous acceleration

How do you find the distance travelled on a velocity time graph?

• the area under the graph

What is Newton’s second law?

the accelortaion of a body is directly proportional to the resultant force acting on inversely proportional to the mass of the body

What is inertia?

• the tendency of an object to resist acceleration

• Inertial mass is the same as the mass

Why does drag increase on falling objects?

• object is travelling faster so collides with more air per second

Why is weight constant for a falling object?

• because the mass and gravitational field strength don’t change

Relationship between resultant force of a falling object and its speed?

• when one increases the other decreases

• If no resultant form then the object stops accelerating

What is thinking distance ?

• the distance travelled during during the reaction time

• During reaction time the car is travelling at a constant speed

• Thinking distance = speed x reaction time

• Directly proportional to the speed of the car and to the reaction time of the driver

What affects thinking distance ?

• stimulants (drug alcohol)

• Using device

• Weather (poor visibility)

• Age and tiredness

• Speed

What is breaking distance ?

• distance traveled under breaking force

• Depends on size of develoration

• Greater force means shorter breaking distance e

• Greater mass means larger breaking distance

• Doubling the speed increase the breaking distance by a factor of 4

What effects breaking distance?

• speed

• Mass

• Road conditions (ice)

• Tire grip and brake conditions

What does the momentum of an object depend on?

• proportional to its mass

• Proportional to its velocity

Units for momentum?

Kgm/s

What type of quantity is momentum?

• vector quantity; has magnitude and direction

Principal of conservation of momentum?

• the total momentum before a collision is equal to the total momentum after the collision