Forces

Question 1

What are the effects that forces can have on different bodies?

Answer 1

• Changes in speed
• Changes in direction
• Changes in shape

Question 2

What are the different types of forces?

Answer 2

• Gravitational (or weight) - Force between objects with mass
• Electrostatic - Force between charged particles
• Thrust - Push from things like car and rocket engines
• Upthrust - Upward force on an object on/in fluid
• Air resistance - Force of friction between objects moving through air
• Reaction force - Like the upwards force from a table on the book

Remember to refer to the force of gravity on an object as weight and not gravity

Question 3

What is the difference between scalar quantities and vector quantities?

Answer 3

• Scalar quantities just have magnitude
• Vector quantities have magnitude and direction
• For example: Distance is a scalar quantity, but displacement is a vector quantity

Question 4

Is force a scalar or a vector quantity

Answer 4

Vector

Question 5

What is the resultant force and how do you work it out?

Answer 5

• When many forces are applied to an object, they can be combined to produce one final force called the resultant, or net force
• The resultant force will determine the direction the object will move, and the magnitude of force the object will experience
• Substract opposite forces from eachother, and add forces working in the same direction to get the resultant force. If opposite forces are equal there is no resultant force

Question 6

What is friction and what causes it?

Answer 6

• The force which opposes the motion of an object, meaning it always act in the opposite direction of the objects movement
• It happens when two surfaces (including air) rub against eachother, such as when the surface’s imperfections push against the moving object

Question 7

What are balanced forces and what are unbalanced forces?

Answer 7

• Balanced forces occur when opposing forces are equal and there is no resultant force
• Unbalanced force occur when there is a stronger force in one direction, causing a resultant force

Question 8

What is the relationship between unbalanced force, mass and acceleration?

(Word and symbol equation)

Answer 8

• F = MA
• Force(Newtons) = Mass(Kilograms) x Acceleration(m/s²)

Question 9

What is weight?

Answer 9

The force acting on an object due to gravitational attraction

Question 10

What is the relationship between weight, mass and gravitational field strength

(Symbol and word equation)

Answer 10

• W = MG
• Weight(Newtons) = Mass(Kilograms) x Gravitational Field Stength(m/s² or N/kg)

Question 11

What is the gravitational field strength on earth equal to?

Answer 11

• 9.81m/s² (or N/kg)
• Writing 10 is fine

Question 12

How do you work out the stopping distance of a vehicle?

Answer 12

Thinking distance (The distance travelled in the time taken for the driver to react) + Braking distance (The distance travelled in the time taken for the car to deccelerate to 0)

Question 13

Which factors can affect stopping distance?

Answer 13

• Vehicle speed - Increases braking distance
• Vehicle mass - Increases breaking distance
• Wet/icy roads - Increase braking distance by reducing friction and therefore traction, making it harder to decelerate
• Drivers reaction time - Increase thinking distance, influenced by levels of fatigue and intoxication from alcohol or drugs

Question 14

What are the two forces which falling objects experience?

Answer 14

• Weight (down)
• Air resistance (up)

Question 15

What happens to the force of air resistance on a falling object as its speed increases?

Answer 15

The force of air resistance increases

Note that the force of weight never changes

Question 16

How is terminal velocity achieved?

Answer 16

• A falling object will accelerate due to the force of its weight being unbalanced with air resistance
• Eventually, it will reach such a velocity (terminal velocity) where the force of air resistance is equal to the force of its weight
• When this happens, the speed will be constant as the forces are balanced (no more acceleration, but still moving down)

Question 17

What is Hookes law?

Answer 17

The extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality

The length of extension per force applied will depend on the elasticity of the material

Question 18

What is the limit of proportionality?

Answer 18

• The limit of a material to obey Hooke’s law
• It is reached when force is applied above a limit and the object does not return to its original shape after removing the material
• This results in the relationship between force applied and extension being non-linear

Question 19

What is elastic deformation?

Answer 19

• Elastic deformation occurs when an elastic object can return to its original shape when the force stretching it is removed
• Objects that undergo elastic deformation include rubberbands and springs

Question 20

What is inelastic deformation?

Answer 20

• Inelastic deformation occurs when an elastic object can not return to its original shape when the force stretching it is removed and the shape is permanently changed
• Objects that undergo inelastic deformation include plastic and clay
• Objects can also be inelastically deformed once the matieral exceeds its limit of proportionality

Question 21

What is the relationship between momentum, mass and velocity?

(Symbol and word equation)

Answer 21

• p = mv
• Momentum(Kg m/s) = Mass(Kilograms) x Velocity(m/s)

This means a stationery object has no momentum

Question 22

Is momentum vector or scalar and why?

Answer 22

• Vector, because it includes velocity so has direction
• This means that if an object travelling to the right has positive momentum, an object travelling to the left has negative momentum

Question 23

What is the principle of conservation of momentum?

Answer 23

In a closed system (no other influencing forces) , the total momentum before an event (such as a collision) is equal to the total momentum after

Question 24

What is the relationship between force, change in momentum and time taken?

(symbol and word equation)

Answer 24

• F = (mv-mu)/T
• Force = (Mass x Final velocity - Mass x Intitial velocity)/Time

• Thus, you can describe force as the rate of change in momentum
• mv - mu just represents the change in momentum

Question 25

Why do safety features like airbags, seatbelts and crash mats work?

Answer 25

* The change in momentum from something like falling, or a truck colliding with a wall will be large * In order to reduce the force of the impact, the contact time must be made higher (F = (mv-mu)/T * By adding cushioning with things like airbags, crashmats and seatbelts, it not only absorbs some of the force, but also changes shape/stretches which increases the contact time, reducing the impact force

Question 26

What is Newton's third law?

Answer 26

When two objects interact, the forces they exert onto each other are equal and opposite ## Footnote * The forces they exert on each other will be of the same type (e.g. gravitational) * It must involve forces acting on two objects, otherwise it is Newton's first law

Question 27

What is a moment defined as?

Answer 27

The turning effect of a force around a pivot ## Footnote Meaning if there is a moment on one side of a beam, it will cause it to turn on that side

Question 28

What is the relationship between the size of a moment, force and perpendicular distance from pivot? | (Symbol and word equation) ## Footnote The pivot is what the object is spinning around, such as the door hinges

Answer 28

* M = F x d * Moment = Force x Perpendicular distance from pivot ## Footnote d is the perpendicular distance between the pivot and the *line of action* of the force, so turning it at an angle will reduce the moment if it reduces the distance between line of action of force and the pivot

Question 29

What is the centre of gravity of an object?

Answer 29

* **The point through which all the weight of the object acts** * It is the balance point of the object if no other forces are acting * If the object is uniform, it will be in the physical center, if not, it will be in the center of mass ## Footnote The distance between the center of mass of an object and the pivot will be included in a moment whereby total object weight x distance from pivot works to calculate the magnitude of the moment