Topic 5 - Forces

Question 1

What is the difference between scalar and vector quantities? Give some examples of each.

Answer 1

Scalar:
— Have magnitude only e.g. distance, time, temperature, speed, mass etc.

Vector:
— Have magnitude and direction e.g. displacement, acceleration, force, velocity etc.

Question 2

What is the definition of a force?

Answer 2

Force:

— A push or pull acting on an object caused by an interaction with another object

Question 3

What are the two types of forces? Give examples of each.

Answer 3

Contact:
— Friction, air resistance, tension, normal contact force

Non-contact:
— Gravitational force, magnetic force, electrostatic force

Question 4

What is the definition for mass in comparison to weight?

Answer 4

Mass:
— The ‘amount of stuff’ in an object

Weight:
— The force acting on an object due to gravity. Weight depends on the gravitational field strengthj of where the object is.

NOTE: weight and mass are directly proportional so if one doubles for example then so does the other

Question 5

How do you calculate weight?

Answer 5

Weight(N) = mass(kg)*gravitational field strength(N/kg)

Question 6

What is the resultant force?

Answer 6

The resultant force on an object is when you replace all the forces acting on an object with a single force. This single force is known as the resultant force.

Question 7

How do you work out the work done when a resultant force moves an object?

Answer 7

Work done(J) = Force(N) * distance(m)

Question 8

What is the difference between elastic and inelastic deformation?

Answer 8

When an object has been elastically deformed, it will return back to its original shape and length once the applied force has been removed whereas with inelastic deformation, once the object has been stretched, bent or compressed, it does not return back to its original shape and length.

Question 9

What is the equation that links force, spring constant and extension?

Answer 9

Force(N) = spring constant(N/m) * extension(m)

NOTE: extension is directly proportional to force

Question 10

What does the limit of proportionality indicate and how is it shown on a graph?

Answer 10

Limit of proportionality indicates the extension is no longer directly proportional to the force. This can be shown on an extension-force graph where the graph begins to curve.

Question 11

How do you calculate the pressure at the surface of a fluid?

Answer 11

Pressure(Pa) = force(N) / area(m^2)

Question 12

How do you calculate the pressure of a liquid at a certain depth due to the column of liquid?

Answer 12

Pressure(Pa) = height aka depth(m) * density(kg/m^3) * gravitational field strength(N/kg)

Question 13

What is upthrust?

Answer 13

Upthrust is the resultant force upwards when the force exerted on the bottom of an object is larger than the force exerted from the top. Upthrust is equal to the weight of the object that has been displaced.

Question 14

What factors influence an object floating and sinking?

Answer 14

An object floats if the weight of the object is equal to the upthrust. It sinks if the object weighs more than upthrust because it is more dense.

Question 15

How does atmospheric pressure change with changhung altitude?

Answer 15

As altitude increases, atmospheric pressure decreases. This is due to the fact that as altitude increases, the atmosphere gets less dense.

Question 16

What are some typical values of speed?

Answer 16

Walking - 1.5 m/s
Running - 3 m/s
Cycling - 6 m/s
Car - 25 m/s
Train - 55 m/s
Plane - 250 m/s
Sound in air - 330 m/s

Question 17

What is the equation that links speed, distance and time?

Answer 17

distance travelled(m) = speed(m/s) * time(s)

Question 18

How do you calculate the speed of an object on a distance-time graph when the object is:

• – moving at a constant speed in a straight line?
• – changing speed?

Answer 18

— Work out speed by finding gradient of the straight line on the graph where it is moving at a constant speed.

— Draw a tangent at a particular point on the curve and work out the gradient of the tangent

Question 19

How do you work out acceleration and distance travelled on a velocity-time graph?

Answer 19

Acceleration:
— Finding gradient of the graph

Distance travelled:
— Finding the area under the graph

Question 20

What is the equation for acceleration and uniform acceleration (constant acceleration)?

Answer 20

Acceleration:
— acceleration(m/s^2) = change in velocity(m/s) / time(s)

Uniform acceleration:
— (final velocity)^2 - (initial velocity)^2 = 2accelerationdistance

Question 21

What is terminal velocity? Link terminal velocity to the idea of a parachute.

Answer 21

Terminal velocity:
— The maximum speed an object reaches when falling.

—With a parachutist, when the parachute is first opened the force of weight is larger than air resistance which causes the parachutist to accelerate. As speed increases, air resistance builds up and eventually the force of air resistance is equal to the force of weight. This causes the parachutist to fall at a steady speed meaning it has reached its terminal velocity.

Question 22

What is Newton’s first law of motion?

Answer 22

Newtons first law:
— If the resultant force on a stationary object is zero then it will remain stationary and if the resultant force on a moving object is zero, then it will continue to move at a constant speed in the same direction therefore the velocity of the object will remain the same.

Question 23

What is inertia? Also, what is inertial mass?

Answer 23

Inertia:
— The tendency of objects to continue in their state of rest or uniform motion.

In other words:
— The resistance of objects to change their motion.

Inertial mass:
— A measure of how difficult it is to change an object’s velocity.

Question 24

What is Newton’s second law of motion and the equation linked to this?

Answer 24

Newton’s second law:
— The resultant force on an object is directly proportional to the acceleration of an object and the acceleration of an object is inversely proportional to the mass of an object.

Equation:
Resultant force(N) = mass(kg) * acceleration(m/s^2)

Question 25

What is Newton’s third law of motion?

Answer 25

Newton’s third law:
— When two objects interact, the forces they exert on each other are equal and opposite.

NOTE:
— This law only applies if the forces are the same and acting on two different objects.

Question 26

How do you find the stopping distance of an object?

Answer 26

Stopping distance = thinking distance + braking distance

Question 27

What are some typical braking distances you should know?

Answer 27

• – 14m at 30mph
• – 55m at 60mph
• – 75m att 70mph

Question 28

What affects thinking and braking distance?

Answer 28

Thinking distance:

• – Speed: faster speed = greater thinking distance
• – Reaction time: longer reaction time = greater thinking distance

Braking distance:

• – Speed: greater speed = greater braking distance
• – Weather: wet or icy conditions on the road = greater braking distance (anything that causes less grip on the road increases braking distance)
• – Tyre conditions: worn out tyres = greater braking distance
• – Brakes conditions: worn out or faulty brakes = greater braking distance

Question 29

What is a typical reaction time and what things can affect reaction time?

Answer 29

Typical reaction time:
— between 0.2s and 0.9s

Factors affecting reaction time:

• – Distractions
• – Tiredness
• – Drugs or alcohol

Question 30

What is the equation for momentum?

Answer 30

momentum(kg m/s) = mass(kg) * velocity(m/s)

Question 31

What is the conservation of momentum?

Answer 31

Conservation of momentum:

— In a closed system, the total momentum before an event is equal to the momentum after an event.

Question 32

What is the equation that links F = ma and a = change in velocity / change in time for momentum?

Answer 32

Force = (mass * change in velocity) / change in time

NOTE:
— Force equals the rate of change of momentum

Question 33

What are some safety features tthat help reduce risk of injuries?

Answer 33

Cars:

• – Seatbelts: stretch which increases time taken for the wearer to stop
• – Air bags: inflates just before you hit the dashboard so the compressing air slows you down than suddenly hitting the dashboard

Soft mats and gym mats:
— Soft and cushionable which increase the time taken for you to stop if you fall on them.

Bike helmets:
— Have a crushable layer which increases the time taken for your head to stop in a crash as it reduces the impact on your brain.