Topic P5 - Forces

Question 1

Describe a vector quantity.

Answer 1

• Vector quantities have magnitude and direction
• Force is a vector quantity as well as; velocity, displacement, acceleration, momentum, etc…
• Vectors are usually represented by an arrow (the length shows the magnitude and direction of the arrow shows the direction of the quantity)

Question 2

Describe a scalar quantity.

Answer 2

• Scalar quantities only have magnitude and not direction

- Some scalar quantities are; speed, distance, mass, temperature, time, etc…

Question 3

What is the contrast in non-contact and contact forces?

Answer 3

Non-contact forces;
- the objects do not need to be touching
- eg; magnetic, gravitational, electrostatic…
Contact forces;
- the objects will need to be touching
- eg; friction, air resistance, normal contact force
When two objects interact, there is a force produced on both objects.

Question 4

Describe gravitational force.

Answer 4

• on the surface of a planet, it makes all things go towards the ground
• it gives everything a weight

Question 5

Describe weight.

Answer 5

• weight is the force acting on an object due to gravity
• it depends on the strength of the gravitational field at the location of the object
• it is a force that is measured in newtons and is measured by a newtonmeter

Question 6

How do you calculate the weight?

Answer 6

Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)

Question 7

What is a resultant force?

Answer 7

• if you have a number of forces that are acting at a single point, you can replace them with a single force
• the single force is a resultant force
• if a resultant force moves an object, work is done

Question 8

How do you calculate work done?

Answer 8

Work Done (J) = Force (N) x Distance (m)

Question 9

What is an equilibrium?

Answer 9

• an object is an equilibrium if the forces on it are balanced
• (if all of the forces combine to make zero)

Question 10

Describe how to transfer energy by stretching, compressing and bending.

Answer 10

• when you apply a force to an object, it can cause it to stretch, compress and bend.
• to do this you will need more than one force acting on the object, because otherwise it will just move rather than change shape.
• work is done when a force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store of the object.

Question 11

How do you work out force?

Answer 11

Force (N) = Spring Constant (N/m) x Extension (m)

Question 12

How do you work out the elastic potential energy?

Answer 12

Elastic Potential Energy (J) = 1/2 x Spring Constant (N/m) x Extension^2 (m)

Ee = 1/2 x k x e^2

Question 13

Describe distance and displacement

Answer 13

• Distance is scalar because it doesn’t involve direction
• Displacement is a vector
• Displacement measures the distance and direction in a straight line from the object’s starting point to it’s finishing point

Question 14

Describe speed and velocity.

Answer 14

• Speed and velocity both measure how fast you are going
• speed is scalar
• velocity is vector
• this means that you can have objects travelling at a constant speed with a changing velocity.
• objects rarely travel at a constant speed, so in this case you will find the average speed during that time.

Question 15

How do you work out the distance travelled?

Answer 15

Distance Travelled (m) = Speed (m/s) x Time (s)

s = v x t

Question 16

Describe acceleration.

Answer 16

• acceleration is how quickly you’re speeding up
• it is not the same as speed or velocity
• acceleration is the change in velocity in a certain amount of time
• deceleration is just negative acceleration

Question 17

How do you work out the acceleration?

Answer 17

Acceleration (m/s^2) = Change in Velocity (m/s) / Time (s)

a = (triangle V) / t

Question 18

What is a distance-time graph?

Answer 18

• if an object moves in a straight line, it’s distance travelled can be plotted on a distance-time graph
• the steeper the graph, the faster it’s going
• flat sections is where the object has stopped
• if the object is changing speed, you can find it’s speed at a point by finding the gradient of the tangent to the curve at that point.

Question 19

How do you work out the speed?

Answer 19

Speed = Distance / Time

Question 20

What is a velocity-time graph?

Answer 20

• how an object’s velocity changes as it travels can be plotted on a velocity-time graph
• flat sections represent steady speed rather than that the object has stopped.
• if the graph is curved, you can use a tangent to the curve at a point to find the acceleration at that point.

Question 21

Describe friction.

Answer 21

• Friction is always there to slow things down
• it always acts in the opposite direction to movement
• to travel at a steady speed, the driving force will need to balance the frictional forces

Question 22

Describe drag.

Answer 22

• drag is the resistance you get in a gas or liquid. (for example air resistance is a type of drag)
• to reduce drag, you have to keep the shape of the object streamlined

Question 23

What is terminal velocity?

Answer 23

• as the speed of a falling object increases, the friction builds up and then gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force
• it will have then reached it’s maximum speed or terminal velocity and will fall at a steady speed.
• terminal velocity depends on shape and area

Question 24

What is Newton’s first law?

Answer 24

• a resultant force is needed to make something start moving, speed up or slow down.
• a non-zero resultant force will always produce acceleration or deceleration in the direction of the force
• this acceleration can take five different forms; starting, stopping, speeding up, slowing down and changing direction.

Question 25

What is Newton’s second law?

Answer 25

• The larger the resultant force acting on an object, the more it accelerates - the force and acceleration are directly proportional
• acceleration is also inversely proportional to the mass of the object - so an object with a larger mass can accelerate less than one with a smaller mass
• Resultant Force (N) = Mass (kg) x Acceleration (m/s^2)

Question 26

How do you work out the stopping distance?

Answer 26

Stopping Distance = Thinking Distance + Braking distance

Question 27

What is stopping distance affected by?

Answer 27

• the thinking distance which is how far the car travels during the driver’s reaction time
• the braking distance which is how long it takes the car to stop when you have your foot down in the brake

Question 28

What is the thinking distance affected by?

Answer 28

• speed (faster you’re going = further to travel when reacting)
• reaction time (longer reaction time = longer the thinking distance)

Question 29

What is the braking distance affected by?

Answer 29

• speed
• weather or the road surface
• condition of the tyres
• how good the brakes are

Question 30

How does braking rely on friction?

Answer 30

• When the brake pedal is pushed, it causes the brake pads to press on the wheels which causes friction
• the faster a vehicle is going, the more energy in it’s kinetic stores of the wheels to the thermal energy of the brakes

Question 31

How can you measure reaction time?

Answer 31

• the drop test
• you drop a ruler and the other person has to catch it and see how long it took them to react
• you can then calculate how long your reaction time was and repeat the test

Question 32

How do you work out momentum?

Answer 32

Momentum (kg m/s) = Mass (kg) x Velocity (m/s)

p = m x v

Question 33

Describe momentum.

Answer 33

• the greater the mass, or greater the velocity, the more momentum it will have.
• it is a vector quantity as it has size and direction