9: Momentum, Force and Energy

Question 1

What is Newton’s first law?

Answer 1

The velocity of an object will not change unless a resultant force acts on it

Question 2

What is Newton’s second law?

Answer 2

Acceleration is proportional to force. F=ma

The rate of change of momentum of an object is directly proportional to the net force which acts on the object

Question 3

Do all objects accelerate towards the ground at the same rate?

Answer 3

Yes in theory however air resistance means they don’t

Question 4

What is Newton’s third law?

Answer 4

If an object A exerts a force on an object B, then object B exerts an equal but opposite force on object A

Question 5

Where does Newton’s third law come from?

Answer 5

It is a consequence of the conservation of momentum. The resultant force acting means a change in mass or acceleration – which means a change in momentum.
Momentum is always conserved when no external forces act, so whenever one object exerts a force on another, the second object must exert equal size force back on the first object so that the overall change in momentum is zero

Question 6

What is a perfectly elastic collision?

Answer 6

One where momentum is conserved and kinetic energy is conserved

Question 7

What is an inelastic collision ?

Answer 7

One where some of the kinetic energy is converted into other forms during the collision. But momentum is always conserved

Question 8

What are the two types of friction? What are the differences?

Answer 8

Contact friction between solid surfaces

Fluid friction, drag or fluid resistance or air resistance

Question 9

What does ‘fluid’ mean, in the term fluid friction?

Answer 9

A liquid or a gas

Question 10

What does the force of the fluid friction depend on?

Answer 10

This thickness/viscosity of the fluid
It also increases as the speed increases
It also depends on the shape and size of the object moving through it – the larger the area pushing against the fluid, the greater the resistance force

Question 11

Three things you need to know about or frictional forces:
Their direction?
Their affect on the speed of the object?
Which types of energy do they convert?

Answer 11

They always act in the opposite direction to the motion of the object
They can never speed things up or start something moving
They convert kinetic energy into heat

Question 12

You will reach your terminal velocity at some point, if you have…

Answer 12

A driving force that stays the same all the time

A fictional or drag force that increases with speed

Question 13

When does something reach terminal velocity?

Answer 13

When the frictional force equals the driving force

Question 14

Describe the graph of velocity against time for an object reaching terminal velocity

Answer 14

Curve, where the gradient decreases, eventually turning into a horizontal line. Graph starts at the origin and increases

Question 15

Describe the graph of acceleration against time for an object reaching terminal velocity

Answer 15

Acceleration starts off high then decreases. The rate of decrease of acceleration starts slow then speeds up then slows again. The acceleration finally reaches zero. The graph looks like an unexaggerated S shape

Question 16

Describe a parachutist falling through the air before they open their parachute.

Answer 16

A skydiver leaves a plane and will accelerate until the air resistance equals his weight. He will then be travelling at terminal velocity.

Question 17

Describe a parachutist falling through the air, once they’ve opened their parachute

Answer 17

Before reaching the ground he will open his parachute which will immediately increase the air resistance so it is now bigger than his weight
This slows him down until his speed has dropped enough for the air resistance to be equal to his weight again. The new terminal velocity is small enough to survive landing

Question 18

Describe the graph of the velocity against time for the parachutist

Answer 18

First half of the graph looks like the normal velocity against time for an object reaching terminal velocity. Then the graph suddenly drops to a lower velocity and flattens out, where it reaches a new terminal velocity

Question 19

Describe the setup of the experiment finding the terminal velocity of a ball bearing in a viscous liquid

Answer 19

Put a viscous liquid in a glass tube with a stopper in the end, put the tube in some clamps attached to a clampstand
Put elastic bands around the tube at fixed distances using a ruler

Question 20

Describe the experiment of finding the terminal velocity of a ball bearing in a viscous liquid

Answer 20

Drop a ball bearing into the tube, and use a stopwatch to record the time at which it reaches each band. Record results in a table.
Calculate the times taken by the ball bearing to travel between consecutive elastic bands and calculate an average speed reading. Use the average times and the distance between bands to calculate the average velocity between each pair of elastic bands. You should find that the average velocity increases at first, then stays constant. This is the ball bearing’s terminal velocity

Question 21

How can you reduce the effects of random errors in the experiment finding the terminal velocity of a ball bearing?

Answer 21

Repeat this experiment a few times

Question 22

Investigation of the terminal velocity of a ball bearing:

How do you test the effect of changing the liquid, on the terminal velocity?

Answer 22

The terminal velocity will be lower in the more viscous liquid because the drag is greater. Try mixing water into wallpaper paste and see how much the terminal velocity increases when the drag is low

Question 23

Investigation of the terminal velocity of a ball bearing:

How does changing the shape of the thing you’re dropping, change the terminal velocity?

Answer 23

The drag force will be greater on less streamlined shapes, the terminal velocity will therefore be lower

Question 24

Investigation of the terminal velocity of a ball bearing:

Describe the effects of mass on terminal velocity

Answer 24

Heavier objects reach a faster terminal velocity because a greater drag force is needed to balance the extra weight.
Keep the size the same

Question 25

When is work done?

Answer 25

Whenever energy is transferred

Question 26

What is power?

Answer 26

The rate of doing work – it’s the amount of energy transferred from one form to another per second

Question 27

State the principle of conservation of energy

Answer 27

Energy cannot be created or destroyed. Energy can be transferred from one form to another but the total amount of energy in a closed system will not change