1. Forces
After workshop activity

2. Jodrell Bank Discovery Centre
What we are going to do...
In the Forces workshop you discussed movement and forces on Earth and in space.
We are going to look at the forces at different stages of a rocket launch to see how much you have learnt.

3. Before launch...

4. B A Before launch... What are the names of forces A and B?
Click here to reveal the answers! A

5. What are the names of forces A and B?
Before launch...
REACTION FORCE
(Ground pushing back on rocket) B
What are the names of forces A and B?
WEIGHT (or GRAVITY)
(Rocket pushing on ground) A

6. B A Before launch... The rocket is stationary.
REACTION FORCE
(Ground pushing back on rocket) B
The rocket is stationary.
Are these forces balanced or unbalanced?
Click here to reveal the answer!
WEIGHT (or GRAVITY)
(Rocket pushing on ground) A

7. The rocket is stationary. Are these forces balanced or unbalanced?
Before launch...
REACTION FORCE
(Ground pushing back on rocket) B
The rocket is stationary.
Are these forces balanced or unbalanced?
The forces are balanced
WEIGHT (or GRAVITY)
(Rocket pushing on ground) A

8. Jodrell Bank Discovery Centre
Remember...
This is Newton’s first law (part 1) If the forces on a stationary object are balanced, the object will remain stationary until an unbalanced force acts upon it.

9. Launch time!

10. Launch time! The rocket generates thrust and starts to accelerate.
Are the forces balanced or unbalanced?
Click here to reveal the answer!

11. Launch time! The rocket generates thrust and starts to accelerate.
Are the forces balanced or unbalanced?
The forces are unbalanced

12. Thrust = 3.6 Mega Newtons Weight = 3.0 Mega Newtons
Launch time!
What is the resultant force acting on the rocket?
Weight = 3.0 Mega Newtons
Click here to reveal the answer!

13. Thrust = 3.6 Mega Newtons Weight = 3.0 Mega Newtons
Launch time!
What is the resultant force acting on the rocket?
Resultant force = 0.6 Mega Newtons
Weight = 3.0 Mega Newtons

14. Jodrell Bank Discovery Centre
Remember...
This is Newton’s second law Any resultant (unbalanced) force acting on an object will cause that object to accelerate (change it’s motion).

15. Launch time! As the rocket lifts up, fuel gets used.
How does this affect the acceleration of the rocket?
Click here to reveal the answer!

16. Launch time! As the rocket lifts up, fuel gets used.
How does this affect the acceleration of the rocket?
As the rocket loses mass, the acceleration increases

17. This is also Newton’s second law
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Newton found that the acceleration of an object depends on...
The size of the force
(a bigger force produces a bigger acceleration)
The mass of the object
(a bigger mass results in a smaller acceleration)
This is described by this formula:
Force = mass x acceleration
F = m a

18. Force = mass x acceleration
Newton’s second law
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An Arianne 5 rocket can produce 13.5 MN of thrust, but has a weight of 7.6 MN. What is the resultant force at launch?
The mass of the Arianne 5 rocket is 777,000 Kg. What is the acceleration of the rocket?
The force of air resistance increases as the rocket speed increases.
Five seconds into flight the air resistance is 500 KN. What is the new acceleration of the rocket? (assume a constant thrust and mass)
[Harder] As the rocket travels, it uses fuel and its mass decreases. Five minutes into flight the mass of the rocket has reduced to 400,000 Kg, but the air resistance has increased to 4 MN. What is the new acceleration of the rocket? (assume a constant thrust)
Hint: The weight of the rocket has changed. Weight = mass x gravity. Gravity on Earth = 9.8 N/Kg
Force = mass x acceleration
F = m a
Click here to reveal the answers!
MN = Mega Newton
1 MN = 1,000,000 N

19. Newton’s second law Jodrell Bank Discovery Centre
An Arianne 5 rocket can produce 13.5 MN of thrust, but has a weight of 7.6 MN. What is the resultant force at launch? 5.9 Mega Newtons
The mass of the Arianne 5 rocket is 777,000 Kg. What is the acceleration of the rocket? 7.6 m/s2
The force of air resistance increases as the rocket speed increases.
Five seconds into flight the air resistance is 500 KN. What is the new acceleration of the rocket? (assume a constant thrust and mass) 6.9 m/s2
[Harder] As the rocket travels, it uses fuel and its mass decreases. Five minutes into flight the mass of the rocket has reduced to 400,000 Kg, but the air resistance has increased to 4 MN. What is the new acceleration of the rocket? (assume a constant thrust)
Hint: The weight of the rocket has changed. Weight = mass x gravity. Gravity on Earth = 9.8 N/Kg m/s2

20. During launch...

21. During launch...
As the speed of the rocket increases, the force of air resistance increases.
Eventually, the air resistance force balances with the thrust force.
What will happen to the motion of the rocket?
Click here to reveal the answer!

22. During launch...
As the speed of the rocket increases, the force of air resistance increases.
Eventually, the air resistance force balances with the thrust force.
What will happen to the motion of the rocket?
The rocket will continue to move, in a straight line at a constant speed

23. Jodrell Bank Discovery Centre
Remember...
This is Newton’s first law (part 2) If the forces on a moving object are balanced, the object will continue moving at a constant speed in a straight line, until an unbalanced force acts upon it.

24. During launch...

25. During launch... How does the rocket generate thrust?
Remember the two volunteers on the trolleys, with the rope?
How did this explain a rocket launching?
Click here to reveal the answer!

26. During launch... How does the rocket generate thrust?
The rocket pushes on the exhaust gas; forcing it out.
The gas pushes back on the rocket, producing an upwards thrust!

27. Jodrell Bank Discovery Centre
Remember...
This is Newton’s third law Any force acting on a body will experience a reaction force, of equal size acting in the opposite direction.

28. Click here for an example!
Newton’s third law
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How many examples of action-reaction force pairs can you think of?
How many examples can you see in the room around you?
Click here for an example!

29. Newton’s third law
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How many examples of action-reaction force pairs can you think of?
How many examples can you see in the room around you?
Walking: You push on the floor and the floor pushes back, making you move!
Sitting: Your weight pushes on the chair and the chair pushes back, so you don’t fall to the ground!

30. Congratulations! You have completed your investigation into forces
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You have completed your investigation into forces
What have you learnt today?