Newtons Laws

1. Topic – Physics 2a Newtons Laws Key words Force, mass, acceleration, action, reaction, tension

2. Newtons first law: Inertia

3. Newtons first law What is Newton’s first law? • States that a body will either stay at rest or continue with a constant speed in a straight line unless it is acted on by a net force. • Draw Fig 3.15 page 97 How do satellites move at a constant velocity? • Since there is no air in space to retard a craft, a satellite will continue to move at a constant velocity once in orbit as long as it is not acted on by a net force the opposite direction. • This also occurs if forces are involved but there net forces are zero- ∑F=0. They are said to be at equilibrium or forces are balanced

4. Describe how the following show inertia

5. Answers:Describe how the following show newtons first law • Newton’s First Law: Things like to keep doing what they are doing unless acted on by net external force. • Shopping trolley has a large inertia (large mass) so a large resistance to change of motion. When it is moving it wants to keep moving so much harder to stop and you need a large net external force acting in the opposite direction Again, explain Newton’s First Law. Here the china is heavy (large inertia) so wants to stay in place as that is what it is doing. As long as the table cloth is pulled quickly, and the friction reduced, then the china will stay where it is

6. 3 and 4 • Trying to push car: • Newton’s First Law: Things like to keep doing what they are doing unless acted on by net external force. • The car is stationary and has a large inertia (large mass). To overcome the large inertia a very large net external force is required. Passengers in a bus: Newton’s First Law: Things like to keep doing what they are doing unless acted on by net external force. When a passenger is standing on a moving bus, they are moving at the speed of the bus. When the bus stops, unless a net external force is acting on them (seat or hand hold) they will continue to move forward and can fall over when they lose balance.

7. Mass and inertia Activity:Use these ideas to answer the questions below. • The law of inertia states that no force is required to maintain motion. So why do you have to keep peddling your bicycle to maintain motion when riding somewhere? • In this case there is a net external force acting on the bicycle – friction. 2. If you were in a spaceship in space and fired a rocket, how much force would have to be exerted on the rocket to keep it going? • None, there is no external force acting on the spaceship Complete questions 3.3 page 101 Heinemann

8. Newtons second law • Consider a book sitting on your school desk. According to Newton’s First Law, the book will stay there until a net external force is added. Say you now push that book (you supply the net external force), the book moves. Newton’s Second Law of Motion explains the relationship between the force supplied and the acceleration. • Newtons second law states: rate of acceleration is proportional to the force supplied and inversely proportional to the mass of the object.

9. Or more simply, • Acceleration = Force / Mass • Force = mass x acceleration - which is more often stated as F = ma • Newton’s Second Law of Motion is also expressed as follows: The rate of change of momentum of an object is proportional to the applied force and takes place in the direction of that force. • This can be explained by investigating momentum. • F= (mv-mu) = F = m(v-u) ∆t∆t • Now we know that change in velocity divided by time is acceleration so substituting this into the above equation results in F = ma which, as we have said before, is the simple way of expressing Newton’s Second Law.

10. Newtons third law • Newtons third law states: • states that for every force applied (action), an equal and opposite force always appears (reaction), even if no movement results. • The most important aspect of Newton’s Third Law is to be aware of the fact that there are TWO different forces acting. While the value is identical, the directions are reverse so they are different forces. The forces act on different objects, NEVER on the same one, so the forces can’t cancel each other out.

11. E.g. body A exerts a force on B – ACTION body B exerts a force on A – REACTION • Example: Kicking a ball: A common misunderstanding of Newton’s Third Law – if I kick a ball, the ball can’t move as action/reaction forces are the same. But if the ball is kicked it accelerates. Does this contradict Newton’s Third Law? NO!!!!!! • The only force on the ball is the kick, so it can accelerate, the reaction force acts on your foot and slows it down – that is the force from the ball on your foot.

12. Task 3 • Activity: • Why do cars move? • Action force: Reaction Force: • Why do objects fall to earth? • Action force: Reaction Force: • A bug hitting the windscreen of a bus slows the bus down. • Action force: Reaction Force:

13. Task 3 • Activity: • Why do cars move? • Friction of road on tyre exerts a force on the type and the car moves forwards. • Action force: road on tyre Reaction Force: tyre on road • Why do objects fall to earth? • Gravity is obviously pulling the object towards the Earth but likewise, an object is pulling the Earth towards itself as well. It is only that the mass of the Earth is so much more than the object that the object moves a much further distance than the Earth • Action force: Earth on object Reaction Force: Object on Earth • A bug hitting the windscreen of a bus slows the bus down. • Yes, it does. The bug has a very small mass and very large acceleration compared to bus • Action force: bug on bus Reaction Force: bus on bug