Course overview. Forces 5 lessons

Transcription

1 Forces Mechanics

2 Course overview Forces 5 lessons Newton s laws of mo9on (2 lessons) Mass and weight (0.5 lessons) Equilibrium (1 lesson) Unbalanced forces (1.5 lessons) Trolley Lab (DEF)

3 Newton s first law Newton I: Every object con9nues in a state of rest or uniform mo9on in a straight line unless acted upon by an external force.

4 Examples Any sta9onary object! Difficult to find examples of moving objects here on the earth due to fric9on. A possible example could be a puck on ice where the surface is near fric9onless.

5 Transla9onal equilibrium Transla9onal means of or rela9ng to uniform movement without rota9on. If a body is acted upon by a number of forces in the same plane and is in equilibrium (i.e. at rest (sta,c equilibrium) or unaccelerated mo9on (dynamic equilibrium)) then the following condi9on must apply: The components of the forces in both of any two direc4ons must balance. Is the object accelera9ng or not is the key ques9on for all problems you solve based on this. If it is accelera9ng, then Newton s 2nd law applies.

6 Balanced and unbalanced forces Consider a camel standing on a road. What forces are acting on it? Normal reaction force 12/09/13 Weight

7 Balanced and unbalanced forces Reaction What would happen if we took the road away? The camel s weight is no longer balanced by anything, so the camel falls downwards 12/09/13 Weight

8 Balanced and unbalanced forces What would happen if we took the road away? The camel s weight is no longer balanced by anything, so the camel falls downwards 12/09/13

9 Newton s second law (1st vers.) The rate of change of momentum of a body is proportional to the resultant force and occurs in the direction of the force. F = Δp t F = mv mu t

10 Newton s Second Law (2nd vers.) The acceleration of a body is proportional to the resultant force and occurs in the direction of the force. F = ma

11 Mass and Weight Mass and weight are the same thing in day- to- day life. In Physics the understanding of weight W is different: In Physics weight is the gravita9onal pull on an object, it s a force. The amount of ma\er (mass) is measured in kg. W = m g (Remember: F = ma) W: weight, m: mass, g: accelera9on due to gravity or grav. Field strength (on Earth g=10 m/s 2, on the Moon g =1.67 m/s 2 ) 12/09/13

12 Linear Momentum Define linear momentum and impulse. Momentum p of a body is defined as the mass m of the body mul9plied by its velocity v p = mv It is a vector quan9ty Its units are kg m s - 1 or Ns Example: car of 1500kg moving at 20ms - 1. p = mv =1500 x 20 = Ns

13 From Newtons second law Impulse mv mu F = t Ft = mv mu Ft = Δp This quan9ty Ft is called the impulse of the force on the body and it is equal to the change in momentum of a body. Vector quan9ty Units: kg m s - 1 or Ns IB defin9on: Impulse is the product of force and the 9me for which the force acts and is equal to the change in momentum.

14 Impulse: example A bicycle and its rider have a total mass of 100kg. The bike is accelerated over a period of 10s from 0 to 8ms - 1. Find the impulse exerted. Ft = mv mu = 100 x x 0 = 1000 Ns. Note: the 9me taken does not ma\er for the calcula9on of impulse!

15 Impulse and force- 9me graphs Determine the impulse due to a 4me- varying force by interpre4ng a force- 4me graph. The impulse can be determined from the area under the force- 9me graph. Here: Ft = 0.5 x x 2.0 x 2 + ( ) x = Ns

16 Law of Conserva9on of Linear Momentum State the law of conserva4on of linear momentum Solve problems involving momentum and impulse. The law can be stated as When bodies in a system interact the total momentum remains constant provided no external force acts on the system. IB version: the total momentum of a system is constant priveded no external force acts on the system.

17 Newton s third law When two bodies A and B interact, the force that A exerts on B is equal and opposite to the force that B exerts on A. Note: this is about forces between different bodies, not different forces on the same body!

18 Example of Newton s 3 rd Q. According to Newton s third Law what is the opposite force to your weight? A. As your weight is the pull of the Earth on you, then the opposite is the pull of you on the Earth!

19 Newton s 3 rd Law The law is sta9ng that forces never occur singularly but always in pairs as a result of the interac9on between two bodies. For example, when you step forward from rest, your foot pushes backwards on the Earth and the Earth exerts an equal and opposite force forward on you. Two bodies and two forces are involved.