Newton’s Laws of Motion

Newton’s First LawIf there is no unbalanced force an object will move at constant velocity or remain at rest.

Newton’s Second LawFu = ma

Newton’s Third LawEvery force has an equal but opposite reaction force.

Eg. Force of the rocket on the hot gases = - Force of the hot gases on

the rocket

Balanced Forces

Forces are balanced when they are equal in size but opposite in direction.

If everyone pulls with the same strength there will be no movement.

Forces are balanced.pull pull

Buoyancy of water

weight

Forces balanced so no up down movementBigger downward force boat moves down

For balanced forces there is no change in movement.

Force due to water - buoyancy

Force due to gravity - weight

Going down Going up

Balanced Forces

Object will be moving at constant speed or be at rest.

Terminal speedthrust

weight

lift

Air resistance

Parachute weight is constant

weight

weight

Air resistance

No air resistance

weight

Air resistance

Just left the plane

No speed, no air resistance.

Downward unbalanced force – accelerate down

Speeding up, air resistance increasing.

Unbalanced force is smaller so less acceleration.

Maximum speed ( terminal velocity)

Air resistance = - weight so no unbalanced force

No acceleration

Draw a speed time graph for a parachute jumpSky diver

weight

Air resistance

weight

Air resistance

Land !!! Speed is now zero

As speed decreases so does air resistance until the forces are balanced and terminal velocity is reached.

The parachute is opened and air resistance increases.

Upward unbalanced force, so the parachute decelerates and slows down

Tutorial Qus page 5/6 Free falling objects.

Example

1. A car of mass 5000 kg is travelling at constant speed. The engine exerts a force of 20 N.

(a) What is the magnitude of the friction forces?

(b) The engine force increases to 100N. What is the car’s acceleration? Assume the friction forces stay constant.

Tutorial Questions pages 14 and 15

Qus 1 to 8

Rockets

2. A rocket of mass 10 000kg was launched from the Earth.

(a) If it had an initial acceleration of 2ms-2, what would the force of the rockets be?

(b) As it moves away from the Earth, the acceleration increases, give two reasons why.

Lifts

3. A person of mass 60 kg stands on scales inside a lift. The lift accelerates at 4 ms-2 downwards. What will be the reading on the scales?

Tutorial Questions 15 to17 Qus 9 to15

Purple book Chp 2.5

Connected Masses4. A force of 20 N is used to pull the blue

box, mass 5 kg. Friction is 1N on each mass.

The green box has a mass of 1.5 kg and the grey box has a mass of 2 kg.

What is the acceleration of the blocks?

What will be the tension between the green and blue boxes?

Tutorial Questions pages18 and 19 Qus 16 to 19

Connected Masses 2

5. Assume no friction

(a)What is the acceleration of the 0.5 kg trolley?

(b)What is the tension in the string?

0.5 kg

1 kg

Tutorial Questions page 19 Qu 20

Purple book Ex 2.2

Forces at Angles40 N

450

6. If the object has mass, 1500 Kg, what will its acceleration be?

Tutorial Questions pages 19 to 22 Qus1 to 5

Forces and Slopes

200

7. A mass of 10 kg is sliding down a slope. If it accelerates at 2ms-2, find:-

(a) The component of the weight acting down the slope.

(b) The friction between the mass and the slope.

Tutorial Questions pages 19 to 22 Qus1 to 9

Purple book Chp 2.4

Fw

200

Fc