Science /BIOLOGY online and physical classes

Seeing forces

1. CAMBRIDGE PRIMARY SCIENCE: Physics Year 7 Unit 9: Forces and Motion Isini Sehansa Amarathunga Bsc(Hons) in Biomedical Science 9.1 Seeing forces

2. 9.1 Seeing forces Must understand that forces can’t be seen Should be able to explain the effects of force Could be able to label force arrows Learning objectives

3. Force

4. What is force?

5. Effects of force

6. Effects of force

7. Effects of force

8. Effects of force

9. Effects of force

10. The direction of each arrow shows the direction of each force. A force diagram uses labelled arrows to show all the forces acting on the object. What is the force diagram for this ball when it first starts to fall? air resistance weight How can forces be represented? The forces acting on any object can be shown using a force diagram . The length of each arrow is proportional to the size of the force. Forces are measured in newtons ( N )

11. thrust What forces are acting on this fish as it swims at a constant speed? friction Thrust is the forwards force created by the fish. Upthrust is the upwards force on the fish caused by the water around the fish. This is sometimes called buoyancy . weight upthrust What forces act on a swimming fish?

12. What forces are acting on this accelerating boat? What forces act on a moving boat? thrust drag weight upthrust air resistance Air resistance is friction caused by movement through air.

13. What are resultant forces? There are usually several different forces acting on an object. The overall motion of the object will depend on the size and direction of all the forces. The motion of the object will depend on the resultant force . This is calculated by adding all the forces together, taking their direction into account. 50 N 30 N Resultant force on the crate = 50 N – 30 N = 20 N to the left

14. 9.2 Forces big and small Must know that Newton is the unit of force Should understand that force can be measured Could know how to use force meter to measure force Learning objectives

15. Force A force can be a  push   or a  pull . For example, when you push open a door you have to apply a force to the door. You also have to apply a force to pull open a drawer.

16. Measuring forces Forces can be measured using a force meter . Force meters contain a spring connected to a metal hook. The spring stretches when a force is applied to the hook. The bigger the force applied, the longer the spring stretches and the bigger the reading The unit of force is called the newton , and it has the symbol N . So 100 N is a bigger force than 5 N.

17. Measuring forces

18. 9.3 Mass and weight Must know that weight is a force Should distinguish between mass measured in kilograms (kg) and weight measured in Newtons . Could recognise and use units of mass and weight and identify the direction in which forces act. Learning objectives

19. What conclusions can you make about this? Introduction 56kg * 10N

20. People often confuse mass and weight. Remember that, Weight is a force, and is measured in newtons , N. Mass is measured in kilograms , kg. Weight, mass and gravity

21. Mass The mass of an object is the  amount of matter  or "stuff" it contains. The more matter an object contains, the greater its mass . An elephant contains more matter than a mouse, so it has a greater mass. Mass is measured in kilograms ,  kg . Remember an object's mass stays the same wherever it is

22. All objects have a force that attracts them towards each other. This is called gravity . Gravitational force increases when: 1. the masses are bigger 2. the objects are closer We are pulled down towards the ground because of gravity . The gravitational force pulls in the direction towards the centre of the Earth. Gravity

23. Weight Weight is a force caused by gravity . The weight of an object is the gravitational force  between the object and the Earth. The more mass the object has the greater its weight will be. On the surface of the Earth an object with a mass of 1 kg has a weight of about 10 N.

24. Mass and weight The mass of an object stays the same wherever it is, but its weight can change . This happens if the object goes somewhere where gravity is stronger, or weaker, such as the Moon. The Moon has less mass than the Earth , so its gravity is less than the Earth's gravity. This means that objects weigh less on the Moon than they do on the Earth. Remember that mass is measured in kilograms, kg, and weight is measured in newtons, N.

25. Mass and weight The Moon's gravity is one sixth of the Earth's gravity. A 120 kg astronaut weighs 1200 N on Earth. On the Moon they would weigh only 200 N. The astronaut's  mass  is 120kg wherever they are.

26. 9.4-Friction -an important force Friction is a force that appear when two objects are in contact with each other.

27. Steps Set up: Attach a force meter to a block and place it on a surface. Static Friction: Pull the force meter slowly until the block begins to move, record that force. Friction (Optional): Keep pulling at a constant speed, record the new force reading. Analyze: Write down the friction forces (static & kinetic if measured) for calculations.

28. 9.5-Air resistance