Forces

Forces

• Aidhm• What are forces • What are they measured in

Do objects move on their own?

No. Something must cause it to move

A force is something that causes an object to accelerate

Forces

• Cause an object at rest to move• Cause a moving object to change its direction

(velocity)

Isaac Newton

Unit of force is called the Newton after Isaac Newton

Different Types of Forces•Pushing•Pulling•Weight•Friction•Magnetic

Can you think of any other types?

What type of force is this?

Pulling

Can a ball move without a force been applied to it?

Yes: due to the force of gravity

FrictionFriction is the force which opposes motion

between two objects in contact

Puck on ice

Can you think of any other places you will see the effect of friction?

The advantages and disadvantages of friction

The advantages and disadvantages of friction

Advantages• Walking• Holding objects• Wheels of cars

and bikes

Disadvatages•Slows things down•Wastes energy•Gases in atmosphere effect surface of space shuttle

How do we reduce friction?

Lubrication

Oil: reduces friction in cars Synovial fluid: in joints in the body

How would you measure friction?Newton meter

Block of wood

Friction between block and table

http://www.youtube.com/watch?v=OSJlL4wqLGo&feature=related

Moon walk

Weight

The weight of an object is the force of gravity on itWeight is a force. So what is it measured in?

Newtons

Do astronauts have the same weight on the moon as they do on the earth?

NoThe force of gravity is not the same on the moon as

on the earth

Would you weigh the same if you stood on Mount Everest and stood at normal ground

level?

No. Weight depends on distance between two objects (person and the centre of the earth)

Mass versus Weight

1.Weight of an object is the force of gravity on it2.Measured in Newtons3.Changes •depending on the force of gravity (moon and earth)•Depending on distance between objects (person on Mount Everest)

1.Mass is the amount of matter in an object2.Measured in kilograms3.Never changes no matter where you are

Are you finding the mass or weight of an object using scales?

You are finding the mass

Force = Mass × Acceleration

Formulae you need to know

Newtonskg m/s2

Weight = Mass × 10

NewtonsKg

When you drop an apple, its speed will increase 10metres in 1 second. Its acceleration is 10m/s2

m/s2

(a) What is the weight of an object whose mass is1. 2 kg2. 5 kg3. 9 kg4. 500 g (hint 1000 g = 1 kg)

(b) What is the mass of an object that has a weight of5. 30 N6. 4.5 N

(c) What force is needed to give an object of mass 3 kg the following accelerations

7. 3m/s28. 5 m/s2

(d) What force is needed to give an object an acceleration of 4 m/s2 when they have the following masses

9. 7 kg10.8kg

What acceleration is needed to give an object1.A force of 12 N, mass of 3 kg2.A force of 15 N , mass of 3 kg

What mass is an object if3.A force of 20 N and travels at an

acceleration of 10 m/s24.A force of 50 N and travels at an

acceleration of 25 m/s2

Elastic/Stretching Forces

When a force is put on an elastic object it changes shapeWhen the force is removed it returns to its original shape

http://www.youtube.com/watch?v=NfedS1tbV2oAt 4 mins onwards

Hookes Law

The extension of a spring is directly proportional to the force that is

stretching it

1. When icy weather is expected, the Co. Council or Corporation often spread sand on the road. Why do they do this?

2. If you slide down a rope, your hands can get burned. What causes this?

3. A spring has a 3N weight hanging from it and the total length of the spring is 20cms. When the 3N weight is replaced by an 8N weight, the total length of the spring is 40cms. What would the length of the spring be when there is no weight on it?