Building a Catapult: Physics Prior Knowledge Recap

Physics Concepts:Turning Effects of Forces

Momentum

Before embarking on your journey for survival…

• Turning Effects of Forces

• Momentum

Recall the Physics concepts

Needed

• lever• Dead weight• Scoop

Use these concepts to research on

your plans to create your war

machine

Use Google Sketchup to draw

out your 3D modelsDesign your catapult!!!

PHYSICS CONCEPTS TO HELP YOU BUILD YOUR CATAPULT

The Turning Effects of Forces

Turning Effects of Forces

Moments Balancing Moments

Centre of mass and Stability

The turning effect of a force is called a moment.

• A moment is NOT a period of time. Nor is it the same as momentum.

Moment is the product (two numbers multiplied together) between the force and the perpendicular distance from the force and the pivot.

Moments

look at the picture:      

Line of action of force

Perpendicular distance

      

moment (newton metre, Nm) = force (newton, N)× perpendicular distance (metre, m)

Worked Examples

• Question 1 A wheel nut is tightened to a moment of 100 Nm. A motorist has to undo the nut with a wheel wrench which is 0.40 m long. What force must he apply? Answer

• Question 2 How can the force applied be reduced?Answer

Balancing Moments

Moments have two possible directions, clockwise or anti-

clockwise.

If the clockwise moment is bigger

than the anticlockwise

moment, then the object will turn

clockwise.

If the anticlockwise moment is bigger

than the clockwise moment, then the

object will turn anticlockwise.

If the clockwise moment is equal to

the clockwise moment, then the

object will stay where it is.

Therefore…

Principle of Moments: If the clockwise moment = anticlockwise moment, the system is in equilibrium

Can you show that the seesaw in the picture above is balanced? Answer

 

F2F

d 2dClockwiseAnticlockwis

e

Pivot

           

The seesaw above is balanced. What is the force F?Answer

 40 N F

0.8 m 1.8 mClockwiseAnticlockwise

Pivot

            

Centre of Mass

In Physics we find it a lot easier to think of objects as point masses. All objects have a point at which they balance, called the centre of mass. We think of all the mass as being concentrated at the centre of mass.

The centre of mass is the point at which the weight of the object is said to act. The green arrow is the line of action of the force from the centre of mass. Force due to gravity on a mass is the weight. Note that it is called the centre of mass not centre of weight. This is because if the object were in space, it would still have a centre of mass, even though it were weightless. Sometimes the centre of mass is called the centre of gravity.

Pivot

Centre of mass

Line of action

F

                

Stability

Stability is the extent to which an object resists toppling

over.

Stable objects do not topple over easily.

When designing vehicles, engineers try to design so that the centre of mass is as low as

possible.

This makes vehicles less likely to turn over when

going round corners.

You should have gotten an ideaabout how you are going to

Structure your catapult by now…Use these concepts to your

advantage.We are going to learn about

momentum next and this would help you with the concepts on

firing your projectile

PHYSICS CONCEPTS TO HELP YOU BUILD YOUR CATAPULT

Momentum

Definition

All objects have mass; so if an object is moving, then it has momentum - it has its mass in motion dependant variables: how much stuff is moving, how fast the stuff is moving

Equation

• momentum of an object is equal to the mass of the object times the velocity of the object.

Momentum = mass • velocity

p = m • v

p- momentum (kg•m/s)

m- mass (kg)

v- velocity (m/s)

The momentum equation can help us to think about how a change in one of the two variables might affect the momentum of an object.

Use this equation to determineHow you are going to fire your

projectiles!!!

Now get to work!!!Your Survival Depends On It!!!

Resources

• http://www.antonine-education.co.uk/physics_gcse/Unit_3/Topic_1/topic_1.htm

• http://www.physicsclassroom.com/class/momentum/u4l1a.cfm