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Science 

Exercise 4 - Speed, Forces, Friction and Pressure

Speed

The Speed of an object is a measure of how fast it is moving.
The Velocity of an object is a measure of how fast it is moving and in which direction it is moving.

For example.

Speed = 10 metres per second
Velocity = 10 metres per second northwards.

To find the speed of a specific object we need to know the distance it travelled and how long it took to travel that distance. Then we can calculate the speed. Of course there is an easy formula, which details the relationship between speed, distance and time taken.
 

		Distance
Speed	=
		Time

If drawn as a formula triangle, this formula would appear as:
 

	D = Distance
	S = Speed
	T = Time

 

Using the formula above, can you answer the following questions?
1) A cyclist is travelling at 20 km/hour. How long will it take the cyclist to complete his 100 km journey?
	Select One 2 KM 20 SECONDS 720 METRES 50 KM/HOUR 160 KM 4 METRES/SECOND 20 METRES/SECOND 5 HOURS
2) A tennis ball is thrown across a playground. It takes 10 seconds for the ball to travel across the playground, which is 200 metres wide. What is the speed of the tennis ball?
	Select One 2 KM 20 SECONDS 720 METRES 50 KM/HOUR 160 KM 4 METRES/SECOND 20 METRES/SECOND 5 HOURS
3) A bus is travelling at 40 km/ hour. How far will the bus travel in 4 hours?
	Select One 2 KM 20 SECONDS 720 METRES 50 KM/HOUR 160 KM 4 METRES/SECOND 20 METRES/SECOND 5 HOURS
4) Jenny ran the 400 metres in 100 seconds? What speed was she running at?
	Select One 2 KM 20 SECONDS 720 METRES 50 KM/HOUR 160 KM 4 METRES/SECOND 20 METRES/SECOND 5 HOURS
5) It takes 6 hours for Mr Smith to travel by car to a conference 300 km away. What average speed was Mr Smith travelling at?
	Select One 2 KM 20 SECONDS 720 METRES 50 KM/HOUR 160 KM 4 METRES/SECOND 20 METRES/SECOND 5 HOURS
6) How long will it take a cannonball to travel 500 metres at a speed of 25 metres/ second?
	Select One 2 KM 20 SECONDS 720 METRES 50 KM/HOUR 160 KM 4 METRES/SECOND 20 METRES/SECOND 5 HOURS
7) If Ella has an average swimming speed of 4 metres/second, how far will she swim in 3 minutes?
	Select One 2 KM 20 SECONDS 720 METRES 50 KM/HOUR 160 KM 4 METRES/SECOND 20 METRES/SECOND 5 HOURS
8) Joshua walks at a speed of 8 km/ hour. How far will he walk in 15 minutes?
	Select One 2 KM 20 SECONDS 720 METRES 50 KM/HOUR 160 KM 4 METRES/SECOND 20 METRES/SECOND 5 HOURS

Forces

Forces can be applied to objects and can cause the object to:

• Start or stop moving

• Speed up or slow down

• Change direction

Most forces are applied through physical contact. However you need to be aware of two forces that can be applied to an object without any physical contact being applied.
These two forces are gravity and magnetism.

Measuring Forces

Force can be measured using a Newton meter. The Newton meter consists of a spring and a scale; the scale measures how much the spring stretches when a force is applied to it. The larger the force the more the spring extends. Force is measured in Newtons. An average sized apple has a weight of approximately 1N (Newton).

Newton Meter

Forces and Acceleration

The size of acceleration is dependent on the size of the force and the size of the mass of the object. The four main rules to remember are:

• A small force will give a small acceleration.

• A LARGE force will give a large acceleration.

• A small mass will lead to a large acceleration.

• A LARGE mass will lead to a small acceleration.

Balanced and Unbalanced Forces

If several forces are applied to the same object, they may cancel each other out. In this case the forces are balanced. When balanced forces are applied to an object they will have no effect on its motion. If the object in this case is stationary, it will remain stationary. If balanced forces are applied to a moving object, the object will continue moving at the same speed and in the same direction.

Examples of balanced forces include simple things such as a book resting on a table. The weight of the book (gravity - downward force) equals the reaction from the table (upward force) leading to the object being stationary.

If the forces applied to an object do not cancel each other out, they are unbalanced and hence will have an effect on the motion of the object.

Friction and Air Resistance

Whenever an object tries to move, friction is present. Friction is present when two surfaces are gripping. This is called static friction. When two surfaces are sliding over each other the friction present is called sliding friction. Friction acts in the direction that opposes motion. Therefore if a car is moving forward the friction occurs in the reverse direction between the tyres and the road surface.

Friction can be reduced through streamlining and lubrication. Streamlining is where the shape of the object is such that it cuts through the air with less resistance. Lubrication is where a substance is applied between the two surfaces to reduce friction.

Continuous friction between surfaces can lead to them wearing away. In addition friction can cause an increase in temperature (heat).
 

Comprehension on Forces and Friction Can you answer the following questions on forces correctly?

1) Name two forces that are not dependent on physical contact.

2) Which instrument do we use to measure the size of a force?

3) How do we measure force?

4) If balanced forces are applied to an object, how will that object change direction?

5) Name two ways in which force can affect an object.

6) If the forces on an object are unbalanced, will the object be affected by the force?

7) If a large force is applied to a small mass, will the rate of acceleration be large or small?

8) If a small force is applied to large mass, will the rate of acceleration be large or small?

9) Friction can be reduced by two factors. What are they?

10) Friction can cause two factors? What are they?

Pressure

Pressure is a measure of how concentrated or spread out a force is.

If a force is applied over a small area it creates a large pressure.
Alternatively, if a force is applied over a large area it creates a small pressure.

Examples of this phenomenon are present in every aspect of life.

Consider pushing a drawing pin into a wall. The area is incredibly small (the sharp point of the drawing pin) and hence the pressure will be large. Consider a skier. If the skier is walking in the snow, his feet will sink quite quickly. However if he is wearing skis, he will be able to glide on the snow instead. This is due to the area of the skis being considerably larger than that of the skier's shoes. Hence the pressure is larger on the small area (the skier's shoes) and the skier sinks into the snow.

We are able to calculate the pressure created by a force using the following equation:

		Force
Pressure	=
		Area

Pressure is measured in Pascals (Pa).

(remember area = breadth x length).

If drawn as a formula triangle, this formula would appear as
 

	F = Force
	P = Pressure
	A = Area

Using the formula above we can consider the following question.

A crate weighing 500 N is standing upright on one of its sides, which measures 2 m x 2m. Calculate the pressure created by the crate.

To answer this question we must firstly consider the area of the crate.
The area is 2m x 2m = 4m².

Now we know the area we can apply the rest of the formula for pressure.

Pressure = Force = 500 = 125 Pa.
Area 4

The correct answer is 125 Pa.

Now can you answer the following questions?

 

Using the formula above, can you answer the following questions?
1) A force of 50N is applied to an area of 2.5m². What is the value of pressure?
	Select One 10m² 320N 60N 50 Pa 5m² 20 Pa
2) A pressure of 6 Pa is present over an area of 10m². What is the value of the force?
	Select One 10m² 320N 60N 50 Pa 5m² 20 Pa
3) A force of 100N is present over an area leading to a pressure of 20 Pa. What is the size of the area?
	Select One 10m² 320N 60N 50 Pa 5m² 20 Pa
4) What force, when applied to an area of 4m² will lead to a pressure of 80Pa?
	Select One 10m² 320N 60N 50 Pa 5m² 20 Pa
5) What is the resulting pressure when a force of 200N is applied to an area of 4m²?
	Select One 10m² 320N 60N 50 Pa 5m² 20 Pa
6) A force of 250N is present over an area leading to a pressure of 25 Pa. What is the size of the area?
	Select One 10m² 320N 60N 50 Pa 5m² 20 Pa

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