E-Activity 7

1. Copy and complete the table using the results of all six experiments shown in the diagrams.
2. What do you notice about the first and last columns in the table?
3. Look at the examples in the picture.
Use the formula to work out the missing items.
The first one is done for you.

Force, mass and acceleration

Applying a force
Students investigate the relationship between force, mass and acceleration in the laboratory.
• How do the students apply a constant force to accelerate the trucks?
• When they apply the same force to different masses, which accelerates faster-the larger mass or the smaller mass?
• If they keep the mass constant but increase the force, how does the acceleration change?
• Give one reason why it’s difficult to do the experiment accurately.
Experiments in space
A spaceship is a better place to measure the accelerations produced by different forces.
• Why is a spaceship a good place to do experiments on force and acceleration?
How force affects acceleration
The diagrams show some experiments in a spaceship.
The same 1 kg mass is accelerated using different forces.
• How much acceleration does a 1 N force give tgo the 1 kg mass?
• How much acceleration does a 2 N force give?
Copy and complete the sentence.
• When the force on an object is doubled, its acceleration is _______________
How mass affects acceleration
A 1 N force is then used to accelerate different masses.
• How much acceleration do you get when the mass is twice as big?
• How much acceleration do you get when the mass is three times as big?
Copy and complete the sentence.
• If the force stays the same, an object with twice as much mass is given half as much __________
A force of1 N acting on a mass of 1 kg produces an acceleration of 1 m/s2.

Friction V-Speed and Safety

Friction and the motor car

In order that a car can change its speed or direction, there must be friction between its tyres and the road surface. A new tyre with lots of tread has a rough surface and will provide lots of grip. A worn tyre with a smoother surface will provide less grip and the cart will be much more difficult to control and stop.
Having tyres which are in food condition is important if you need to stop quickly, but there are several other important factors which will affect how quickly a car can stop. These are:
• The reaction time of the driver,
• The efficiency of the breaking system of the car,
• The speed of the car,
• The weather/road conditions,
Figure illustrates how the speed of a car affects the total distance it will travel before it stops. The total stopping distance is equal to the thinking distance + the breaking distance.
Braking force
When a car driver brakes, the braking force between the brakes and the wheels slows the wheels down.
Explain why you must brake harder to stop in the same direction when you are traveling at higher speed.
Tyres must grip
Friction between the tyres and the road makes the tyres grip the road. When you brake, the wheels slow down. The friction with the road must then increase to slow the whole car. The friction force depends on the tyre design and the road surface.
• Why does the car skid if you brake too hard?
• Why must the driver brake carefully when the road is wet?
• Why must the driver brake very carefully when there is ice on the roads?
• Why is it safer to have a rough road surface before a pedestrian crossing?
Why do tyres have tread?
You need good tyres to stop quickly. Tyres can grip the road only if they are touching it. They lose their gripwhen the road is wet. The tread on a tyre is designed to push away the water. In dry conditions, the tread doesn’t help. In dry weather, racing cars use tyres with no tread.
Why do racing drivers stop to change their tyres when it starts raining?
Why do worn tyres increase the chance of a skid?
You can’t stop instantly
If someone steps out in front of a car, it takes time for the driver to react. This is called the reaction time. The distance the car travels during the reaction time is called the thinking distance.
Look at the diagram. Why does it take time to react?
Look at the table.
• What is the thinking distance when traveling at 30 miles per hour?
• What happens to the thinking distance if the speed is doubled?
When the driver presses the brake pedal, it takes time for the brakes to slow the car down. During this time, the car travels a distance called the braking distance.
Look at the table.
• What is the braking distance for a speed of 30 miles per hour?
• What happens to the braking distance if the speed is increased?
• Copy and complete the formula.
• Stopping distance = thinking distance + _________ distance
After drinking alcohol, people may feel perfectly normal but their reactions are actually much slower.
• Why is it a bad idea for people to drive after drinking alcohol?
• What other factors could affect the reaction time?

Friction IV-Speed time graph for a skydiver

Friction III-Terminal velocity

Friction II-Drag

Drag

Whenever an object moves through the air it experiences frictional forces or drag. Which try to prevent its motion. The faster the object moves, the greater the drag. To reduce these forces objects such as cars, trains and aircrafts are shaped so that they cut through the air. They are streamlined to reduce air resistance.
Animals such as dolphins, sharks and penguins have streamlined shaped so that when they move through the water, frictional forces are as small as possible.

Friction I

One of the most common forces which can act upon an object is friction. Whenever an object moves or tries to move, friction is present. Friction is a force which opposes motion.
On some occasions friction can prove very useful. For example, when you walk or run, you push yourself forward by pushing backwards on the ground. Friction between your foot and the floor helps you to do this. If there was no friction, i.e. like on a slippery ice rink, your feet would slip!
Smooth surfaces reduce the friction between objects while rough surfaces increase the frictional forces. Trainers and football boots are designed to prevent your foot from slipping by increasing the frictional forces between you and the ground. In contrast, skates and skis are designed with smooth surfaces which keep friction to a minimum.
Where there is contact between surfaces, friction can be reduced using a lubricant such as oil, friction between two surfaces can cause the surfaces to wear away and become hot.

E-Activity 6

1. Copy and complete the sentences.
(a) When a bicycle moves through the _______________, a force of friction acts in the opposite _____________.
This force ______________ the bicycle down.
When a canoe moves through the water, a force of friction acts in the _______ direction.
This ___________ the canoe down.
(b) An ice rink is almost _________________ free. If there was no friction at all, a curling stone would carry on moving at ____________ speed in a ______________ line. It would _____________ stop.

2. What is the resultant force on the sailboard when it is traveling at constant speed?
3. What force balances the air resistance on the moving jet?
4. What is the resultant force on the jet?
5. When was the space probe Voyager 1 launched?
6. How fast is Voyager traveling away from the Sun?
7. Voyager has no engine, so why does it keep moving?
8. Explain why Voyager is now moving in an almost straight line.

6-Moving at constant speed

Why do things slow down?
If you stop pedaling your bicycle, air resistance and other friction forces slow you down. Friction forces always act in the opposite direction to movement. A similar thing happens if you stop paddling a canoe-the main friction force on a canoe is water resistance.

Keeping going
When there is friction, you need another force to keep going at constant speed.
When a sailboard is moving, water resistance acts against it. To move at a steady speed, the force of the wind on the sail must balance the friction force.
These examples show that if the resultant force acting on a moving body is zero, the body continues to move at the same speed and in the same direction.

Life without friction
Without friction, a cyclist could freewheel forever in a straight line. She would not need to pedal to keep going at a constant speed.
You can get some idea of what life would be like without friction at an ice rink.

Motion in space
Space is an almost perfect vacuum. There is no air to cause friction. In deep space, where the gravity of stars and planets is negligible, a spacecraft will travel in a straight line at constant speed.

Mass

Inertia

Acceleration Part II

Acceleration Part I

Speed

Gravity

Weight vs. Mass

Mass and weight are different. You have the same mass wherever you are because you have the sameamount of body. But your weight depends on the strength of gravity where you are.