# Forces - Forces and their Effects

This GCSE Physics quiz takes a look at forces and their effects. Having a good understanding of forces can help you understand how the world around you moves. In some cases, it can be very useful as it can help you to do physical tasks more easily but most of the time, you will use forces without even thinking about them. Forces can be used to transfer energy and energy can be used to create forces. The forces used to slow a vehicle transfer kinetic energy into heat energy and the chemical energy in your body can be transferred into kinetic energy to move your muscles, creating forces that are useful to your body.

Forces behave predictably, which is extremely useful to humans. Engineers can predict how forces will affect the materials of building materials; vehicle manufacturers can work out the forces required to safely stop their vehicles; rock climbers use the forces of friction and learn how to use turning forces to help to keep them from falling; tennis players learn how to control the tennis ball by changing the forces they apply using their racquet.

Knowing about forces is much more than just learning a few ideas for your physics GCSE!

Unbalanced forces can cause changes to the shape or motion of an object - they can move in a straight line at a constant speed when the forces are balanced or change their shape, speed and / or direction (accelerate or decelerate). Graphs can be used to describe the movement of an objects, the two that you need to learn about are distance-time graphs and velocity-time graphs. For your GCSE, you need to be able to use your skills, knowledge and understanding to interpret data from tables and graphs relating to speed, velocity and acceleration in a wide variety of contexts. Two of the main contexts they use are about cars and skydiving.

For cars, you need to evaluate the effects of alcohol and drugs on stopping distances and evaluate how the shape and power of a vehicle can be altered to increase the vehicle's top speed. The key points to remember about stopping distances are that they involve a thinking distance and a braking distance. Anything that slows down a person's thinking such as drugs and alcohol will increase the thinking distance - the car will have travelled further. If a car's brakes are worn or otherwise in a poor condition, or the tyres are worn, the braking system will not work as effectively, increasing the braking distance. You also need to know that adverse conditions that reduce the frictional force of the tyres on the road will also increase the braking distance. The key points to remember about increasing a vehicle's top speed are that air resistance, mass and driving force from the engine are all involved.

You will be expected to be able to draw and interpret velocity-time graphs for objects that reach terminal velocity, including a consideration of the forces acting on the object. The sort of thing that you may be asked to do is to sketch a graph showing the speed and velocity of a skydiver before and after they have opened their parachute. For cars, you may have to interpret the forces acting on the car based on a graph. You need to know how to work out acceleration and distance travelled from graphs too and there are a few simple equations that you need to learn.

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1. Which of the following surfaces will have the greatest frictional force?
Drivers need to take more care when braking on smooth road surfaces especially when they are wet as friction is lower
2. The equation that links mass, force and acceleration is ...
This is Newton's second law. You will need to be able to rearrange it to work out the mass or acceleration when given the other two figures
3. In a vacuum, all bodies accelerate at what?
What stops a feather falling at the same rate as a stone on Earth is the amount of air resistance each object feels. In a vacuum, this restrictive force is removed and both objects will fall at the same rate. This was famously demonstrated by the Apollo 15 astronaut, David Scott, at the very end of their final Moon walk
4. What can affect the speed of a car?
It is all about resultant forces and inertia
5. When is the terminal velocity of an object reached?
This happens when the resistance of the fluid is the same magnitude (size) as the force causing the movement
6. As the speed of a car increases, what happens to the strength of the air resistance?
Air resistance is caused by the car hitting the molecules of the gases that make up the atmosphere
7. If the resultant force acting on an object is zero, what happens to the object?
It depends on the state of the object when the resultant force is zero. This often sounds odd as instinct tells us that zero force would mean no movement. That's true but this is about resultant forces which can change at any time. So if it changes to zero when the object is moving, it will continue to move exactly as it was when the resultant force hit the zero mark
8. When a resultant force acts upon an object, what determines the size of the acceleration of that object?
Larger masses will be accelerated more slowly for the same resultant force. Using a larger resultant force on a given mass will cause it to accelerate faster (and vice-versa of course for both cases!)
9. How can the acceleration of an object be measured on a velocity time graph?
The gradient is measuring how the velocity of the object has changed over a time period which is what acceleration is all about
10. How can distance travelled be calculated by using a velocity time graph?
If you sum up the area underneath a velocity-time graph, you have calculated the distance an object has travelled in a given time

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