Category Science

We have seen that forces can change the shape of things. But sometimes the changes are not always permanent. A rubber band will stretch, but as soon as you let go it will return to its original shape. As it does so, it exerts a force — called an ‘elastic force’.  Metals are harder to stretch but they can exert a greater elastic force. Metals are often coiled to make springs —to be used for machinery parts or trampolines, or to make seats and mattresses more comfortable for example. Elastic forces are also used to absorb a large force — like breaking the fall of a bungee jumper.

When a rubber ball is dropped onto the ground, it is squashed. The ground exerts a force which pushes the ball upwards and back into the air. The ball then returns to its original shape.

As you stretch or compress an elastic material like a bungee cord, it resists the change in shape. It exerts a counter force in the opposite direction. This force is called elastic force. The farther the material is stretched or compressed, the greater the elastic force becomes. As soon as the stretching or compressing force is released, elastic force causes the material to spring back to its original shape.

After the bungee jumper jumps, he accelerates toward the ground due to gravity. His weight stretches the bungee cord. As the bungee cord stretches, it exerts elastic force upward against the jumper, which slows his descent and brings him to a momentary stop. Then the bungee cord springs back to its original shape and the jumper bounces upward.

Bedsprings provide springy support beneath a mattress. The spring in a door closer pulls the door shut. The spring in a retractable ballpoint pen retracts the point of the pen. The spring in a pogo stick bounces the rider up off the ground.

Friction makes it difficult to rub two dry, rough surfaces together. The thin blades of ice skates move easily on ice because there is little friction between the two smooth surfaces. Sometimes, friction can be used to make a surface smoother. For example, sandpaper is rubbed over wood to wear away the rough edges.

Friction can also wear away moving parts in a machine, eventually ruining them. To prevent this, a lubricant, such as oil, is used. Oiled door hinges will move against each other easily and there will be little wear. Some machines, like aeroplanes and cars, are also designed to reduce friction between the body and surrounding air particles.

If you magnify two surfaces which look smooth you can see that they are actually quite rough. As you rub the surfaces together they scrape against each other. Friction slows down their movement and wears them away.

Putting oil between surfaces (like the parts in a car engine) helps to make them smooth and wet. The materials can now be moved quickly and easily against each other, which reduces friction, and prevents the surfaces from being worn away.

Sports cars have a streamlined design to reduce friction between the moving vehicle and air particles.

When two objects rub against each other they cause ‘friction’. Friction is vitally important in our lives. Friction between our shoes and the ground stops us from slipping over when we walk. Friction between tyres and the road allows cars and Lorries to move forwards and prevents them from skidding. Friction also causes heat — you can start a fire by rubbing two sticks together.

Friction also slows things down. A ball rolling along the ground will gradually get slower until it stops, because of friction between the ball and the ground. Car and bicycle brakes also use friction to slow a moving vehicle down.

Friction is the resistance to motion of one object moving relative to another. It is not a fundamental force, like gravity or electromagnetism. Instead, scientists believe it is the result of the electromagnetic attraction between charged particles in two touching surfaces.

Scientists began piecing together the laws governing friction in the 1400s, but because the interactions are so complex, characterizing the force of friction in different situations typically requires experiments and can’t be derived from equations or laws alone.

For every general rule about friction, there are just as many exceptions. For instance, while two rough surfaces (such as sandpaper) rubbing against each other sometimes have more friction, very smoothly polished materials (such as plates of glass) that have been carefully cleaned of all surface particles may actually stick to each other very strongly. 

There are two main types of friction, static friction and kinetic friction. Static friction operates between two surfaces that aren’t moving relative to each other, while kinetic friction acts between objects in motion. In liquids, friction is the resistance between moving layers of a fluid, which is also known as viscosity. In general, more viscous fluids are thicker, so honey has more fluid friction than water.

The atoms inside a solid material can experience friction as well. For instance, if a solid block of metal gets compressed, all the atoms inside the material move, creating internal friction. In nature, there are no completely frictionless environments: even in deep space, tiny particles of matter may interact, causing friction.

A force can change the shape of an object and the greater the force, the greater the change it brings about. Although you can dent a wall with a hammer, a metal ball could knock a whole building down.

When you lift yourself up onto a wall you push down on the top of the wall with your hands. As you push, you exert a force which acts downwards on the wall, and the wall pushes against your hands, lifting you upwards. This force is equal to the force exerted by your hands, but it acts in the opposite direction. Gymnasts use a similar action to perform. Opposite forces in action can also be seen when a rocket takes off.

The study of rockets is an excellent way for students to learn the basics of forces and the response of an object to external forces. The motion of an object in response to an external force was first accurately described over 300 years ago by Sir Isaac Newton, using his three laws of motion. Engineers still use Newton’s laws to design and predict the flight of full scale rockets.

Forces are vector quantities having both a magnitude and a direction. When describing the action of forces, one must account for both the magnitude and the direction. In flight, a rocket is subjected to four forces; weight, thrust and the aerodynamic forces, lift and drag. The magnitude of the weight depends on the mass of all of the parts of the rocket. The weight force is always directed towards the center of the earth and acts through the center of gravity, the yellow dot on the figure. The magnitude of the thrust depends on the mass flow rate through the engine and the velocity and pressure at the exit of the nozzle. The thrust force normally acts along the longitudinal axis of the rocket and therefore acts through the center of gravity. Some full scale rockets can move, or gimbal, their nozzles to produce a force which is not aligned with the center of gravity. The resulting torque about the center of gravity can be used to maneuver the rocket. The magnitude of the aerodynamic forces depends on the shape, size, and velocity of the rocket and on properties of the atmosphere. The aerodynamic forces act through the center of pressure, the black and yellow dot on the figure. Aerodynamic forces are very important for model rockets, but may not be as important for full scale rockets, depending on the mission of the rocket. Full scale boosters usually spend only a short amount of time in the atmosphere.

In flight the magnitude, and sometimes the direction, of the four forces is constantly changing. The response of the rocket depends on the relative magnitude and direction of the forces, much like the motion of the rope in a “tug-of-war” contest. If we add up the forces, being careful to account for the direction, we obtain a net external force on the rocket. The resulting motion of the rocket is described by Newton’s laws of motion.

Forces change the way things move. The force of the wind will alter the direction of a hot air balloon. A moving ball, with no forces acting on it, will continue moving in the same direction and at the same speed until a force acts upon it.  A force can also change the shape of an object. A giant crusher can change the shape of a car — even your hand can exert a force to shape and mould certain objects. Whenever we find that the speed or direction of a moving object is changing, or the shape is changing, we say that forces are acting to cause these changes.

Forces change motion and shape. The force of a foot kicking a ball speeds the ball up. The force of a parachute on a skydiver slows the skydiver down. The force of a string on a whirling ball constantly changes the direction of motion, keeping it moving in a circle. Combinations of forces applied to materials can stretch, twist, and crush them.

            Illustrates the force exerted on a steel ball by the flick of a finger. We can see that the force sets the ball moving, and when the force stops, the ball continues in a straight line at a constant speed, unless another force acts on it. When a magnet is held near the moving ball, it exerts a pulling force on it – changing the direction of the ball. This is because magnets attract steel.

Although forces occur naturally, they can also be produced by people or machines. Often these forces are comprised of pushes, pulls or twists. You probably use forces like these every day — like when you push a door bell, open a drawer or twist a bottle top.

We also use machines to produce forces on our behalf. Machines can be made to exert a greater force than we can produce ourselves. For example, a tractor can pull a trailer full of hay, a bulldozer can push forward a mound of earth and a potter’s wheel turns and helps to shape a vase out of a lump of clay. Look around you and you will see many forces in action.

You can exert a force by pulling. Pulling a rope attached to a sledge will make the sledge move forwards. You use many pulling forces every day-like doing up a zip, pulling the curtains, putting socks on and pulling a door closed.

Sometimes you exert a force by pushing – you can push a table across a room, push a swing door to open it, push a trolley or push a parcel through letter box. When you push and pull you use your weight to exert a force onto an object.

You can also exert a number of forces by twisting – you might wring out wet clothes to dry them, turn a wheel, open a jam jar, and turn a screw – driver or wind up a clock. Twisting forces are usually conducted with your arms and your hands.