Where a good grip is needed
Have you seen a tennis match on television? The players come out from their dressing rooms with their rackets and bags. They sit on chairs by the side of the court, take a drink and then warm up by knocking a tennis ball to each other over the net. After a few minutes, the umpire asks the crowd to be quiet and the match begins. The winner will be the most skilful but friction also plays its part.

There are two main places where this friction is important: the friction between the racket and the hand and the friction between the soles of the tennis shoes and the court. When a tennis player grips the handle of the tennis racket the surface of the skin presses close to the surface of the handle. Ridges in one surface push down into grooves in the other surface. When the racket is swung to hit the ball, it pulls on the hand and frictional force develops to prevent the racket slipping in the hand. The frictional force develops because of the way the grooves and ridges hold together when the surface of the handle tries to move along the surface of skin.

As the match progresses, the players become hot and start to sweat. When they rest between games they dry their hands on a towel and sometimes rub the surface of their handle to remove any water. In a long game, in hot weather, sweat may build up on the handle before a player has time to wipe it away. When this happens the player is in danger of losing their grip - possibly when they are about to make an important swing. The sweat flows into the grooves on the surfaces and starts to fill them up. This means that the ridges cannot sink so deeply into the grooves and if a particularly powerful swing is to be made the friction will not hold the handle in place. When the swing is made the racket slides a little in the hand and the player makes a weak return shot. The ball may fall into a good position for the other player who then makes such a powerful shot that it cannot be returned and the game is lost.

If a player can survive with a wet racket handle until there is a break, the racket can be changed but there is another danger which may affect the outcome of the game - a slippery court.

Tennis shoes have soles, which make a good grip with the court. When the player stands on the court, the surface of the soles presses against the grassy surface of the court. The ridges on one surface press into the grooves of the other. As a player waits for the other player to serve he or she may stand still. When this happens, the weight of the player pushes down on the ground and the ground pushes back to hold the player in place. As soon as the ball is served and the player moves, friction comes into the game. When you take a stride, like a tennis player coming forwards to play the ball, you keep one foot on the ground and raise the other and move it forwards. At the same time you push backwards on the sole of your foot in contact with the ground. A force of friction develops to match the backward push so you do not slip. When the sole of the other foot is down, you push on it, too, as you raise your other leg and friction develops again so you can move forwards.

Sometimes, during a long game, there may be a light shower of rain. There may be too few drops to stop the game but they are covering the surface of the court. The rainwater fills in the microscopic grooves in the court just as the sweat does on the surface of the handle. If a player has to turn fast to return a difficult shot, the force they make on the foot may be too much for the friction on the wet surface to hold and the shoe slides. The player loses their position, swings wildly and misses the ball and maybe loses the game.
It does not matter who plays a game of tennis, friction always plays its part and can decide who wins and loses.

When we stand up why does the Earth push on our feet?
When one object makes a force which pushes on another object the other object always pushes back with the same size of force. Think what would happen if this did not occur. When you lean on a wall you push on it. If there were nothing pushing back you would push the wall over. This means that as the wall stays in position it must also make a force, which pushes back on you. If the force were just a bit weaker the wall would bend away from you. If the force were a bit stronger the wall would bend towards you. The reason it stays where it is, is because the force it makes just balances your pushing force. The same thing happens when you stand up. The force of your weight pushing down is balanced by the force from the earth pushing back. If this force were weaker than your weight you would sink into the ground.

Does the weight of the person affect the friction between their feet and the ground?
Yes, it does. A heavy person presses down more strongly on the ground than a person with a lighter weight. This means that the ridges are pressed deeper into the grooves. They take much more force to remove them from the grooves and this increases the force of friction which can be made.

Football players have studs in their boots. How do the studs help to grip?
The studs dig into the ground. Friction between the surface of the studs and the surface of the holes helps the players keep their feet even when the ground is wet.

Racing cars have different tyres for dry and wet conditions. Why?
In dry conditions racing cars have tyres without treads. The surface of these tyres can be as close as possible to the surface of the road. This allows as many ridges and grooves as possible to interlock and let a strong frictional force develop which allows the wheels to push firmly on the road.

In wet conditions the water fills the grooves between the smooth tyres and the road and makes the wheels spin or slide. The tyres used in wet weather have treads. The treads channel water away from the road surface so the remaining tyre surface can make as good a grip as possible in the wet conditions.