Forces-­‐2 CONTACT FORCES: Friction Whenever an object moves against another object, it feels frictional forces. These forces act in the opposite direction to the movement. Friction makes it harder for things to move. Helpful frictional forces Friction can be useful: • friction between our shoes and the floor stop us from slipping • friction between tires and the road stop cars from skidding • friction between the brakes and wheel help bikes and cars slow down Frictional forces are much smaller on smooth surfaces than on rough surfaces, which is why we slide on ice. Unhelpful frictional forces Friction can also be unhelpful. If you don't lubricate your bike regularly with oil, the friction in the chain and axles increases. Your bike will be noisy and difficult to pedal. When there is a lot of friction between moving parts, energy is lost to the surroundings as heat. Think of what happens when you rub your hands together quickly. The friction warms them up. Upthrust Upthrust is the force that pushes an object up and makes it seem to lose weight in a fluid (which can be a liquid or gas). It is also called buoyancy. Upthrust or buoyancy keeps this ship afloat. Upthrust or buoyancy keeps this balloon up in the air. Air resistance Air resistance is caused by the frictional forces between a moving object and air. Air resistance slows down a moving object (such as a car) or a falling object (such as a parachute). The faster the vehicle moves, the bigger the air resistance becomes. The bigger the surface area of a parachute, the bigger the air resistance. The top speed of a vehicle is reached when the force from the cyclist or engine is balanced by air resistance. Which parachutist do you think will descend slower? Why? page 1 Forces-­‐2 Streamlining One way of decreasing air resistance is to streamline. For example, racing cyclists crouch down low on their bikes to reduce the air resistance on them. This helps them to cycle faster. They also wear streamlined helmets. These have special, smooth shapes that allow the air to flow over the cyclist more easily. Modern cars are also streamlined. Their smooth shapes make the air resistance smaller, which allows them to travel further on the same amount of fuel. A streamlined racing cyclist Which car do you think will go faster? Why? Surface Tension Surface tension is the skin-­‐like surface of a liquid due to the cohesion or attraction between the particles of the liquid. It makes the surface of the water act like an elastic or stretchable sheet that can support objects that are denser than water. It is the force that allows pins and paper clips float on water and insects to seemingly walk or glide on water! So any time you have water, you have surface tension. It's why water comes out of the faucet in drips, why water beads up on a surface, and why water beads up on a person's skin when they take a shower. A water strider “standing” on water. page 2 A paper clip floating on water. Forces-­‐2 Pressure When entering the gym, it is preferable to wear flat shoes rather than shoes with high, pointy heels. Why do you think that is the case? You may get told off if you sit with one leg of the chair touching the floor instead of all four. Why? This is because both instances are examples of an object exerting too much pressure on the floor. Why? Working out pressure Simply, pressure is the amount of force exerted on a given are. To work out pressure, we need to know two things: 1. the force or weight applied 2. the area over which the force or weight works. This is the equation for working out pressure: pressure =
force
N
The unit is 2 or pascal area
m
Pascals Notice that the unit of pressure here is N/m2 (newtons per square meter). Sometimes you will see another unit being 2
used. This is called the pascal, Pa. 1 Pascal = 1 N/m . Question A force of 20N acted over an area of 2m2 (two square metres). What is the pressure? pressure =
20N
N
= 10 2 or 10 pa 2
2m
m
Using pressure Thumbtacks have a large round end for you to push. The round end has a large area, so it applies a low pressure to your thumb. The sharp end has a very small area. The same pushing force produces a high pressure there, so it pushes into the notice board. page 3 Forces-­‐2 REMEMBER… The smaller the surface area, the higher the pressure applied. So, answer this question…what footwear will you use to walk on sand? Shoes with pointy heels or flat sandals? Why? Moments Forces can make objects turn if there is a pivot. Think of a playground see-­‐saw. The pivot is the thing in the middle of it. When two people applying the same amount of force are seated on opposite sides of the see-­‐saw equidistant (same distance) from the pivot, then equilibrium or balance is achieved [anti-­‐clockwise force = clockwise force]. However, if one person leaves or is replaced by one that applies more force, one side goes up [anti-­‐clockwise force ≠ clockwise force]. Turning forces around a pivot are called moments. It is possible to balance the see-­‐saw again if someone else gets onto the other end and sits in the correct place. This is because the turning forces are balanced. We say the moments are equal and opposite. Working out moments To work out a moment, we need to know two things: • the distance from the pivot that the force is applied. • the size of the force applied This is the equation for working out a moment: moment = force × distance page 4 Forces-­‐2 Example Imagine that a force of 10 N acted on a see-­‐saw 2m from the pivot. This is how we would work out the moment: • moment = force × distance • 10 × 2 = 20 Nm Here is an example of balanced moments. 10N at 2m from the pivot is balancing 20N at 1m from the pivot. The objects create moments of 20Nm that are equal and opposite, so the see-­‐saw is balanced. Moments on a balanced see-­‐saw Using moments •
•
•
A see-­‐saw will balance if the moments on each side of the pivot are equal. This is why you might have to adjust your position on a see-­‐saw if you are a different weight from the person on the other end. If a nut is difficult to undo with a short spanner, a longer spanner will help. This is because there will be a bigger moment on the nut, when the same force is applied further from the pivot. Using the same principle you can increase the moment applied by a lever or a crowbar, and this can help you move heavy objects more easily. Which door would be easier to push open? Why? page 5