Showing Forces 2

In this worksheet, students will be helped to view forces in a different way, using arrows to describe their direction and relative strength. By following this system students have a chance to broaden their understanding of how forces work.

Key stage:  KS 3

Curriculum topic:   Physics: Motion and Forces

Curriculum subtopic:   Motion and Forces

Difficulty level:

QUESTION 1 of 10

One way in which forces can be represented is by the use of labelled arrows; for example look at this swimmer:

As you can see he propels himself through the water using his arms and legs, pushing the water out of the way which creates RESISTANCE to the forward motion in the form of friction.

These forces are represented by a THRUST arrow as the swimmer pushes himself forward and then a FRICTION arrow acting in the opposite direction, pushing against the swimmer.

Notice that the THRUST arrow is bigger than the FRICTION arrow, meaning that the swimmer must be speeding up as he is overcoming the water resistance acting against him. If the two arrows were the same size the forces would be equal, so he would be moving at a steady speed.

OK, use this idea in the questions on this worksheet.

Jenny was out riding her bike:

Here are three possible pictures representing Jenny riding on a level road at a steady speed.

Which one do you think is correct?

A

B

C

Jenny nows speeds up by pedalling harder.

Which of these pictures do you think correctly represents this?

A

B

C

To slow herself down Jenny pulls on her brakes. Which diagram correctly represents this?

A

B

C

When Jenny gets off the bike and stands on the road she stays there because gravity is pulling her towards the centre of the Earth.

OK, so what's the best explanation for why does she NOT get pulled through the road by gravity?

gravity only acts when you jump up

gravity runs out eventually

the road pushes up against Jenny with a force equal to gravity

Mrs. Bates's science class are doing some weighing experiments.

They are weighing objects on forcemeters and then lowering them into water to see what effect it has on the reading.

Here are their results:

Looking at their results you can see that the reading for each object decreases when it is placed in water.

What do you think is the best explanation for this?

the objects in water are not pulled down by gravity

the water takes some of the weight of the object

the water pushes up against the object

Here are the results again:

Looking at the table, which object do you think has a set of readings you feel most confident in; in other words which object had the most consistent results?

Toy car

Scissors

China cat

Here's the table one more time:

Group E found that when they lowered the spoon into the water their reading for weight in water was 0N. Although this result cannot be correct, what does it suggest the spoon did?

floated

sank slowly

sank quickly

Here are the results for just one object: the plasticine ball:

You'll notice that although some of the results for air are the same, others are a little different.

The same is true for the results in water.

Why do you think this is (tick all you think apply)?

the young scientists cannot measure accurately

they kept making mistakes

some forcemeters were different to others

the forcemeter pointer didn't stay steady

the scale looked different depending on where they were standing

some of the plasticene balls may have been different

What do you think the young scientists should do to make their results more reliable?

try lots more different objects

weigh the same object several times

take an average of the different groups' results

Here are the plasticine ball results once more:

Which of these three pictures best represents the forces acting on the plasticine ball when it has been lowered into the water?

A

B

C

• Question 1

Jenny was out riding her bike:

Here are three possible pictures representing Jenny riding on a level road at a steady speed.

Which one do you think is correct?

A
EDDIE SAYS
B is tempting - the arrow's showing Jenny's force as she's pedalling but ... there's no force against her: no air resistance and no road friction, so it cannot be right. Similar sort of problem with C, but A shows opposite and equal forces, showing she's pedalling along at a steady speed, not speeding up or slowing down, so no change in the forces.
• Question 2

Jenny nows speeds up by pedalling harder.

Which of these pictures do you think correctly represents this?

A
EDDIE SAYS
B shows only a forward force with no friction acting against the bike; C shows an increased force AGAINST Jenny (she's either braked or the wind has started to blow stronger) so A is the answer: it shows an increase in the thrust showing Jenny speeding up (as she does so the air resistance will eventually increase so that she reaches a new, higher, steady speed).
• Question 3

To slow herself down Jenny pulls on her brakes. Which diagram correctly represents this?

C
EDDIE SAYS
You might think that A was right as it shows a force opposing Jenny's motion, but there's no forward arrow and she's still moving forward (even though she's not pedalling; B has equal and opposite arrows, so she's going at a steady speed, which means that C is right as it shows an increased friction force as Jenny is braking, but still moving forward.
• Question 4

When Jenny gets off the bike and stands on the road she stays there because gravity is pulling her towards the centre of the Earth.

OK, so what's the best explanation for why does she NOT get pulled through the road by gravity?

the road pushes up against Jenny with a force equal to gravity
EDDIE SAYS
Not at all easy to understand! Are you sitting on a chair? Well, the chair is providing an upward force AGAINST gravity to keep you there. "But I'll float off" you'll say! No, gravity keeps pulling you down, but the chair force equals gravity so you stay there. Try blowing up a balloon and placing it between you and the chair - then you'll see BOTH forces acting! Try the same when you lean against a wall: put a balloon in between the wall and your hand and you'll see that the wall DOES provide a force against your push!
• Question 5

Mrs. Bates's science class are doing some weighing experiments.

They are weighing objects on forcemeters and then lowering them into water to see what effect it has on the reading.

Here are their results:

Looking at their results you can see that the reading for each object decreases when it is placed in water.

What do you think is the best explanation for this?

the water pushes up against the object
EDDIE SAYS
In fact gravity DOES act through water (which is why things sink) but water is a much 'thicker' substance than air (there are lots more particles in each cube of water compared to air) and so these push hard against an object with a force called UPTHRUST. This upward force reduces the reading of gravity which is why things seem to weigh less in water.
• Question 6

Here are the results again:

Looking at the table, which object do you think has a set of readings you feel most confident in; in other words which object had the most consistent results?

Toy car
EDDIE SAYS
If you look at the results for the toy car you can see that the results in water are all the same: 0.5N, whilst those in air are nearly all the same, with just one only 0.5N higher. Those for the scissors and the china cat vary rather more than this.
• Question 7

Here's the table one more time:

Group E found that when they lowered the spoon into the water their reading for weight in water was 0N. Although this result cannot be correct, what does it suggest the spoon did?

floated
EDDIE SAYS
When an object floats it appears to be weightless as the upthrust seems to cancel out gravity: if you imagine it, the forcemeter would keep going lower, in fact below the object that was floating, so there would be no reading.
• Question 8

Here are the results for just one object: the plasticine ball:

You'll notice that although some of the results for air are the same, others are a little different.

The same is true for the results in water.

Why do you think this is (tick all you think apply)?

some forcemeters were different to others
the forcemeter pointer didn't stay steady
the scale looked different depending on where they were standing
some of the plasticene balls may have been different
EDDIE SAYS
Whilst it may be true that they cannot measure accurately and make mistakes, the other explanations are more related to the experiment: some forcemeters may not have started at zero and the pointer does wobble. It's certainly true that the scale looks different depending on where you are (try filling a measuring jug looking from above, level and below: three different readings!). The plasticene balls may well have been different sizes and shapes having been played with!
• Question 9

What do you think the young scientists should do to make their results more reliable?

weigh the same object several times
EDDIE SAYS
Any balding scientist with white hair will tell you that for reliable results you must repeat your experiment again and again! It's so easy for things to go wrong, so the only way of being sure that your answer is THE answer is to check it again and again. Now for school science this just isn't possible (there isn't time), so generally several groups do the same test and those results are compared.
• Question 10

Here are the plasticine ball results once more:

Which of these three pictures best represents the forces acting on the plasticine ball when it has been lowered into the water?

A
EDDIE SAYS
The upthrust of the water is shown by the upward arrow but notice that the downward arrow (due to gravity) shows a stronger force: the ball is sinking. If you look at the results you'll see that there is still a force of around 4N when the ball's in the water, showing it still being pulled down by gravity.
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