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P5 S) Investigating Motion
P5 S) Investigating Motion
We can carry out an experiment to test the relationship of the three variables in Newton’s second law. Newton’s second law is shown below.
We can use the experiment that we will look at in this section to investigate two things:
We will look at how we can modify the experiment to investigate these two things later on in this section.
We set up the apparatus for the experiment like what is shown below.
- the effect that mass has on acceleration when the force is constant
- the effect that force has on acceleration when the mass is constant
We will look at how we can modify the experiment to investigate these two things later on in this section.
We set up the apparatus for the experiment like what is shown below.
The experiment works by using a piece of string to connect a trolley to a hook with some masses/ weight on. The piece of string that connects the trolley and the hook will be place over a pulley. We then mark a start line onto the table and place a light gate a certain distance from the start line. The light gate will be connected to a laptop or computer. The weight of the hook and the masses attached to the hook will provide the force that causes both the trolley and the hook with masses on to accelerate. We can work out the weight of the hook and masses by multiplying the mass by the gravitational field strength (w = m x g). More masses on the hook results in a greater weight/ force. The final step is to add a piece of card onto the top of the trolley – the shape of the card is shown below.
The ends of the card are higher up than the middle of the card. As the trolley passes through the light gate, these two higher bits of card will interrupt the light in the light gate. We measure the lengths of these higher bits of card and input the lengths into the light gate software on the laptop. The light gate software can work out the speed of the trolley by dividing the distance of the higher bits of card by the time that the light in the light gate is interrupted for.
The light gate will have two values for speed from the first and second higher bit of card. The software can then calculate the acceleration rate by using the formula below.
v in the above formula is the final velocity (from the second higher bit of card), u is the initial velocity (from the first higher bit of card) and t is time.
The light gate software will usually just give us the values for acceleration.
Whenever we are carrying out this experiment, we need to ensure that the trolley is placed at the same starting point – this is so that the trolley travels the same distance to the light gate every time we complete the experiment. Also, we need to ensure that the string is taut (which means that the string must be stretched/ pulled tight and is not slack i.e. not touching the table). Furthermore, we will complete the experiment 3 times for each of the different conditions and then work out the mean acceleration.
We are now going to have a look at how we modify the experiment to investigate the effect that mass and force have on acceleration.
The light gate software will usually just give us the values for acceleration.
Whenever we are carrying out this experiment, we need to ensure that the trolley is placed at the same starting point – this is so that the trolley travels the same distance to the light gate every time we complete the experiment. Also, we need to ensure that the string is taut (which means that the string must be stretched/ pulled tight and is not slack i.e. not touching the table). Furthermore, we will complete the experiment 3 times for each of the different conditions and then work out the mean acceleration.
We are now going to have a look at how we modify the experiment to investigate the effect that mass and force have on acceleration.
Investigating Mass
We can investigate the effect that mass has on the rate of acceleration by adding masses to the trolley. We need to make sure that we keep the mass/ weight on the hook the same so that the force is constant throughout the whole experiment, thus meaning that the change in acceleration that we observe is solely down to the change in mass on the trolley and no other factors.
When you undertake this experiment to investigate the effect that mass has on acceleration, you will observe that a greater mass on the trolley results in a lower acceleration rate (and vice versa: a lower mass on the trolley results in a greater rate of acceleration). This is because mass and acceleration are inversely proportional to each other.
We can investigate the effect that mass has on the rate of acceleration by adding masses to the trolley. We need to make sure that we keep the mass/ weight on the hook the same so that the force is constant throughout the whole experiment, thus meaning that the change in acceleration that we observe is solely down to the change in mass on the trolley and no other factors.
When you undertake this experiment to investigate the effect that mass has on acceleration, you will observe that a greater mass on the trolley results in a lower acceleration rate (and vice versa: a lower mass on the trolley results in a greater rate of acceleration). This is because mass and acceleration are inversely proportional to each other.
The graph of how acceleration changes as we increase the mass on the trolley is shown below. From the graph, you can see that mass and acceleration are inversely proportional to each other.
Force on Acceleration
We can also investigate the effect that force has on acceleration. When we investigate the effect that force has on acceleration, we need to make sure that we keep the mass of the whole system the same throughout the whole experiment; the system includes the trolley, the string, the hook and any masses (on both the trolley and the hook). We start with all of the masses on the trolley. We then transfer some of the masses from the trolley to the hook, which increases the force acting on the system (when we transfer masses, the mass of the whole system stays the same as we are just changing the location of the masses in the system).
When you undertake this experiment, you will observe that a greater force (more masses on the hook) will result in a greater acceleration rate (and vice versa; when the force is smaller (fewer masses on the hook), the trolley will have a lower acceleration rate). This is because force and acceleration are directly proportional to each other.
We can also investigate the effect that force has on acceleration. When we investigate the effect that force has on acceleration, we need to make sure that we keep the mass of the whole system the same throughout the whole experiment; the system includes the trolley, the string, the hook and any masses (on both the trolley and the hook). We start with all of the masses on the trolley. We then transfer some of the masses from the trolley to the hook, which increases the force acting on the system (when we transfer masses, the mass of the whole system stays the same as we are just changing the location of the masses in the system).
When you undertake this experiment, you will observe that a greater force (more masses on the hook) will result in a greater acceleration rate (and vice versa; when the force is smaller (fewer masses on the hook), the trolley will have a lower acceleration rate). This is because force and acceleration are directly proportional to each other.
The graph of how acceleration changes as we increase the force is shown below. From the graph, you can see that force and acceleration are directly proportional to each other.
Reducing the Effects of Friction
Friction may affect your results. Friction is the resistive force that one surface or object experiences when moving over another surface. There will be friction between the wheels on the trolley and the table. Friction will decrease the acceleration rate because friction will be acting in the opposite direction to the direction of motion of the trolley (friction is a resistive force). In the diagram of the experiment, the motion of the trolley is towards the right and friction will be acting towards the left (the opposite direction). Friction increases as the velocity of the trolley increases.
We can reduce the effects that friction has on the experiment by using an air tack. Air is pumped through small holes in the air track, which causes the vehicle (trolley) to hover above the air track; this reduces the amount of friction acting on the vehicle/ trolley. An air track is essentially like an air hockey table that you would find at an arcade.
Friction may affect your results. Friction is the resistive force that one surface or object experiences when moving over another surface. There will be friction between the wheels on the trolley and the table. Friction will decrease the acceleration rate because friction will be acting in the opposite direction to the direction of motion of the trolley (friction is a resistive force). In the diagram of the experiment, the motion of the trolley is towards the right and friction will be acting towards the left (the opposite direction). Friction increases as the velocity of the trolley increases.
We can reduce the effects that friction has on the experiment by using an air tack. Air is pumped through small holes in the air track, which causes the vehicle (trolley) to hover above the air track; this reduces the amount of friction acting on the vehicle/ trolley. An air track is essentially like an air hockey table that you would find at an arcade.
Two Light Gates
This experiment can be carried out with two light gates instead of one. When this is the case, the card on top of the trolley will just be a rectangle (rather than having 2 higher bits like before). The setup of the experiment with two light gates is shown below.
This experiment can be carried out with two light gates instead of one. When this is the case, the card on top of the trolley will just be a rectangle (rather than having 2 higher bits like before). The setup of the experiment with two light gates is shown below.
The two light gates will give us two values for speed, which can then be used to calculate the acceleration of the trolley. The experiment with two light gates works exactly the same as the experiment with one light gate – it is just a slightly different way to set up the experiment.