Transcription
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Hi, my name is Malcolm and I'm a psre science specialist here at the Pig Lab. Welcome to another episode of PSRE Science Made Simple. In this video, I'll be going through a past year examination question on the topic of forces. I've also prepared this question for you to download for free by clicking a link in the description box below. So, let's get started.
Question 9. The diagram below shows you a runner's shoes at the start of a race. Now, they ask, which arrow P, Q, R, S indicates the direction of frictional force acting on the runner's shoe when he was running?
Now, let's recall whenever we talk about frictional force, what is the direction it will act in? We know that frictional force always opposes motion. So, I'm going to write this down. It will always be the opposite of motion. So, let's look at this runner. You can see this runner is running, and in which direction is he going to move in? We know the runner is going to move towards the left. So, if he moves towards the left, what is the direction of frictional force?
I know what some of you are thinking. Since he is moving towards the left, frictional force must be in the opposite direction. So, you chose S as your answer and you chose option four as the correct answer. Am I right? Now, those of you who chose option four, you can give yourself a cross. Four is actually wrong.
Now, whenever you tackle questions like this, you must be very careful. Before you jump into finding the opposite direction of the motion, we must first recall. Frictional force is a what kind of force? Frictional force is a contact force. That means it must always be between something and something else. In this case, they ask what is the direction of frictional force acting on which object? It was acting on the runner's shoe when he was running. So, can we look at the motion of the runner? No. Instead, we should look at the motion of his shoe.
So, think about this. If you want the runner to move forward towards the left, how is his shoe going to move? Now, the best way I can show you this is by showing you a diagram. So, now I'm going to draw out two diagrams of the runner. I want you to tell me which diagram here is correct.
Now, if you look at the diagram of the runner on the left, when he was running, what was he doing? You can see he's kicking his foot forward. Versus when you look at the runner on the right, how was he kicking his foot? Foot this time. Did the runner kick his foot forward or backward? You can see he's kicking the foot backwards.
Now, I want you to try running on the spot right now. When you are running forward, do you kick your foot forward like the runner on the left, or do you kick your foot backwards like the runner on the right? Usually, we will kick our foot backwards so that we will have traction and we can run forward. So, similarly, if if this runner was running forward towards the left, how should he kick his shoe? Is he going to kick it forward or backwards? He's going to kick it backwards.
So, I'm going to draw this out. We know that the motion of the shoe should be towards the right. And since we know the motion of the shoe, finding out the frictional force will be much simpler. So, if the motion of the shoe is towards the right, can the direction of frictional force also be towards the right? No. That's why S is definitely wrong. Instead, what's the opposite of the right? Frictional force must be acting towards the left-hand side.
So, from this, I think it becomes very obvious which arrow shows you the direction of frictional force. Is it P, Q, R, or S? Answer should be P. Therefore, the correct answer shouldn't be S, it should be option one.
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