Forces
What is a force?
A push or pull acting on an object
What does a force arrow represent on a force diagram?
A force acting on the object. The direction of the arrow shows the direction of the force.
What is friction?
A force that opposes motion between surfaces in contact.
Which Newton's Law states that an object will remain at rest or continue moving at constant velocity unless acted upon by an unbalanced force?
Newton's First Law.
What does the equation F = ma allow us to calculate?
The force acting on an object when its mass and acceleration are known.
What is the unit used to measure force?
Newtons (N)
Draw a force diagram for a book sitting stationary on a table.
Two forces:
The forces should be equal and opposite.
Name one way friction can be increased and one way it can be reduced.
Increase:
Reduce:
What is inertia?
The tendency of an object to resist changes in its motion.
A 5 kg object accelerates at 2 m/s². Calculate the resultant force.
F = ma
F = 5 × 2
F = 10 N
What is the difference between a contact force and a non-contact force? Give one example of each.
A contact force requires objects to be touching, e.g. friction.
A non-contact force acts without physical contact, e.g. gravity or magnetism.
A student pushes a box to the right across a floor. Name the four forces that could act on the box.
Applied force →
Friction ←
Weight/gravity ↓
Normal/support force ↑
Why do car tyres have tread?
To increase friction/grip between the tyres and road, helping the car accelerate, brake and turn.
A passenger in a car continues moving forwards when the car suddenly brakes. Which Newton's Law explains this? Explain why.
Newton's First Law. The passenger's inertia causes them to continue moving forwards while the car slows down.
A 10 kg object experiences a resultant force of 50 N. What is its acceleration?
F = ma
a = F/m
a = 50/10
a = 5 m/s²
An object has a 10 N force pushing it to the right and a 10 N force pushing it to the left. What can you say about the forces and the object's motion?
The forces are balanced and the resultant force is 0 N. The object will either remain stationary or continue moving at constant velocity.
A box has a 40 N force acting to the right and a 25 N force acting to the left. What is the resultant force?
15 N to the right.
Why are racing cars designed to be streamlined?
To reduce air resistance (drag), allowing the car to move more easily through the air.
A swimmer pushes water backwards with their arms, and the water pushes the swimmer forwards. Which Newton's Law explains this?
Newton's Third Law — for every action force, there is an equal and opposite reaction force.
An object accelerates at 4 m/s² when a resultant force of 20 N acts on it. What is its mass?
F = ma
m = F/a
m = 20/4
m = 5 kg
A bicycle has 150 N of thrust forwards and 80 N of friction backwards. What is the resultant force, and what will happen to the bicycle?
Resultant force = 70 N forwards.
The forces are unbalanced, so the bicycle will accelerate forwards.
A parachutist is falling. Their weight is 700 N downward and air resistance is 700 N upward. Draw the force diagram and explain what happens to their motion.
Equal and opposite forces → resultant force = 0 N.
The parachutist falls at constant velocity (terminal velocity).
A cyclist wants to travel faster without increasing the force produced by their legs. Give TWO design changes they could make and explain how each affects the forces acting on the cyclist.
A rocket pushes gases downwards at high speed. The rocket accelerates upwards. Explain this using Newton's Third Law.
The rocket exerts a downward force on the gases. The gases exert an equal and opposite upward force on the rocket, causing it to accelerate upwards.
A 1200 kg car accelerates from rest at 3 m/s². The driving force is 4000 N forwards. What is the total resistive force acting backwards?
First calculate the resultant force:
F = ma
F = 1200 × 3
F = 3600 N
The car has 4000 N forwards but only 3600 N resultant.
Therefore:
4000 − resistive force = 3600
Resistive force = 400 N backwards.